Symmetry Breaking in Iron(ii) Spin-crossover Molecular Crystals

This review provides an up to date survey of a singular class of iron(II) spin crossover (SCO) molecular materials that undergo high-spin (HS) Ø low-spin (LS) phase transitions accompanied by crystallographic symmetry breaking (CSB). Particular interest has been focused on a variety of complexes that exhibit one-step or stepwise SCO behavior and CSB. Most of them afford excellent examples of well-ordered 1HS-1LS, 2HS-1LS or 1HS-2LS intermediate phases (IP) and represent an important platform to disclose microscopic mechanisms responsible for cooperativity and ordering in such multistable materials.


Introduction
Spin crossover (SCO) complexes represent an important class of switchable molecular materials [1][2][3][4].In these complexes, the energy gap between the high-spin (HS) and the low-spin (LS) states is of the order of magnitude of the thermal energy.Consequently, they can adopt both spin states and change from one state to the other state in a reversible, detectable and controllable way through temperature or pressure gradients, light irradiation and chemical stimuli.The LS Ø HS switch is accompanied by distinctive changes in magnetic, optical and dielectric properties.The SCO behavior involves an electronic redistribution between the t 2g and e g orbitals, strongly coupled with structural changes, which mainly affect the coordination core of the metal complex.In the solid state, these structural changes are cooperatively transmitted through elastic interactions and, in favorable cases, first-order spin transition occasionally accompanied by hysteretic behavior confers a binary function to the material.The elastic interactions depend somehow on the intermolecular contacts developed between SCO centers (i.e., van der Waals, hydrogen bonding, π-π interactions, coordination bonds, etc.) and determine the cooperative character of the SCO, namely steepness, hysteresis and steps.Theoretically explained as the result of a competition between antagonistic short-range and long-range elastic interactions [5][6][7][8][9], observation of steps has been considered for many years an intriguing and relatively rare event.However, the exponential growth experienced by the field in the last decade has uncovered an increasing number of stepwise SCO events featuring ordered intermediate phases (IP) well characterized from a structural viewpoint.There are two distinct situations that account for stepwise transitions.One is observed when different SCO sites, characterized by distinct transition temperatures (T 1/2 or T c ), are present in the solid [10][11][12][13][14][15][16][17].The other arises from subtle balances between different elastic interactions in the crystal lattice, which spontaneously generate two or more distinct SCO sites.This spontaneous change may occur without significant modification of the unit cell, other than the typical structural changes associated with the LS Ø HS conversion.However, a growing number of examples, particularly in the area of Fe II complexes, has been found to exhibit crystallographic symmetry breaking (CSB).CSB is characterized by a change of space group type and eventually of crystal system in the IP.Although the occurrence of phase transitions with CSB is ubiquitous in Nature, it is still considered a rare event and an important opportunity to learn new fundamental aspects of the SCO behavior.As far as we know, apart from the Fe II complexes here reviewed only one complex exhibiting SCO and CSB has been described in the chemistry of Co II [18] and Mn III [19], while there are five reports concerning Fe III complexes [20][21][22][23][24].
CSB has been partially reviewed by Shatruk et al. in a broader context which, includes displacive transitions, phase transitions caused by disorder, polymorphism, and first order transitions without space group changes [25].The aim of the present contribution is to update and systematically review the Fe II SCO systems that manifest half-step, one-step, two-step and multi-step SCO transitions accompanied by CSB.The next three sections are organized according to the nature of the systems: monomeric (Section 2), polynuclear (Section 3), or polymeric (Section 4).Section 2 has been divided in two subsections, one dedicated to mononuclear SCO complexes exhibiting stepwise transitions and ordering in the IP, the other, dedicated to complexes that exhibit complete one-step SCO with CSB.Section 2 essentially describes a series of significant dinuclear SCO compounds that feature CSB.Section 3 is organized according to the one-, two-or three-dimensional nature of the coordination polymer that shows SCO and CSB behavior.

Stepwise SCO Behavior
The prototypical mononuclear [Fe II (2-pic) 3 ]Cl 2 ¨EtOH (pic = picolylamine) was the first two-step SCO complex investigated extensively [26].Its structure was analyzed at 115 K [27] and 90 K in the LS state and at 150 K and 298 K in the HS state [28].Shortly after, Gütlich et al. observed for this compound the occurrence of a two-step behavior characterized by a small plateau (ca. 10 K wide) centered at about 119 K, which separates two half spin transitions with T c1 = 114 K and T c2 = 124 K (Figure 1) [29][30][31].The existence of the intermediate plateau was rationalized in terms of concurrence of "ferromagnetic-like" long-range and "antiferromagnetic-like" short-range elastic interactions in the crystal [32].Almost 20 years after the discovery of this two-step SCO behavior, a detailed structural reinvestigation by Bürgi et al. disclosed the nature of the double phase transition and hence of the plateau in [Fe II (2-pic) 3 ]Cl 2 ¨EtOH].In the LS and HS states the unit cell is monoclinic, space group P2 1 /c, and contains one crystallographically independent Fe II site.In contrast, in the plateau (IP), the unit cell doubles and accommodates two crystallographically independent Fe II sites, which corresponds to a 1:1 mixture of LS and HS states (IP = 1HS-1LS).In the IP, layers of HS molecules alternate with layers of LS molecules defining an ordered ¨¨¨HS-LS¨¨¨pattern.These layers lie approximately in the bc plane and stack along the a-axis (Figure 1) [33].
A less common pattern in the IP has been described by Létard, Howard and Halcrow et al. for [Fe(bmpzpy) 2 ](BF 4 ) 2 ¨xH 2 O (x = 0, 1/3; bmpzpy = 2,6-bis{3-methylpyrazol-1-yl}pyridine).This system undergoes an incomplete SCO where only 1/2 of the Fe II centers switches to the LS while the other 1/2 remains HS.This so-called half SCO occurs in two cooperative steps centered at T c2 = 147 K and T c1 = 105 K with ∆T c2 = 6 K and ∆T c1 = 18 K wide hysteresis loops, respectively (Note: half SCO) (Figure 2).This 50% mixed spin state is observed when usual temperature scan rates (1-2 K/min) are employed at low temperatures, due to slow HS Ø LS kinetics.However, it relaxes to the fully LS state in 2 h at 100 K.The crystal structures of the anhydrous and the hydrate species (x = 0, 1/3) were investigated at 200 K, 130 K and 90 K.At 200 K, the crystals are monoclinic (space group C2/c, (Z = 8)) and contain one crystallographically independent Fe II site, which is HS.At 130 K, the anhydrous form remains in the same space group type but triples the unit cell (Z = 24) to accommodate three distinct Fe II sites, one LS and two HS (IP = 2HS-1LS), a fact consistent with the magnetic behavior.In this ordered state, the unit cell shows two types of chains running along the c-axis.One is defined by two Fe II sites, •••Fe1(LS)-Fe3(HS)•••, while the other contains a unique Fe2(HS) site.Both kinds of chains alternate along the a-axis and superpose along the b-axis (Figure 2).At 90 K, after relaxing the mixed state at 100 K, the complex shows one crystallographically independent Fe II site, which is LS.At 200 K and 130 K, the crystal structures of the hydrated and anhydrous compounds are essentially the same since the inclusion of 1/3 H2O molecules seems to not perturb too much the crystal packing.However, in contrast to the anhydrous compound, all the Fe II centers of the hydrated derivative remain in the HS state at 130 K.At 90 K, the structure of the hydrated derivative is similar to that of the anhydrous compound at 130 K [34].
The  The crystal structures of the anhydrous and the hydrate species (x = 0, 1/3) were investigated at 200 K, 130 K and 90 K.At 200 K, the crystals are monoclinic (space group C2/c, (Z = 8)) and contain one crystallographically independent Fe II site, which is HS.At 130 K, the anhydrous form remains in the same space group type but triples the unit cell (Z = 24) to accommodate three distinct Fe II sites, one LS and two HS (IP = 2HS-1LS), a fact consistent with the magnetic behavior.In this ordered state, the unit cell shows two types of chains running along the c-axis.One is defined by two Fe II sites, •••Fe1(LS)-Fe3(HS)•••, while the other contains a unique Fe2(HS) site.Both kinds of chains alternate along the a-axis and superpose along the b-axis (Figure 2).At 90 K, after relaxing the mixed state at 100 K, the complex shows one crystallographically independent Fe II site, which is LS.At 200 K and 130 K, the crystal structures of the hydrated and anhydrous compounds are essentially the same since the inclusion of 1/3 H2O molecules seems to not perturb too much the crystal packing.However, in contrast to the anhydrous compound, all the Fe II centers of the hydrated derivative remain in the HS state at 130 K.At 90 K, the structure of the hydrated derivative is similar to that of the anhydrous compound at 130 K [34].
The The crystal structures of the anhydrous and the hydrate species (x = 0, 1/3) were investigated at 200 K, 130 K and 90 K.At 200 K, the crystals are monoclinic (space group C2/c, (Z = 8)) and contain one crystallographically independent Fe II site, which is HS.At 130 K, the anhydrous form remains in the same space group type but triples the unit cell (Z = 24) to accommodate three distinct Fe II sites, one LS and two HS (IP = 2HS-1LS), a fact consistent with the magnetic behavior.In this ordered state, the unit cell shows two types of chains running along the c-axis.One is defined by two Fe II sites, ¨¨¨Fe1(LS)-Fe3(HS)¨¨¨, while the other contains a unique Fe2(HS) site.Both kinds of chains alternate along the a-axis and superpose along the b-axis (Figure 2).At 90 K, after relaxing the mixed state at 100 K, the complex shows one crystallographically independent Fe II site, which is LS.At 200 K and 130 K, the crystal structures of the hydrated and anhydrous compounds are essentially the same since the inclusion of 1/3 H 2 O molecules seems to not perturb too much the crystal packing.However, in contrast to the anhydrous compound, all the Fe II centers of the hydrated derivative remain in the HS state at 130 K.At 90 K, the structure of the hydrated derivative is similar to that of the anhydrous compound at 130 K [34].
The same IP = 2HS-1LS has been recently reported by Bao and Tong et al. for the polymorph A of complex [Fe(bpmen)(NCSe) 2 ] (bpmen is the tetradentate ligand N,N 1 -dimethyl-N,N 1 -bis(2pyridylmethyl)-1,2-ethanediamine).This compound undergoes a complete cooperative two-step SCO characterized by a first drop in the thermal dependence of the χ M T product (χ M is the molar magnetic susceptibility and T temperature) from 3.2 cm 3 ¨K¨mol ´1 (HS state) to 2.6 cm 3 ¨K¨mol ´1 (IP state).This step, characterized by a hysteresis loop 5 K wide centered at T c2 = 117.5 K, reflects the conversion of 1/3 of the HS centers to the LS state.The second drop in χ M T (between 2.2 cm 3 ¨K¨mol ´1 and 0.2 cm 3 ¨K¨mol ´1), characterized by a hysteresis loop 10 K wide centered at T c1 = 101 K, involves the remaining 2/3 of the HS species.The crystals of this polymorph are orthorhombic, its space group being Aba2, and contain one asymmetric unit defined by half a molecule with a two-fold axis passing through the Fe II site.The molecules are arranged in layers that stack along the a-axis and extend parallel to the plane bc.At 100 K (IP state) the unit cell triples displaying a HS-HS-LS pattern.However, the order in this IP = 2HS-1LS is produced in a different way to that described in the previous example, namely a layer constituted of Fe2(LS) sites is separated by two consecutive identical layers made up of Fe1(HS) sites (Figure 3) [35].However, the order in this IP = 2HS-1LS is produced in a different way to that described in the previous example, namely a layer constituted of Fe2(LS) sites is separated by two consecutive identical layers made up of Fe1(HS) sites (Figure 3) [35].Red and orange octahedrons correspond to LS and HS sites, respectively.
The multicomponent mononuclear complex [Fe II (dpp)2][Ni(mnt)2]2•MeNO2, (dpp = 2,6bis(pyrazol-1-yl)pyridine (see Appendix Figure A1) and mnt = maleonitriledithiolate) displays a complicated SCO behavior.Above 180 K, the Fe centers are HS but abruptly change to the LS state below this temperature.In contrast, in the heating mode the population of the Fe II HS species increases showing three steps defining an irregular hysteresis.Although crystallographic evidence of order in the intermediate states could only be partially demonstrated for the first plateau (1HS-2LS), where the unit cell triples to contain three distinct Fe II centers, it could be concluded from magnetic susceptibility and Mössbauer data that the steps corresponds to the 1 : 2 2 : 1 sequence [36].Despite the nature of the SCO in [Fe II (2-pic)3]Cl2•EtOH] has attracted much attention for two decades, the first structurally investigated two-step SCO with CSB was reported by Tuchagues et al. for the complex [Fe II {5-NO2-sal-N(1,4,7,10)}], where 5-NO2-sal-N(1,4,7,10) is a six-coordinating ligand prepared by Schiff base condensation of 5-NO2-salicylaldehyde with 1,4,7,10-tetraazadecane [37].The magnetic data showed the occurrence of two cooperative half spin transitions centered at Tc2 = 176 K and Tc1 =142 K with hysteresis loops 3 K and 8 K wide, respectively (Figure 4).In the HS state, the crystal is monoclinic, its space group being P2/c, and is characterized by the presence of one Fe II HS site.The high temperature step, HS → IP, occurs simultaneously with a change of the crystal to the space group P2.The loss of the inversion center in the IP also involves the generation of two crystallographically independent Fe II centers.In the subsequent IP → LS step the crystal loses the two-fold symmetry axes and becomes triclinic (space group P1).Contrary to expectations, the average metal-to-ligand bond length of the [FeN4O2] coordination center is virtually the same for the HS and the IP states and only 0.1 Å larger than that observed for the LS state at 103 K. Consequently, no crystallographic evidence of IP with an ordering •••HS-LS••• was afforded for this complex.This problem was reinvestigated by Gütlich et al. but no clear conclusion was achieved [32].The multicomponent mononuclear complex [Fe II (dpp) 2 ][Ni(mnt) 2 ] 2 ¨MeNO 2 , (dpp = 2,6-bis (pyrazol-1-yl)pyridine (see Figure A1) and mnt = maleonitriledithiolate) displays a complicated SCO behavior.Above 180 K, the Fe centers are HS but abruptly change to the LS state below this temperature.In contrast, in the heating mode the population of the Fe II HS species increases showing three steps defining an irregular hysteresis.Although crystallographic evidence of order in the intermediate states could only be partially demonstrated for the first plateau (1HS-2LS), where the unit cell triples to contain three distinct Fe II centers, it could be concluded from magnetic susceptibility and Mössbauer data that the steps corresponds to the LS step1 Ø 1HS : 2LS step2 Ø 2HS : 1LS step3 Ø HS sequence [36].Despite the nature of the SCO in [Fe II (2-pic) 3 ]Cl 2 ¨EtOH] has attracted much attention for two decades, the first structurally investigated two-step SCO with CSB was reported by Tuchagues et al. for the complex [Fe II {5-NO 2 -sal-N(1,4,7,10)}], where 5-NO 2 -sal-N(1,4,7,10) is a six-coordinating ligand prepared by Schiff base condensation of 5-NO 2 -salicylaldehyde with 1,4,7,10-tetraazadecane [37].The magnetic data showed the occurrence of two cooperative half spin transitions centered at T c2 = 176 K and T c1 =142 K with hysteresis loops 3 K and 8 K wide, respectively (Figure 4).In the HS state, the crystal is monoclinic, its space group being P2/c, and is characterized by the presence of one Fe II HS site.The high temperature step, HS Ñ IP, occurs simultaneously with a change of the crystal to the space group P2.The loss of the inversion center in the IP also involves the generation of two crystallographically independent Fe II centers.In the subsequent IP Ñ LS step the crystal loses the two-fold symmetry axes and becomes triclinic (space group P1).Contrary to expectations, the average metal-to-ligand bond length of the [FeN 4 O 2 ] coordination center is virtually the same for the HS and the IP states and only 0.1 Å larger than that observed for the LS state at 103 K. Consequently, no crystallographic evidence of IP with an ordering ¨¨¨HS-LS¨¨¨was afforded for this complex.This problem was reinvestigated by Gütlich et al. but no clear conclusion was achieved [32].The PF 6 ´derivative displays a half SCO (T c = 122 K) accompanied by a change of space group from P2 1 /n (HS state) to P2 1 (IP state).In the pure HS state, there is one crystallographically independent Fe II site (Fe-N av = 2.190(3) Å), whereas in the IP the complex shows two equally populated Fe II sites, one HS (Fe-N av = 2.186(5) Å) and other LS (Fe-N av = 1.994(5)Å).A similar situation is observed when PF 6 ´is formally replaced by AsF 6 ´.Nevertheless, the resulting compound undergoes a two-step SCO centered at T c2 =122 K and T c1 = 85 K.The low-temperature step is incomplete due to slow HS Ø LS kinetics.In this step, when cooling at 1 K/min, χ M T drops from 1.70 cm 3 ¨K¨mol ´1 at 100 K (IP = 1HS-1LS) down to 1.10 cm 3 ¨K¨mol ´1 at 5 K.However, after keeping the sample at 85 K for 12 h the AsF 6 ´derivative relaxes completely to the LS state (χ M T decreases down to 0.24 cm 3 ¨K¨mol ´1).
In this process the complex recovers the space group P2 1 /n.The crystal packing of both derivatives consists of two-dimensional networks constructed by NH¨¨¨Cl ´hydrogen bonds between Cl ´and the imidazole NH groups of three neighboring [Fe II H 3 L Me ] 2+ cations, whereas XF 6 ´(X = P, As) acts as an isolated anion filling the space between two consecutive supramolecular layers.The layers are racemic with the ∆ and Λ configurations of [Fe II H 3 L Me ] 2+ interconnected through hydrogen bonds in an alternating way.In the ordered IP the layers can be described as a stacking of chains of HS sites that alternate with chains of LS sites.Interestingly, the HS and LS sites adopt the ∆ and Λ configurations, respectively [39].
In the search for new Fe II SCO complexes based on polydentate ligands with imidazole-pendant groups, the same research group has reported an important number of complexes of general formula [Fe(H 2 L 2´Me ) 2 ](X)(Y)¨Solvent, where H 2 L 2´Me is the tridentate ligand ((2-methylimidazol-4 -yl)methylidene)histamine.Among them the system {[Fe(H 2 L 2´Me ) 2 ](X)(ClO 4 )¨1EtOH} (X = Cl, Br) shows a gradual two-step SCO behavior with T c1 (T c2 ) = 75 (255) K for X = Cl ´and 100 (260) K for X = Br ´.The HS Ñ IP transformation is accompanied by a change of the unit cell from monoclinic, space group P2 1 /n, to triclinic, space group P1,.For X = Br, the crystal structure of the IP confirms the occurrence of an ordering that can be described as made up of alternating undulated layers of LS or HS complexes (Figure 6).Interestingly, no re-entrant phase transition was observed in the LS state since the crystal remains P1 [40].
and Tc1 = 85 K.The low-temperature step is incomplete due to slow HS ↔ LS kinetics.In this step, when cooling at 1 K/min, χMT drops from 1.70 cm 3 •K•mol −1 at 100 K (IP = 1HS-1LS) down to 1.10 cm 3 •K•mol −1 at 5 K.However, after keeping the sample at 85 K for 12 h the AsF6 − derivative relaxes completely to the LS state (χMT decreases down to 0.24 cm 3 •K•mol −1 ).In this process the complex recovers the space group P21/n.The crystal packing of both derivatives consists of two-dimensional networks constructed by NH•••Cl − hydrogen bonds between Cl − and the imidazole NH groups of three neighboring [Fe II H3L Me ] 2+ cations, whereas XF6 − (X = P, As) acts as an isolated anion filling the space between two consecutive supramolecular layers.The layers are racemic with the ∆ and Λ configurations of [Fe II H3L Me ] 2+ interconnected through hydrogen bonds in an alternating way.In the ordered IP the layers can be described as a stacking of chains of HS sites that alternate with chains of LS sites.Interestingly, the HS and LS sites adopt the ∆ and Λ configurations, respectively [39].
In the search for new Fe II SCO complexes based on polydentate ligands with imidazole-pendant groups, the same research group has reported an important number of complexes of general formula [Fe(H2L 2−Me )2](X)(Y)•Solvent, where H2L 2−Me is the tridentate ligand ((2-methylimidazol-4yl)methylidene)histamine.Among them the system {[Fe(H2L 2−Me )2](X)(ClO4)•1EtOH} (X = Cl, Br) shows a gradual two-step SCO behavior with Tc1 (Tc2) = 75 (255) K for X = Cl − and 100 (260) K for X = Br − .The HS → IP transformation is accompanied by a change of the unit cell from monoclinic, space group P21/n, to triclinic, space group 1 ,.For X = Br, the crystal structure of the IP confirms the occurrence of an ordering that can be described as made up of alternating undulated layers of LS or HS complexes (Figure 6).Interestingly, no re-entrant phase transition was observed in the LS state since the crystal remains 1 [40].).The AsF 6 ´derivative undergoes an asymmetric two-step SCO behavior.At 250 K, in the HS state, the crystal is orthorhombic (space group P22 1 2 1 ) and contains one crystallographically independent Fe II site (Fe-N av = 2.187(3) Å).At 100 K, in the IP, the crystal retains the same unit cell but displays an unusual complex distribution of HS and LS sites.In the LS state (T = 15 K), the crystal changes to P2 1 2 1 2 1 with concomitant quadrupling of the unit cell, which contains two independent Fe II sites (Fe-N av = 2.023(4) Å) [41].In contrast, the two-step transition of the PF 6 ´derivative is practically symmetric.The high-temperature step is gradual (T c2 « 155 K) while the low-temperature step (T c1 « 97 K) is cooperative and exhibits a narrow hysteresis loop 6 K wide (Figure 7).At 250 K, the unit cell P22 1 2 1 contains two crystallographically identical [Fe II (H 2 L* 2´Me ) 2 ] 2+ cations, which consistently with the magnetic data are HS (Fe-N av = 2.190 Å).At 110 K, in the IP, the parameter c doubles and the new unit cell (P2 1 ) contains four cations, two of which are crystallographically unique.One site is mainly HS (Fe1-N av = 2.13(1) Å), and the other is mainly LS (Fe2-N av = 2.04(1) Å) (a rough estimate of the HS fraction on sites 1 (2) was found to be 75 (25)%).The crystal packing is defined by layers of cations and layers of anions in the ab plane alternating along c.Interestingly, in the IP = 1HS-1LS the ordering is defined by layers of cations that follow the pattern ¨¨¨HS-LS-LS-HS¨¨¨(Figure 7).In the pure LS state (80 K) the unit cell exhibits a new space group (P2 1 2 1 2 1 ) defined by two independent LS complex cations [42].7).In the pure LS state (80 K) the unit cell exhibits a new space group (P212121) defined by two independent LS complex cations [42].The remaining 1/3 of HS centers change to the LS state in a strong cooperative low-temperature transition centered at Tc1 = 183 K with a hysteresis 22 K wide (Figure 8).The unit cell is monoclinic, space group C2/c, and contains one independent Fe II site in the HS state.The molecules are held together through π-π interactions and hydrogen bonds defining supramolecular chains running along the c-axis.In the IP, the crystal retains the same space group type but c triples giving rise two independent Fe II sites, namely 1 HS center and 2 equivalent LS centers (IP = 1HS-2LS), which display a •••HS-LS-LS••• ordering (Figure 8).In the LS state the crystal is triclinic, space group 1 , and contains one Fe II site.These steps are accompanied by unusually large overall enthalpy (∆H = 26.7 kJ•mol −1 •K −1 ) and entropy (∆S = 125 J•mol −1 •K −1 ) variations, which clearly account for the SCO and crystallographic phase transition contributions [43].8).The unit cell is monoclinic, space group C2/c, and contains one independent Fe II site in the HS state.The molecules are held together through π-π interactions and hydrogen bonds defining supramolecular chains running along the c-axis.In the IP, the crystal retains the same space group type but c triples giving rise two independent Fe II sites, namely 1 HS center and 2 equivalent LS centers (IP = 1HS-2LS), which display a ¨¨¨HS-LS-LS¨¨¨ordering (Figure 8).In the LS state the crystal is triclinic, space group C1, and contains one Fe II site.These steps are accompanied by unusually large overall enthalpy (∆H = 26.7 kJ¨mol ´1¨K ´1) and entropy (∆S = 125 J¨mol ´1¨K ´1) variations, which clearly account for the SCO and crystallographic phase transition contributions [43].

One-Step SCO Behavior
So far, we have discussed a series of mononuclear complexes featuring two-step and half-step SCO with CSB and ordering.However, reversible CSB is not exclusive of systems undergoing stepwise transitions.Indeed, CSB has been reported for some mononuclear Fe II complexes exhibiting complete "one-step" SCO where obviously no IP exists.In this respect, Kahn et al.It displays a cooperative SCO centered Tc = 164 K with a hysteresis 4 K wide.In the HS state the crystal is monoclinic, C2/c, and changes to triclinic, 1 , in the LS state.Contrarily to that observed in the precedent example, the complex loses the two-fold axis when moving from the HS state to the LS state.Interestingly, in the photo-generated HS* state, the complex does not recover the two-fold axis, what constitutes a rare example photo-induced polymorphism [45].
Some Fe II complexes based on bispyrazolpyridine-ligands have been shown to exhibit SCO and CSB.For example, the complex [Fe(dmpp

One-Step SCO Behavior
So far, we have discussed a series of mononuclear complexes featuring two-step and half-step SCO with CSB and ordering.However, reversible CSB is not exclusive of systems undergoing stepwise transitions.Indeed, CSB has been reported for some mononuclear Fe II complexes exhibiting complete "one-step" SCO where obviously no IP exists.In this respect, Kahn et al.In the HS state the crystal is monoclinic, C2/c, and changes to triclinic, P1, in the LS state.Contrarily to that observed in the precedent example, the complex loses the two-fold axis when moving from the HS state to the LS state.Interestingly, in the photo-generated HS* state, the complex does not recover the two-fold axis, what constitutes a rare example photo-induced polymorphism [45].
Within the family of imidazole-based Fe II SCO complexes, Bréfuel and Matsumoto et al. have found that the perchlorate salt of the aforementioned complex [Fe II (H 2 L* 2´Me ) 2 ] 2+ displays a strong cooperative spin transition centered at T c = 171.5K featuring a hysteresis loop ca. 5 K wide and simultaneous crystallographic phase transition between the space groups P2 1 /n (HS) and P2 1 (LS) [50].
Linert et al. have reported the complex [Fe(i4tz) 6 ](SbF 6 ) 2 (i4tz = 1-isobuthyl-1H-tetrazole), which undergoes a gradual SCO in the temperature interval 60-160 K with T c = 116 K and a crystallographic phase transition.At 200 K, the crystal is triclinic, P1, and contains one crystallographically independent Fe II site, located on an inversion center.The average Fe-N av bond length, 2.189(5) Å, is consistent with a fully populated HS state in agreement with the magnetic data.Upon cooling at 100 K, where the HS molar fraction is about 37%, the space group type remains the same, but the volume of the unit cell triples.The new unit cell contains two different [Fe(i4tz) 6 ] 2+ centers with average bond lengths, Fe1-N av = 2.187(7) Å and Fe2-N av = 2.175(7) Å, typical of the 100% populated HS state [51].

Polynuclear Iron(II) Complexes
In the late 1980s Kahn introduced the idea of coupling two Fe II centers through suitable bridging ligands as a way to modulate the SCO behavior by means of magnetic and/or elastic interactions [52].Kahn and Zarembovitch et al. reported some years later the dinuclear complex {[Fe(bt)(NCS) 2 ] 2 (µ-bpym)} (bt = 2,2 1 -bithiazoline and bpym = bipyrimidine, see Appendix), which became a new paradigm of two-step SCO (2d, [53,54]).It was suggested that this two-step transition reflects the transformation between the three possible "spin-pair" states of the dinuclear species, i.e., LS-LS Ø LS-HS Ø HS-HS.In a subsequent collaborative work Real, Gütlich, Bousseksou, Collet and coworkers confirmed, from Mössbauer and Raman spectroscopies and crystallographic investigations combined with LIESST effect, the selective transformation between the "spin-pair" states in the bpym-bt dinuclear species [55][56][57][58][59]. Unfortunately, no direct evidence of the IP = 1HS-1LS state could be obtained, since the crystal structure of this complex in the IP state is an average of the HS and LS states.
The interesting properties of the bpym-bt derivative motivated the search for new dinuclear SCO complexes [60].In this context, Kaizaki et al. reported a new singular dinuclear complex {[Fe II (NCBH 3 ) (4-phpy)] 2 (µ-bpypz) 2 } (bpypz = 3,5-bis(2-pyridyl)-pyrazolate and 4phpy = 4-phenylpyridine).This compound does not follow the aforementioned transformation between "spin-pair" states.The magnetic properties show the occurrence of a gradual two-step SCO with a plateau centered at ca. 200 K (Figure 9).According to the magnetic data, the crystal structure was investigated at 296 K, 200 K and 100 K where, respectively, states HS-HS, LS-HS and LS-LS should be completely populated.At 296 K and 100 K, the crystal is triclinic, P1, and contains one crystallographically independent Fe II site with Fe-N bond lengths typical of the HS and LS states, respectively.At 200 K the space group type remains the same, but the volume of the unit cell doubles to accommodate two Fe II sites, one HS and the other LS.However, the HS and LS sites belong to different dinuclear molecules and consequently, the IP is made up of HS-HS and LS-LS dinuclear species defining a ¨¨¨HS-HS¨¨¨LS-LS¨¨¨pattern.The dimers interact each other via strong π-π intermolecular interactions through the 4-phpy ligands.These intermolecular interactions extend in two dimensions defining layers (Figure 9).The authors consider that compared to the bpym-bt system, the more rigid nature of the tetradentate µ-bpypz ĺigand and the strong π-π intermolecular interactions developed in the crystal, favour the generation of HS-HS and LS-LS spin-pairs instead of the LS-HS ones in the IP [61].
The first dinuclear complex with an ordered IP = LS-HS was reported by Brooker and Murray et al. for the complex [Fe II (PMAT)] 2 (BF 4 ) 4 ¨DMF (PMAT is a 1,2,4-triazole based acyclic tetradentate ligand).Magnetic and Mössbauer studies show that this dinuclear complex undergoes a half SCO behavior.At 298 K, the asymmetric unit is made up of one half of the dinuclear complex with the other half generated by a center of inversion.The average bond length Fe II -N av = 2.188(5) Å is characteristic of the HS state.However, at 123 K, the asymmetric unit contains a complete dimeric unit with two independent Fe II centers, one HS (Fe II -N av = 2.202(4) Å) and the other LS (Fe II -N av = 1.998(4)Å) (Figure 10).The HS-HS Ñ HS-LS transition involves doubling of the triclinic, P1, unit cell [62].Further evidence confirming the prevalence of the LS-HS state in Fe II dinuclear SCO complexes was found by Kepert and Murray et al. for the complex [Fe 2 (ddpp) 2 (NCS) 4 ]¨4CH 2 Cl 2 (ddpp = 2,5-di (2 1 ,2"-dipyridylamino)pyridine, see Appendix).This compound displays a gradual two-step SCO with T c2 = 180 K and at T c1 = 80 K separated by a plateau ca.30 K wide in the temperature interval 140-110 K.The crystal structure investigated at 250 K, 123 K and 25 K confirmed the presence of the HS-HS, LS-HS and LS-LS states, respectively.The crystal is triclinic, its space group being P1, but in contrast the asymmetric unit contains two independent Fe II sites at all temperatures.Consequently, no CSB occurs in this system [63].HS-HS, LS-HS and LS-LS states, respectively.The crystal is triclinic, its space group being 1 , but in contrast the asymmetric unit contains two independent Fe II sites at all temperatures.Consequently, no CSB occurs in this system [63].In the temperature interval 250-120 K, χMT is equal to 7.2 cm 3 •K•mol −1 and consistent with two Fe II centers in the HS state.Below 120 K, χMT gradually drops to 4.7 cm 3 •K•mol −1 , a value ca.15% higher than expected for a half SCO (Figure 11).Below 70 K, no further spin conversion occurs, most probably due to kinetic effects.The unit cell is monoclinic, its space group being P21/n, with an inversion center located on the 4,4′-bipy bridge (phase I).This structure, characterized by one independent HS Fe II site, is stable in the temperature interval 298-120 K.At ca. 115 K, a crystallographic phase transition coincides with the onset of the SCO.Consequently, in the new Cc space group the unit cell volume quadruples, and contains four crystallographically independent HS Fe II centers segregated in two distinct dinuclear units (phase II).At 30 K, where χMT is consistent with a 1:1 mixture of HS and LS states, the crystal is triclinic, 1 , and the unit cell volume HS-HS, LS-HS and LS-LS states, respectively.The crystal is triclinic, its space group being 1 , but in contrast the asymmetric unit contains two independent Fe II sites at all temperatures.Consequently, no CSB occurs in this system [63].In the temperature interval 250-120 K, χMT is equal to 7.2 cm 3 •K•mol −1 and consistent with two Fe II centers in the HS state.Below 120 K, χMT gradually drops to 4.7 cm 3 •K•mol −1 , a value ca.15% higher than expected for a half SCO (Figure 11).Below 70 K, no further spin conversion occurs, most probably due to kinetic effects.The unit cell is monoclinic, its space group being P21/n, with an inversion center located on the 4,4′-bipy bridge (phase I).This structure, characterized by one independent HS Fe II site, is stable in the temperature interval 298-120 K.At ca. 115 K, a crystallographic phase transition coincides with the onset of the SCO.Consequently, in the new Cc space group the unit cell volume quadruples, and contains four crystallographically independent HS Fe II centers segregated in two distinct dinuclear units (phase II).At 30 K, where χMT In the temperature interval 250-120 K, χ M T is equal to 7.2 cm 3 ¨K¨mol ´1 and consistent with two Fe II centers in the HS state.Below 120 K, χ M T gradually drops to 4.7 cm 3 ¨K¨mol ´1, a value ca.15% higher than expected for a half SCO (Figure 11).Below 70 K, no further spin conversion occurs, most probably due to kinetic effects.The unit cell is monoclinic, its space group being P2 1 /n, with an inversion center located on the 4,4 1 -bipy bridge (phase I).This structure, characterized by one independent HS Fe II site, is stable in the temperature interval 298-120 K.At ca. 115 K, a crystallographic phase transition coincides with the onset of the SCO.Consequently, in the new Cc space group the unit cell volume quadruples, and contains four crystallographically independent HS Fe II centers segregated in two distinct dinuclear units (phase II).At 30 K, where χ M T is consistent with a 1:1 mixture of HS and LS states, the crystal is triclinic, P1, and the unit cell volume halves.Despite this, the unit cell contains two distinct asymmetric dinuclear units displaying a LS-HS state (phase III) [64,65].halves.Despite this, the unit cell contains two distinct asymmetric dinuclear units displaying a LS-HS state (phase III) [64,65].

One-Dimensional Complexes
As far as we know the complex [FeL1(  3 ′} and azpy is the bismonodentate bridging ligand 4,4′azopyridine).The magnetic behavior of this complex shows a complete two-step transition with steps, centered at Tc2 = 326 K and Tc1 = 152 K, separated by a large plateau (IP).The crystal structure was solved in the IP, at 200 K, where the crystal is orthorhombic, space group P212121, and displays a perfect ordering HS-LS along the chains (Figure 12).In the LS state, the crystal changes to monoclinic P21 (130 K) but, unfortunately, complete analysis of the structure could not be performed due to twinning during the phase transition [66].Apparently, the crystallographic phase transition predisposes the system to exhibit a half SCO.This compound crystallizes in the hexagonal space group P63/m.However, it changes to the trigonal space group 3 at 170 K.In the hexagonal phase there is one independent Fe II site, which is HS, while the trigonal   31 } and azpy is the bismonodentate bridging ligand 4,4 1 -azopyridine).The magnetic behavior of this complex shows a complete two-step transition with steps, centered at T c2 = 326 K and T c1 = 152 K, separated by a large plateau (IP).The crystal structure was solved in the IP, at 200 K, where the crystal is orthorhombic, space group P2 1 2 1 2 1, and displays a perfect ordering HS-LS along the chains (Figure 12).In the LS state, the crystal changes to monoclinic P2 1 (130 K) but, unfortunately, complete analysis of the structure could not be performed due to twinning during the phase transition [66].

One-Dimensional Complexes
As far as we know the complex [FeL1(azpy)] reported by Weber et al., represents the only example of one-dimensional polymeric SCO displaying an ordered IP state and CSB (L1 is the tetradentate Schiff-like base {diethyl(E,E)-2,2′-[1,2-phenyl-bis(iminomethylidyne)]bis[3-oxobutanoate]-(2-)-N,N′,O 3 ,O 3 ′} and azpy is the bismonodentate bridging ligand 4,4′azopyridine).The magnetic behavior of this complex shows a complete two-step transition with steps, centered at Tc2 = 326 K and Tc1 = 152 K, separated by a large plateau (IP).The crystal structure was solved in the IP, at 200 K, where the crystal is orthorhombic, space group P212121, and displays a perfect ordering HS-LS along the chains (Figure 12).In the LS state, the crystal changes to monoclinic P21 (130 K) but, unfortunately, complete analysis of the structure could not be performed due to twinning during the phase transition [66].Apparently, the crystallographic phase transition predisposes the system to exhibit a half SCO.This compound crystallizes in the hexagonal space group P63/m.However, it changes to the trigonal space group 3 at 170 K.In the hexagonal phase there is one independent Fe II site, which is HS, while the trigonal Haasnoot and Gamez et al.
have reported the 1D coordination polymer [Fe(btzx) 3 ][CF 3 SO 3 ] 2 ¨CH 3 CN, (btzx = m-xylylenebis(tetrazole), see Appendix), a singular example in which SCO and crystallographic phase transition do not occur simultaneously.Apparently, the crystallographic phase transition predisposes the system to exhibit a half SCO.This compound crystallizes in the hexagonal space group P6 3 /m.However, it changes to the trigonal space group P3 at 170 K.In the hexagonal phase there is one independent Fe II site, which is HS, while the trigonal phase is characterized by the presence of four crystallographically independent Fe II HS sites distributed in two distinct chains.The chain 1 runs along a C 3 axis and contains the sites Fe 1 and Fe 2 with Fe-N av bond lengths 2.193(4) Å and 2.192(4) Å, respectively.Chain 2 is on a S 6 symmetry with Fe 3 (Fe-N av = 2.178(4) Å) and Fe 4 (Fe-N av = 2.185(4) Å) sites.Below 150 K, the Fe 2 and Fe 4 centers undergo a gradual SCO (T c = 110 K) generating an ordered IP = 1HS-1LS [67].

Two-Dimensional Coordination Polymers
The compound [Fe 2 (2,2 1 -bipyridine)(imidazolate) 4 ] reported by Reiff, Storr, and Thompson et al. is a singular coordination polymer constituted of extended 2D double-layer sheets of alternating tetrahedral and octahedral Fe II centers.The tetrahedral Fe II centers are surrounded by four imidazolate ligands, which bridge to other Fe II .The hexacoordinate Fe II centers complete their coordination sphere with a 2,2 1 -bipyridine ligand.This compound undergoes two structural phase transitions centered at T c2 = 151 and T c1 = 135 K defining three phases (α, β, and γ).The high-temperature phase, α, is consistent with 100% Fe II centers in the HS state (χ M T = 3.4 cm 3 ¨K¨mol ´1), the transition to the intermediate phase, β, features a very limited change of χ M T (ca.3%), while the subsequent transformation to the low temperature phase, γ, involves a variation of χ M T ca.16% characterized by a thermal hysteresis loop 4 K wide.The three phases are triclinic, P1, but the volume of the unit cell becomes 6 times larger when the system moves from the α phase to the β phase and, then, reduces to the half when the β phase transforms into the γ phase.In the γ phase there are six crystallographically independent Fe II sites (three octahedrons and three tetrahedrons).In perfect agreement with the magnetic and Mössbauer data, the crystal structure of the γ phase shows that only one octahedrally coordinate Fe II site is LS while the other five sites remain in the HS state [68].
Kepert et al. have reported the doubly interpenetrated 2D porous coordination polymer [Fe(bpe) 2 (NCS) 2 ]¨3(acetone), (bpe is 1,2-bis(4-pyridyl)ethane, see Appendix), which undergoes a complete two-step SCO.Above 155 K, a constant χ M T value, ca.3.2 cm 3 ¨K¨mol ´1, characterizes the HS state while below 60 K the compound is essentially LS (χ M T « 0.35 cm 3 ¨K¨mol ´1).The two half SCO, separated by a plateau 20 K wide (χ M T centered at 1.6 cm 3 ¨K¨mol ´1), occurs at T c2 = 133 K and T c1 = 80 K.In the HS state, there are two independent Fe II sites with distinct angular distortions and Fe-N values.Upon cooling, the less distorted HS site changes to the LS state, thereby generating a 1HS-1LS checkerboard-like arrangement in the plateau (IP).This structural reorganization occurs without crystallographic phase transition.However, upon further cooling, the low-temperature step 1HS-1LS Ñ LS occurs simultaneously with a crystallographic phase transition from the orthorhombic space group P2 1 2 1 2 to the tetragonal space group P42 1 c [69].In a related system, Real et al. have recently reinvestigated the related porous coordination polymer [Fe(tvp) 2 (NCS) 2 ]¨Guest (tvp = trans(4,4 1 -vinylenedi-pyridine, see Appendix).This compound is made up of perpendicular interpenetration of two identical stacks of [Fe(tvp) 2 (NCS) 2 ] n layers, which define large square-sectional channels where the guest molecules are located.The magnetic and calorimetric data indicate a guest-dependent SCO behavior accompanied by crystallographic phase transition in the temperature interval 170-215 K.In particular, for Guest = 2¨benzonitrile and 4CH 3 CN¨4H 2 O, the crystallographic phase transition occurs between the tetragonal space group P4/ncc in the HS state and the orthorhombic space group Pccn in the LS state.In addition to the spin-state change, this phase transition features a change of the angle of interpenetration of the [Fe(tvp) 2 (NCS) 2 ] n layers from strictly orthogonal in the HS state to oblique in the LS state [70].

Adams and Real et al.
have reported a series of compounds formulated [Fe(4,4 1 -bipy) 2 (NCX) 2 ]¨Guest (4,4 1 -bipy = 4,4 1 -bipyridine, X = S, Se), which are closely related to the two precedent examples.However, the slightly shorter length of the bridging ligand 4,4 1 -bipy with respect to the ligands bpe and tvp prevents interpenetration of the coordination polymer [Fe(4,4 1 -bipy) 2 (NCX) 2 ] and, consequently, the crystal is constituted of a parallel stack of layers.Nevertheless, the space between the layers can be filled with different guest solvent molecules.For Guest = 4CHCl 3 , the S derivative experiences a cooperative SCO two steps centered at T c2 = 146.9K and T c1 = 101.7 K) and hysteresis loops 6.6 K and 21 K wide, respectively (Figure 13).The corresponding Se derivative undergoes a cooperative half SCO centered at T c = 165 K characterized by a hysteresis 6.6 K wide.For both derivatives, the high-temperature HS Ñ 1HS-1LS step is accompanied by a crystallographic phase transition from the space group C2/c to the space group P2/c affording an ordered checkerboard like arrangement.The low-temperature step for the S derivative presumably involves a re-entrant crystallographic phase transition from IP = 1HS-1LS (P2/c) phase to the LS (C2/c) phase [71].14).This decrease of Tc is accompanied by a slight increase of cooperativity, which is specially pronounced for the EtOH derivative.In the HS state, the unit cell is monoclinic, space group P21/n, and contains one independent Fe II site, while in the plateau the unit cell is triclinic, space group 1 , and contains two independent Fe II sites with HS and LS spin states, respectively.This IP = 1HS-1LS defines the same checkerboard-like arrangement described for [Fe(bpe)2(NCS)2]•3(acetone) and [Fe(4,4′-bipy)2(NCX)2]•4CHCl3.A re-entrant transition to the monoclinic P21/n space group is observed when the crystal goes to the LS state [72].
Bronisz et al. have reported the cationic 2D complex [Fe(bbtr)3](ClO4)2 (bbtr = 1,4-bis(1,2,3-triazol-1-yl)butane, see Appendix) which features an abrupt one step SCO with a hysteresis loop 8 K wide centered at Tc = 105 K. Furthermore, this compound displays upon cooling a crystallographic phase transition at 125 K that induces a change from trigonal (space group 3 ), to triclinic (space group 1 ).Interestingly, the isostructural Zn II derivative undergoes the same structural phase transition at 151 K with similar structural modifications as the Fe II compound.This circumstance was used to prepare solid solutions, [FexZn1−x(bbtr)3](ClO4)2, and separate both events, namely crystallographic phase transition and SCO.Indeed, an increase of Zn II content induces a proportional increase of the phase transition's temperature and a decrease of the SCO temperature without modifying its cooperative nature.This case is an example in which a crystallographic phase transition does not  14).This decrease of T c is accompanied by a slight increase of cooperativity, which is specially pronounced for the EtOH derivative.In the HS state, the unit cell is monoclinic, space group P2 1 /n, and contains one independent Fe II site, while in the plateau the unit cell is triclinic, space group P1, and contains two independent Fe II sites with HS and LS spin states, respectively.This IP = 1HS-1LS defines the same checkerboard-like arrangement described for [Fe(bpe) 2 (NCS) 2 ]¨3(acetone) and [Fe(4,4 1 -bipy) 2 (NCX) 2 ]¨4CHCl 3 .A re-entrant transition to the monoclinic P2 1 /n space group is observed when the crystal goes to the LS state [72].
Bronisz et al. have reported the cationic 2D complex [Fe(bbtr) 3 ](ClO 4 ) 2 (bbtr = 1,4-bis(1,2,3 -triazol-1-yl)butane, see Appendix) which features an abrupt one step SCO with a hysteresis loop 8 K wide centered at T c = 105 K. Furthermore, this compound displays upon cooling a crystallographic phase transition at 125 K that induces a change from trigonal (space group P3), to triclinic (space group P1).Interestingly, the isostructural Zn II derivative undergoes the same structural phase transition at 151 K with similar structural modifications as the Fe II compound.This circumstance was used to prepare solid solutions, [Fe x Zn 1´x (bbtr) 3 ](ClO 4 ) 2 , and separate both events, namely crystallographic phase transition and SCO.Indeed, an increase of Zn II content induces a proportional increase of the phase transition's temperature and a decrease of the SCO temperature without modifying its cooperative nature.This case is an example in which a crystallographic phase transition does not trigger directly the SCO but affords the necessary structural conditions [73].Red and orange octahedrons correspond to LS and HS sites, respectively.
The three remaining examples belong to the family of heterobimetallic complexes derived from cyanometallate bridging building blocks.In these compounds the Fe II is axially coordinated by two monodentate pyridine-like ligands, while the equatorial positions are occupied by the cyanide moieties of four [M(CN)2] − (M = Ag, Au) or [Pd(CN)4] 2− groups that act as bridges defining infinite layers.Two different SCO behaviors have been described for the complex {Fe(3Fpy)2[Au(CN)2]} (3Fpy = 3-fluoropyridine).On one hand, the "form 1" reported by Real et al., displays a half SCO characterized by an asymmetric hysteresis loop 5 K wide centered at Tc = 142.5 K.At room temperature, the monoclinic (space group P21/c) unit cell contains one independent Fe II site, which is in the HS state.At 120 K, the triclinic (space group 1 ) unit cell consists of two independent LS and HS Fe II sites displaying a checkerboard like arrangement (Figure 15).On the other hand, the "form 2", reported by Kitazawa et al., shows a complete two-step SCO with critical temperatures centered at Tc2 = 147.9K and Tc1 = 108.4K (hysteresis loop ∆Tc1 = 20.4K) (Figure 15).The crystal structures of the HS and the IP = 1HS-1LS reported by these authors for "form 2" are consistent with the structure reported for "form 1".The crystal structure of "form 2" solved at 80 K shows that in the LS state the crystal recovers the monoclinic (space group P21/c) unit cell.Whatever the temperature, the packing is dominated by the occurrence strong aurophilic interactions between consecutive layers.Although the magnetic behavior of these samples suggests the existence of two polymorphs, it is difficult to justify them from a crystallographic viewpoint.Interestingly, application of low pressures (0.16 GPa) induces complete two-step spin conversion in the "form 1" [74][75][76].´(M = Ag, Au) or [Pd(CN) 4 ] 2´g roups that act as bridges defining infinite layers.Two different SCO behaviors have been described for the complex {Fe(3Fpy) 2 [Au(CN) 2 ]} (3Fpy = 3-fluoropyridine).On one hand, the "form 1" reported by Real et al., displays a half SCO characterized by an asymmetric hysteresis loop 5 K wide centered at T c = 142.5 K.At room temperature, the monoclinic (space group P2 1 /c) unit cell contains one independent Fe II site, which is in the HS state.At 120 K, the triclinic (space group P1) unit cell consists of two independent LS and HS Fe II sites displaying a checkerboard like arrangement (Figure 15).On the other hand, the "form 2", reported by Kitazawa et al., shows a complete two-step SCO with critical temperatures centered at T c2 = 147.9K and T c1 = 108.4K (hysteresis loop ∆T c1 = 20.4K) (Figure 15).The crystal structures of the HS and the IP = 1HS-1LS reported by these authors for "form 2" are consistent with the structure reported for "form 1".The crystal structure of "form 2" solved at 80 K shows that in the LS state the crystal recovers the monoclinic (space group P2 1 /c) unit cell.Whatever the temperature, the packing is dominated by the occurrence strong aurophilic interactions between consecutive layers.Although the magnetic behavior of these samples suggests the existence of two polymorphs, it is difficult to justify them from a crystallographic viewpoint.Interestingly, application of low pressures (0.16 GPa) induces complete two-step spin conversion in the "form 1" [74][75][76].
the HS and the IP = 1HS-1LS reported by these authors for "form 2" are consistent with the structure reported for "form 1".The crystal structure of "form 2" solved at 80 K shows that in the LS state the crystal recovers the monoclinic (space group P21/c) unit cell.Whatever the temperature, the packing is dominated by the occurrence strong aurophilic interactions between consecutive layers.Although the magnetic behavior of these samples suggests the existence of two polymorphs, it is difficult to justify them from a crystallographic viewpoint.Interestingly, application of low pressures (0.16 GPa) induces complete two-step spin conversion in the "form 1" [74][75][76]. .This compound undergoes a strong cooperative two-step SCO, each one involving 50% of Fe II centers, with critical temperatures centered at T c1 = 144.9K and T c2 = 162.1 K characterized by a hysteresis loop 7 K and 5 K wide, respectively.Above 160 K, the HS state is characterized by an orthorhombic (space group Pnc2) unit cell, which contains one independent Fe II site, while in the IP (T = 150 K) the unit cell changes to orthorhombic (space group Pmna) where the 1HS-1LS state adopts the same orderly distribution described for most of the aforementioned two-dimensional coordination polymers (Figure 16).In the second step the crystal recovers the original Pnc2 space group characterized by one Fe II LS site [77].The same research group has described the complex {Fe(DEAS) 2 [Ag(CN) 2 ] 2 } (DEAS = 4 1 -diethylaminostilbazole).This compound undergoes an incomplete SCO where 2/3 of the Fe II centers remain in the HS state.The SCO, characterized by a hysteresis loop 6 K wide centered at T c = 141 K, is accompanied by a reversible crystallographic phase transition in which the volume of the unit-cell doubles in the low temperature phase.At 100 K, there are six Fe II atoms per unit cell, two Fe1 and four Fe2.The average bond lengths, Fe1-N av = 1.969(4)Å and Fe2-N av = 2.176(4) Å, agree well with the Fe-N bond distances typically found for the Fe II ion in the LS and HS states, respectively.This singular fact perfectly correlates with the reported magnetic data at low temperature.In this IP = 2HS-1LS state the layers can be defined as generated by a sequence of ¨¨¨LS-HS-HS¨¨¨columns (Figure 17) [78]. .This compound undergoes a strong cooperative two-step SCO, each one involving 50% of Fe II centers, with critical temperatures centered at Tc1 = 144.9K and Tc2 = 162.1 K characterized by a hysteresis loop 7 K and 5 K wide, respectively.Above 160 K, the HS state is characterized by an orthorhombic (space group Pnc2) unit cell, which contains one independent Fe II site, while in the IP (T = 150 K) the unit cell changes to orthorhombic (space group Pmna) where the 1HS-1LS state adopts the same orderly distribution described for most of the aforementioned twodimensional coordination polymers (Figure 16).In the second step the crystal recovers the original Pnc2 space group characterized by one Fe II LS site [77].The same research group has described the complex {Fe(DEAS)2[Ag(CN)2]2} (DEAS = 4′-diethylaminostilbazole).This compound undergoes an incomplete SCO where 2/3 of the Fe II centers remain in the HS state.The SCO, characterized by a hysteresis loop 6 K wide centered at Tc = 141 K, is accompanied by a reversible crystallographic phase transition in which the volume of the unit-cell doubles in the low temperature phase.At 100 K, there are six Fe II atoms per unit cell, two Fe1 and four Fe2.The average bond lengths, Fe1-N av = 1.969(4)Å and Fe2-N av = 2.176(4) Å, agree well with the Fe-N bond distances typically found for the Fe II ion in the LS and HS states, respectively.This singular fact perfectly correlates with the reported magnetic data at low temperature.In this IP = 2HS-1LS state the layers can be defined as generated by a sequence of •••LS-HS-HS••• columns (Figure 17) [78].

Three-Dimensional Coordination Polymers
To the best of our knowledge, two examples of 3D coordination polymers displaying SCO and CSB have been reported so far.The compound {Fe(5Brpmd  The chains are interconnected through [Ag(CN) 2 ] ´thus generating an intricate three-dimensional framework.The unit cell is monoclinic, C2/c, in the HS state and displays one independent Fe II site with Fe-N av = 2.166( 8) Å (T = 293 K).However, at 120 K the monoclinic, P2 1 /c, unit cell contains, in agreement with the magnetic data, two independent Fe II sites: one is HS (Fe-N av = 2.174(5) Å) and the other is LS (Fe-N av = 1.955(5)Å).The HS and LS sites alternate along the infinite ¨¨¨Fe-(5Brpmd)-Fe¨¨l inear chains, which also define layers parallel to the ac plane (Figure 18) [79].In the interval 300-140 K, χMT = 3.30 cm 3 •K•mol −1 is consistent with the HS state of Fe II (state 1).In a first step, centered at 138 K, the system reaches a plateau, ca. 10 K wide, with a χMT value of 1.54 cm 3 •K•mol −1 , which is consistent with a IP1 = 1HS-1LS (state 2).Then, a second drop of χMT in the temperature interval 130-125 K defines a new plateau with a χMT value equal to 1.01 cm 3 •K•mol −1 (state 3).The χMT value of this second plateau is consistent with a IP2 = 1HS-2LS.Below ca.112 K, the practically zero value of χMT indicates a complete transformation to the LS state (state 4).In the heating mode the three-step behavior occurs ca.20 K higher displaying an irregular hysteresis loop.In state 1, the system is characterized by a monoclinic, P2/a, unit cell defined by one The complex [Fe(dpsme)Pt(CN) 4 ]¨2/3dpsme¨xEtOH¨yH 2 O (dpsme = 4,4 1 -di(pyridylthio) methane), reported by Kepert et al., is a unique example of cooperative three-step SCO transition displaying CSB.In the interval 300-140 K, χ M T = 3.30 cm 3 ¨K¨mol ´1 is consistent with the HS state of Fe II (state 1).In a first step, centered at 138 K, the system reaches a plateau, ca. 10 K wide, with a χ M T value of 1.54 cm 3 ¨K¨mol ´1, which is consistent with a IP 1 = 1HS-1LS (state 2).Then, a second drop of χ M T in the temperature interval 130-125 K defines a new plateau with a χ M T value equal to 1.01 cm 3 ¨K¨mol ´1 (state 3).The χ M T value of this second plateau is consistent with a IP 2 = 1HS-2LS.Below ca.112 K, the practically zero value of χ M T indicates a complete transformation to the LS state (state 4).In the heating mode the three-step behavior occurs ca.20 K higher displaying an irregular hysteresis loop.In state 1, the system is characterized by a monoclinic, P2/a, unit cell defined by one independent Fe II site, which is HS.In state 2, the unit cell changes to triclinic, P1, where two equally populated independent sites defining an ordered IP = 1LS:1HS characterized by alternating rows of LS and HS states as described for the related Fe(3-Clpy) 2 [Pd(CN) 4 ] (see Figure 16).The change from state 2 to state 3 involves doubling of the P1 unit cell along the a-axis and concomitant generation of three crystallographically independent Fe II sites, i.e., two LS (Fe 1 -N av = 1.944Å and Fe 2 -N av = 1.991Å), one HS (Fe 3 -N av = 2.113 Å) with occupation factors 0.5, 1 and 0.5, respectively.From a crystallographic point of view state 3 corresponds well with a IP 2 = 1HS:3LS, which is not consistent with the magnetic data.The authors suggest that a solid solution region with varying modulation along the a-axis may exist with conversion from 1HS-2LS to 1HS:3LS [80].

Conclusions
Here we have reviewed a variety of Fe II complexes that undergo reversible SCO and CSB, giving special emphasis to those systems displaying stepwise and ordering behavior.Complete two-step HS Ø LS conversion characterized by a 1HS-1LS symmetric distribution of both spin states in the IP can be considered the most common situation.A few examples of complete two-step SCO with asymmetric 2HS-1LS and 1HS-2LS distributions in the IP have also been described.In addition, incomplete SCO characterized by 1HS-1LS or 2HS-1LS stable low-temperature phases are also known.Thus far, only one example of three-step SCO involving successive generation of 1HS-1LS and 1HS-2LS IPs between the pure HS and LS phases has been described.
The IP states, easily identifiable as plateaus from the thermal dependence of the χ M T product, represent macroscopic manifestations of precise microscopic distributions of HS and LS centers in the crystal, which can only be evidenced by X-ray diffraction analysis.The examples of order here highlighted represent an excellent platform to investigate the microscopic mechanisms, most likely based on cooperative and anticooperative elastic interactions, which could explain and predict the occurrence of different HS-LS patterns in the IP state.Understanding of these mechanisms is relevant for the control of the responsiveness of SCO materials and their possible integration in functional devices.We hope that this review will stimulate creative collaboration between the different involved areas of research, including coordination chemistry, materials science and solid-state physics.

Figure 1 .
Figure 1.[Fe II (2-pic)3]Cl2•EtOH]: Two-step SCO behavior in the χMT vs. T plot (χM is the molar magnetic susceptibility and T is the temperature) (left).Projection of the unit cell down the c-axis in the intermediate phase (IP) (1HS-1LS) showing alternance of the HS (orange) and LS (red) complex cations (Cl − anions and EtOH solvent molecules have been omitted) (right).

Figure 1 .of 22 Figure 1 .
Figure 1.[Fe II (2-pic) 3 ]Cl 2 ¨EtOH]: Two-step SCO behavior in the χ M T vs. T plot (χ M is the molar magnetic susceptibility and T is the temperature) (left).Projection of the unit cell down the c-axis in the intermediate phase (IP) (1HS-1LS) showing alternance of the HS (orange) and LS (red) complex cations (Cl ´anions and EtOH solvent molecules have been omitted) (right).
T temperature) from 3.2 cm3 •K•mol −1 (HS state) to 2.6 cm3 •K•mol −1 (IP state).This step, characterized by a hysteresis loop 5 K wide centered at Tc2 = 117.5 K, reflects the conversion of 1/3 of the HS centers to the LS state.The second drop in χMT (between 2.2 cm 3 •K•mol −1 and 0.2 cm 3 •K•mol −1 ), characterized by a hysteresis loop 10 K wide centered at Tc1 = 101 K, involves the remaining 2/3 of the HS species.The crystals of this polymorph are orthorhombic, its space group being Aba2, and contain one asymmetric unit defined by half a molecule with a two-fold axis passing through the Fe II site.The molecules are arranged in layers that stack along the a-axis and extend parallel to the plane bc.At 100 K (IP state) the unit cell triples displaying a HS-HS-LS pattern.
At 110 K, in the IP, the parameter c doubles and the new unit cell (P21) contains four cations, two of which are crystallographically unique.One site is mainly HS (Fe1-N av = 2.13(1) Å), and the other is mainly LS (Fe2-N av = 2.04(1) Å) (a rough estimate of the HS fraction on sites 1 (2) was found to be 75 (25)%).The crystal packing is defined by layers of cations and layers of anions in the ab plane alternating along c.Interestingly, in the IP = 1HS-1LS the ordering is defined by layers of cations that follow the pattern •••HS-LS-LS-HS••• (Figure

Figure 7 .
Figure 7. [Fe II (H2L* 2−Me )2](PF6)2: Spin crossover behavior (adapted with permission from Reference [42]) (left).Projection on the ac plane of the crystal packing in the IP showing the sequence of •••HS-LS-LS-HS••• layers parallel to the ab plane (the layers of PF6 − have been removed); Red and orange octahedrons correspond to LS and HS sites (right).

Figure 7 .
Figure 7. [Fe II (H 2 L* 2´Me ) 2 ](PF 6 ) 2 : Spin crossover behavior (adapted with permission from Reference [42]) (left).Projection on the ac plane of the crystal packing in the IP showing the sequence of ¨¨¨HS-LS-LS-HS¨¨¨layers parallel to the ab plane (the layers of PF 6 ´have been removed); Red and orange octahedrons correspond to LS and HS sites (right).

Figure 8 .
Figure 8. [Fe(bapbpy)(NCS)2]: Spin crossover behavior (adapted with permission from Reference [43]) (left); and projection on the ac plane of the crystal packing in the IP showing the LS-LS-HS sequence of molecules running along the c-axis.Red and orange octahedrons correspond to LS and HS sites, respectively (right).

Figure 8 .
Figure 8. [Fe(bapbpy)(NCS) 2 ]: Spin crossover behavior (adapted with permission from Reference [43]) (left); and projection on the ac plane of the crystal packing in the IP showing the LS-LS-HS sequence of molecules running along the c-axis.Red and orange octahedrons correspond to LS and HS sites, respectively (right).

Figure 10 .
Figure 10.Molecular structure of a dimeric {[Fe II (PMAT)]2} 4+ unit (left) and projection of the unit cell on bc (right) (BF4 − anions and DMF inclusions have been omitted).Red and orange octahedrons correspond to LS and HS sites, respectively.

Figure 10 .
Figure 10.Molecular structure of a dimeric {[Fe II (PMAT)]2} 4+ unit (left) and projection of the unit cell on bc (right) (BF4 − anions and DMF inclusions have been omitted).Red and orange octahedrons correspond to LS and HS sites, respectively.

Figure 10 .
Figure 10.Molecular structure of a dimeric {[Fe II (PMAT)] 2 } 4+ unit (left) and projection of the unit cell on bc (right) (BF 4 ´anions and DMF inclusions have been omitted).Red and orange octahedrons correspond to LS and HS sites, respectively.

Figure 12 .
Figure 12. [FeL1(azpy)]: Spin crossover behavior (adapted with permission from Reference [66]) (left); and fragment of a chain displaying the alternate order 1HS-1LS in the IP at 200 K (right).Red and orange octahedrons correspond to LS and HS sites, respectively.Haasnoot and Gamez et al. have reported the 1D coordination polymer [Fe(btzx)3][CF3SO3]2•CH3CN, (btzx = m-xylylenebis(tetrazole), see Appendix), a singular example in which SCO and crystallographic phase transition do not occur simultaneously.Apparently, the crystallographic phase transition predisposes the system to exhibit a half SCO.This compound crystallizes in the hexagonal space group P63/m.However, it changes to the trigonal space group 3 at 170 K.In the hexagonal phase there is one independent Fe II site, which is HS, while the trigonal

Figure 12 .
Figure 12. [FeL1(azpy)]: Spin crossover behavior (adapted with permission from Reference [66]) (left); and fragment of a chain displaying the alternate order 1HS-1LS in the IP at 200 K (right).Red and orange octahedrons correspond to LS and HS sites, respectively.Haasnoot and Gamez et al. have reported the 1D coordination polymer [Fe(btzx)3][CF3SO3]2•CH3CN, (btzx = m-xylylenebis(tetrazole), see Appendix), a singular example in which SCO and crystallographic phase transition do not occur simultaneously.Apparently, the crystallographic phase transition predisposes the system to exhibit a half SCO.This compound crystallizes in the hexagonal space group P63/m.However, it changes to the trigonal space group 3 at 170 K.In the hexagonal phase there is one independent Fe II site, which is HS, while the trigonal

Figure 12 .
Figure 12. [FeL1(azpy)]: Spin crossover behavior (adapted with permission from Reference [66]) (left); and fragment of a chain displaying the alternate order 1HS-1LS in the IP at 200 K (right).Red and orange octahedrons correspond to LS and HS sites, respectively.
the high-temperature HS → 1HS-1LS step is accompanied by a crystallographic phase transition from the space group C2/c to the space group P2/c affording an ordered checkerboard like arrangement.The low-temperature step for the S derivative presumably involves a re-entrant crystallographic phase transition from IP = 1HS-1LS (P2/c) phase to the LS (C2/c) phase[71].

Figure 14 .
Figure 14.[Fe(bdpt) 2 ]¨nSolvent: Spin crossover behavior (adapted with permission from Reference [72]) (left); and fragment of a layer displaying the checkerboard like 1HS-1LS order in the IP (right).Red and orange octahedrons correspond to LS and HS sites, respectively.

Figure 15 .Figure 15 .
Figure 15.{Fe(3Fpy)2[Au(CN)2]}: (a) Spin crossover behavior (form 2); (b) perspective view of the packing of two consecutive layers along c the c-axis in the IP (123 K); and (c) fragment of a layer along the a-axis in the IP (123 K).Red and orange octahedrons correspond to LS and HS Fe II sites, respectively, while green and yellow spheres represent F and Au, respectively.

Figure 16 .
Figure 16.{Fe(3-Clpy)2[Pd(CN)4]}: (a) Spin crossover behavior (b) perspective view of the packing of three consecutive layers seen along the a-axis in the IP (150 K); and (c) fragment of a layer in the IP state (150 K).Red and orange octahedrons correspond to LS and HS Fe II sites, respectively, while green and pink spheres represent Cl and Pd, respectively.

Figure 16 .
Figure 16.{Fe(3-Clpy) 2 [Pd(CN) 4 ]}: (a) Spin crossover behavior (b) perspective view of the packing of three consecutive layers seen along the a-axis in the IP (150 K); and (c) fragment of a layer in the IP state (150 K).Red and orange octahedrons correspond to LS and HS Fe II sites, respectively, while green and pink spheres represent Cl and Pd, respectively.

Figure 16 .
Figure 16.{Fe(3-Clpy)2[Pd(CN)4]}: (a) Spin crossover behavior (b) perspective view of the packing of three consecutive layers seen along the a-axis in the IP (150 K); and (c) fragment of a layer in the IP state (150 K).Red and orange octahedrons correspond to LS and HS Fe II sites, respectively, while green and pink spheres represent Cl and Pd, respectively.

4. 3 .
Three-Dimensional Coordination Polymers To the best of our knowledge, two examples of 3D coordination polymers displaying SCO and CSB have been reported so far.The compound {Fe(5Brpmd) 2 [Ag(CN) 2 ] 2 } (5Brpmd = 5-bromo-pyrimidine), reported by Real et al., displays a strong cooperative half SCO centered at T c = 158 K characterized by a hysteresis loop 18 K wide.The Fe II centers are bridged by the bismonodentate ligand 5Brpmd generating stacks of infinite ¨¨¨Fe-(5Brpmd)-Fe¨¨¨linear chains running parallel to the a-axis.
Magnetochemistry 2016, 2, 16 16 of 22 pyrimidine), reported by Real et al., displays a strong cooperative half SCO centered at Tc = 158 K characterized by a hysteresis loop 18 K wide.The Fe II centers are bridged by the bismonodentate ligand 5Brpmd generating stacks of infinite •••Fe-(5Brpmd)-Fe••• linear chains running parallel to the a-axis.The chains are interconnected through [Ag(CN)2] − thus generating an intricate threedimensional framework.The unit cell is monoclinic, C2/c, in the HS state and displays one independent Fe II site with Fe-N av = 2.166(8) Å (T = 293 K).However, at 120 K the monoclinic, P21/c, unit cell contains, in agreement with the magnetic data, two independent Fe II sites: one is HS (Fe-N av = 2.174(5) Å) and the other is LS (Fe-N av = 1.955(5)Å).The HS and LS sites alternate along the infinite •••Fe-(5Brpmd)-Fe••• linear chains, which also define layers parallel to the ac plane (Figure 18) [79].

Figure 18 .
Figure 18.{Fe(5Brpmd)2[Ag(CN)2]2}: Spin crossover behavior (adapted with permission from Reference [79]) (left); and projection of the unit cell on the ab plane in the IP (120 K) (right).Red and orange octahedrons correspond to LS and HS Fe II sites, respectively, while green and light-blue spheres represent Br and Ag, respectively.

Figure 18 .
Figure 18.{Fe(5Brpmd) 2 [Ag(CN) 2 ] 2 }: Spin crossover behavior (adapted with permission from Reference [79]) (left); and projection of the unit cell on the ab plane in the IP (120 K) (right).Red and orange octahedrons correspond to LS and HS Fe II sites, respectively, while green and light-blue spheres represent Br and Ag, respectively.