Perplexing Coordination Behaviour of Potentially Bridging Bipyridyl-Type Ligands in the Coordination Chemistry of Zinc and Cadmium 1 , 1-Dithiolate Compounds

The X-ray structural chemistry of zinc and cadmium 1,1-dithiolates (for example, xanthate, dithiophosphate and dithiocarbamate) with potentially bridging bipyridyl-type ligands (for example, 4,4′-bipyridine) is reviewed. For zinc, the xanthates and dithiophosphates uniformly form one-dimensional coordination polymers, whereas the zinc dithiocarbamates are always zero-dimensional, reflecting the exceptional chelating ability of dithiocarbamate ligands compared with xanthates and dithiophosphates. For cadmium, one-dimensional coordination polymers are usually found, reflecting the larger size of cadmium compared with zinc, but zero-dimensional aggregates are sometimes found. Steric effects associated with the 1,1-dithiolate-bound R groups are shown to influence supramolecular aggregation and, when formed, polymer topology in order to reduce steric hindrance; the nature of the bipyridyl-type ligand can also be influential. For the dithiocarbamates of both zinc and cadmium, in instances where the dithiocarbamate ligand is functionalised with hydrogen bonding potential, extended supramolecular architectures are often formed via hydrogen bonding interactions. Of particular interest is the observation that the bipyridyl-type ligands do not always bridge zinc or cadmium 1,1-dithiolates, being monodentate instead, often in the presence of hydrogen bonding. Thus, hydroxyl-O–H . . . N(pyridyl) hydrogen bonds are sometimes formed in preference to M←N(pyridyl) coordinate-bonds, suggesting a competition between the two modes of association.


Introduction
Bioinorganic chemistry, incorporating investigations into the natural functions of metals in biology to the development of metal-based therapeutics and diagnostic agents, has been the mainstay of coordination chemistry, involving transitions metals, main group elements and lanthanides, for many decades.However, this dominance is increasingly challenged by the development of coordination polymers of various dimensions up to and including three-dimensions, that is, metal-organic framework structures.This interest is reflected in the now over 70,000 X-ray crystal structures of coordination polymers [1] included in the Cambridge Structural Database [2].A myriad of potential applications prompt investigations into coordination polymers.While these were initially in the realm of materials science with applications relating to energy and gas storage, photo-responsive materials, catalysis, etc. [3][4][5][6][7], there are increasing applications of coordination polymers relevant to bioinorganic chemists.Examples include coordination polymers functioning as carriers for drug delivery, biosensors and as therapeutics themselves [8][9][10][11].Coordination polymers are now being constructed from biologically relevant materials [12,13].
Coordination polymers are typically neutral and are constructed from a combination of charged and neutral ligands; it is noted that charged coordination polymers are also known.Increasing the functionality of the ligands enhances the probability of attaining the three-dimensional metal-organic frameworks.Charged ligands for complexation are exemplified by carboxylates derived from, for example, acetic acid, terephthalic acid (benzene-1,4-dicarboxylic acid) and trimesic acid (benzene-1,3,5-tricarboxylic acid); that is, a series with increasing bridging capacity.In the same way, potentially bridging bipyridyl-type ligands are prominent in building coordination polymers with the most prominent example being 4,4 -bipyridine.The combination of these and related building blocks have been extensively exploited by the research groups of, for example, Fujita [14,15], Yaghi [16,17] and Zaworotko [18,19].
A class of ligands closely related to carboxylate ligands is the 1,1-dithiolates, having two sulphur atoms connected to the quaternary carbon atom rather than two oxygens.Prominent examples of 1,1-dithiolate ligands include xanthates, dithiophosphates and dithiocarbamates, Figure 1.Despite the chemical similarity of the 1,1-dithiolates with carboxylate, the evaluation of the propensity of these ligands to form coordination polymers is generally lacking.This is perhaps a little surprising given that 1,1-dithiolate ligands are well known to form bridges between metal centres [20][21][22][23][24], especially in the binary 1,1-dithiolates of the zinc-triad elements [25].Indeed, it was the desire to destroy this supramolecular aggregation by the addition of nitrogen-bases that was the original motivation for investigating this chemistry, as the resulting smaller aggregates were found to be more useful as synthetic precursors for chemical vapour deposition of metal sulphide nanomaterials.
Coordination polymers are typically neutral and are constructed from a combination of charged and neutral ligands; it is noted that charged coordination polymers are also known.Increasing the functionality of the ligands enhances the probability of attaining the three-dimensional metal-organic frameworks.Charged ligands for complexation are exemplified by carboxylates derived from, for example, acetic acid, terephthalic acid (benzene-1,4-dicarboxylic acid) and trimesic acid (benzene-1,3,5-tricarboxylic acid); that is, a series with increasing bridging capacity.In the same way, potentially bridging bipyridyl-type ligands are prominent in building coordination polymers with the most prominent example being 4,4′-bipyridine.The combination of these and related building blocks have been extensively exploited by the research groups of, for example, Fujita [14,15], Yaghi [16,17] and Zaworotko [18,19].
A class of ligands closely related to carboxylate ligands is the 1,1-dithiolates, having two sulphur atoms connected to the quaternary carbon atom rather than two oxygens.Prominent examples of 1,1-dithiolate ligands include xanthates, dithiophosphates and dithiocarbamates, Figure 1.Despite the chemical similarity of the 1,1-dithiolates with carboxylate, the evaluation of the propensity of these ligands to form coordination polymers is generally lacking.This is perhaps a little surprising given that 1,1-dithiolate ligands are well known to form bridges between metal centres [20][21][22][23][24], especially in the binary 1,1-dithiolates of the zinc-triad elements [25].Indeed, it was the desire to destroy this supramolecular aggregation by the addition of nitrogen-bases that was the original motivation for investigating this chemistry, as the resulting smaller aggregates were found to be more useful as synthetic precursors for chemical vapour deposition of metal sulphide nanomaterials.In this bibliographic review, the structural chemistry of zinc and cadmium 1,1-dithiolates and bipyridyl-type ligands is surveyed.While one-dimensional coordination polymers are observed, there are no examples of two-let alone three-dimensional coordination polymers, at least not stabilised by bonds involving zinc or cadmium, that is, by a combination of covalent and coordinatebonding interactions.Despite this, an interesting structural chemistry is frequently revealed.For example, often in the presence of hydrogen bonding functionality in the 1,1-dithiolate ligands, putative coordinate-bonding between zinc or cadmium and the bipyridyl-type ligand is suppressed to allow for the formation of competing hydrogen bonding interactions.

Methodology and Organisation
The structures included in this survey were extracted in the form of crystallographic information files (CIFs) from the Cambridge Structural Database (CSD, 2017 release with updates) [2].Data were analysed using PLATON [26] and diagrams drawn with the graphics program DIAMOND [27].The review is divided into a discussion of zinc structures followed by those of cadmium.Within in each category, xanthate species are discussed before dithiophosphates (and lesser known derivatives) and then dithiocarbamates.A diverse variety of bipyridyl-type ligands are represented among the structures and their chemical diagrams with abbreviations are shown in Figure 2. As a general principal, smaller bipyridyl-type ligands are discussed before larger/more complicated analogues and for each type of bipyridyl-type ligand, the molecules/aggregates are described in terms of the bulk of the R/R′ substituents of the 1,1-dithiolate ligand, with smaller groups discussed first.The In this bibliographic review, the structural chemistry of zinc and cadmium 1,1-dithiolates and bipyridyl-type ligands is surveyed.While one-dimensional coordination polymers are observed, there are no examples of two-let alone three-dimensional coordination polymers, at least not stabilised by bonds involving zinc or cadmium, that is, by a combination of covalent and coordinate-bonding interactions.Despite this, an interesting structural chemistry is frequently revealed.For example, often in the presence of hydrogen bonding functionality in the 1,1-dithiolate ligands, putative coordinate-bonding between zinc or cadmium and the bipyridyl-type ligand is suppressed to allow for the formation of competing hydrogen bonding interactions.

Methodology and Organisation
The structures included in this survey were extracted in the form of crystallographic information files (CIFs) from the Cambridge Structural Database (CSD, 2017 release with updates) [2].Data were analysed using PLATON [26] and diagrams drawn with the graphics program DIAMOND [27].The review is divided into a discussion of zinc structures followed by those of cadmium.Within in each category, xanthate species are discussed before dithiophosphates (and lesser known derivatives) and then dithiocarbamates.A diverse variety of bipyridyl-type ligands are represented among the structures and their chemical diagrams with abbreviations are shown in Figure 2. As a general principal, smaller bipyridyl-type ligands are discussed before larger/more complicated analogues and for each type of bipyridyl-type ligand, the molecules/aggregates are described in terms of the bulk of the R/R substituents of the 1,1-dithiolate ligand, with smaller groups discussed first.The discussion is focussed upon a general description of the modes of coordination of the 1,1-dithiolate and bipyridyl-type ligands and describing the resultant aggregation patterns.Details of geometric parameters are available in the original publication and are not generally considered.Similarly, generally descriptions of molecular packing are avoided unless relevant to the putative competition between hydrogen bonding and M←N(pyridyl) coordinate-bonds.In all, there are 57 zinc-containing structures and 31 cadmium analogues.

Zinc Xanthate Structures
There are six structures in this category, 1-6 [28][29][30][31], and general features of these are summarised in Table 1 along with the other zinc structures, 1-57 , included in this survey.The first structure to be described contains the smallest bipyridyl-type ligand, namely pyrazine (pyr), and is a one-dimensional coordination polymer formulated as [Zn(S2COEt)2(pyr)]n (1) [28].In 1, Figure 3a, the zinc atom lies on a special position 2/m and the pyr ligand lies about a two-fold axis with the nitrogen atoms lying on the axis.The xanthate ligands are S,S-chelating and the resulting trans-N2S4 donor set defines a distorted octahedral geometry.The topology of the chain is strictly linear owing to the aforementioned symmetry.It is apposite here to comment on the structure of the parent

Zinc Xanthate Structures
There are six structures in this category, 1-6 [28][29][30][31], and general features of these are summarised in Table 1 along with the other zinc structures, 1-57 , included in this survey.The first structure to be described contains the smallest bipyridyl-type ligand, namely pyrazine (pyr), and is a one-dimensional coordination polymer formulated as [Zn(S 2 COEt) 2 (pyr)] n (1) [28].In 1, Figure 1a, the zinc atom lies on a special position 2/m and the pyr ligand lies about a two-fold axis with the nitrogen atoms lying on the axis.The xanthate ligands are S,S-chelating and the resulting trans-N 2 S 4 donor set defines a distorted octahedral geometry.The topology of the chain is strictly linear owing to the aforementioned symmetry.It is apposite here to comment on the structure of the parent molecule found in 1; that is, [Zn(S 2 COEt) 2 ] n [67,68].Here, each xanthate ligand is bidentate bridging and the resulting two-dimensional structure is conveniently described as being constructed by an infinite pattern of edge-shared squares, with each square comprising zinc atoms and with pairs of adjacent zinc atoms being bridged by a xanthate ligand resulting in ZnS 4 tetrahedra.With the addition of base, pillaring of the layers into a three-dimensional architecture might be envisaged, with suitable adjustments in the coordination geometries about the zinc atoms.Instead, the original two-dimensional architecture of [Zn(S 2 COEt) 2 ] n is disrupted in 1 to form a one-dimensional aggregation pattern.This principle of perturbing the supramolecular architecture observed in the parent 1,1-dithiolate structure is generally applicable to the structures described herein.Although only including six structures, the foregoing offers a microcosm of the variability of structures covered in this review.Thus, different coordination modes of the xanthate ligand are evident leading to distinct coordination geometries, different aggregation patterns, sometimes related to the steric bulk of the substituents, and, when formed variable topologies of coordination polymers and conformational variations in the bridging ligands.

Zinc Dithiophosphate and Related Structures
The first dithiophosphate structure to be described in this section is a centrosymmetric binuclear species similar to that observed for 2, Figure 3b.In {Zn[S2P(OiPr)2]2}2(pyr) (7) [32], one dithiophosphate ligand is chelating while the other, with Zn-S bond lengths differing by nearly 1 Å, is coordinating in an asymmetric mode or taken to an extreme, is monodentate.The NS3 donor set is distorted tetrahedral with the widest angle at the zinc atom of approximately 132° correlating with the close approach of the non-coordinating sulphur atom.Crystals of 7 were isolated from solutions containing a 1:1 ratio of reagents but only the 2:1 species was isolated [32].As evident from Figure 4a, the environment about the binuclear is congested and steric hindrance is likely the cause for the  The next structures to be described feature the 4,4 -bipyridine (bpy) ligand which is well represented in the present survey.The structure of [Zn(S 2 COiPr) 2 ] 2 (bpy) (2) [29] is shown in Figure 1b and is binuclear; the molecule has two-fold symmetry with the zinc atoms lying on the axis.The xanthate ligands are S,S-chelating and the five-coordinate geometries are completed by a pyridyl-N atom.The NS 4 donor set is heavily distorted with the value of τ computing to 0.63, which compares to 1.0 for an ideal trigonal-bipyramid and 0.0 for an ideal square-pyramid [69].The second zinc xanthate structure with bpy is a coordination polymer as the ratio of components is 1:1 rather than 2:1 as in 2. In [Zn(S 2 COiBu) 2 (bpy)] n (3) [30], one xanthate ligand is S,S-chelating but with disparate Zn-S bond lengths while the other is S,O-chelating with the Zn-O separation being long at 3.11 Å.While comparatively rare, such S,O-chelating modes are known in the structural chemistry of metal xanthates [20].In 3, the resulting N 2 OS 3 coordination geometry is based on a distorted octahedron.The topology of the chain is helical, Figure 1c.A difference in the conformation of the bpy ligand in the structures of 2 and 3 is noted, namely the dihedral angle between the pyridyl rings in 2 is 13.9 • compared with 29.0 • in 3, indicating conformational flexibility.The three remaining structures in this section contain the trans-1,2-bis(4-pyridyl)ethylene (bpe) ligand in a bridging mode [31].In [Zn(S 2 COR) 2 (bpe)] n , for R = Et (4) and nBu (5), both xanthate ligands are S,O-chelating resulting in N 2 O 2 S 2 donor sets which are best described as being skew-trapezoidal bipyramidal in which the pyridyl-N atoms lie over the relatively long Zn-O bonds.The coordination polymer in each of 4 and 5 has a zig-zag topology as illustrated for 4 in Figure 1d.When the steric bulk of the R group in the xanthate ligand is increased, a new aggregation pattern in observed.In [Zn(S 2 COCy) 2 ] 2 (bpe) (6) [31], isolated from analogous 1:1 solutions that yielded 4 and 5, the xanthate ligands revert to S,S-coordination modes and a binuclear aggregate is formed, similar to that shown in Figure 1b.Such steric effects in the coordination chemistry of metal 1,1-thiolates is well documented [25,68,70].With τ = 0.51, the NS 4 coordination geometry in 6 is almost exactly intermediate between the two ideal five-coordinate geometries.The influence of aggregation patterns in 4-6-that is, polymeric versus dimeric-was shown to have a distinct influence upon the solid-state luminescence responses [31].
Although only including six structures, the foregoing offers a microcosm of the variability of structures covered in this review.Thus, different coordination modes of the xanthate ligand are evident leading to distinct coordination geometries, different aggregation patterns, sometimes related to the steric bulk of the substituents, and, when formed variable topologies of coordination polymers and conformational variations in the bridging ligands.

Zinc Dithiophosphate and Related Structures
The first dithiophosphate structure to be described in this section is a centrosymmetric binuclear species similar to that observed for 2, Figure 1b.In {Zn[S 2 P(OiPr) 2 ] 2 } 2 (pyr) (7) [32], one dithiophosphate ligand is chelating while the other, with Zn-S bond lengths differing by nearly 1 Å, is coordinating in an asymmetric mode or taken to an extreme, is monodentate.The NS 3 donor set is distorted tetrahedral with the widest angle at the zinc atom of approximately 132 • correlating with the close approach of the non-coordinating sulphur atom.Crystals of 7 were isolated from solutions containing a 1:1 ratio of reagents but only the 2:1 species was isolated [32].As evident from Figure 4a, the environment about the binuclear is congested and steric hindrance is likely the cause for the lack of polymer formation.
Compounds 8-10 involve bridging bpy ligands.In {Zn[S 2 P(OEt) 2 ] 2 (bpy)} n (8) [33], two independent formula units comprise the asymmetric unit.One of the independent zinc atoms is coordinated within a distorted tetrahedral N 2 S 2 donor set; both dithiophosphate ligands coordinate in the monodentate mode.By contrast, the second independent zinc atom is five coordinate as one of the dithiophosphate ligands is S,O-coordinating.The NO 2 S 2 donor set is intermediate between ideal trigonal-bipyramidal and square-pyramidal geometries (τ = 0.53).There are three independent bpy molecules in the structure with two located about a centre of inversion.For these, the twist between rings is 0 • .For the bpy molecule bridging the independent zinc atoms, the twist is 29 • .The resultant coordination polymer has a zig-zag topology as shown in Figure 4b.To a first approximation, a similar coordination polymer is found in the crystal of {Zn[S 2 P(OiPr) 2 ] 2 (bpy)} n (9) [34].The asymmetric unit of 9 comprises four independent repeat units but, each zinc atom exists within a distorted tetrahedral N 2 S 2 donor set.The dihedral angles for the bpy ligands range from 26 to 45 • .The third structure has bulky cyclohexyl substituents and therefore, owing to steric hindrance, only binuclear {Zn[S 2 P(OCy 2 ) 2 ] 2 } 2 (bpy) (10) [35] could be isolated.Here, the dithiophosphate ligands coordinate in a S,S-mode leading to a NS 4 donor set which tends towards a square-pyramidal geometry with the pyridyl-N in the apical position (τ = 0.44).The steric crowding in the centrosymmetric molecule is apparent from Figure 4c.There are also three structures with bridging bpe ligands.The structure of {Zn[S 2 P(OiPr) 2 ] 2 (bpe)} n (11) [36] with a N 2 S 2 donor sets resembles the zig-zag polymeric structure of 10.However, for {Zn[S 2 P(OR) 2 ] 2 (bpe)} n , for R = iBu (12) [37] and R = Cy (13) [35], distorted octahedral geometries based on trans-N 2 S 4 donor sets are observed leading to effectively linear coordination polymers.Particularly noteworthy is the polymeric structure of 13, the first thus far in this survey featuring bulky cyclohexyl groups.As seen from Figure 4d, the ethene link between the 4-pyridyl rings introduces more space between adjacent zinc centres which readily accommodates the large cyclohexyl rings and thereby, enables the formation of a polymeric chain.Compounds 8-10 involve bridging bpy ligands.In {Zn[S2P(OEt)2]2(bpy)}n (8) [33], two independent formula units comprise the asymmetric unit.One of the independent zinc atoms is coordinated within a distorted tetrahedral N2S2 donor set; both dithiophosphate ligands coordinate in the monodentate mode.By contrast, the second independent zinc atom is five coordinate as one of the dithiophosphate ligands is S,O-coordinating.The NO2S2 donor set is intermediate between ideal trigonal-bipyramidal and square-pyramidal geometries (τ = 0.53).There are three independent bpy molecules in the structure with two located about a centre of inversion.For these, the twist between rings is 0°.For the bpy molecule bridging the independent zinc atoms, the twist is 29°.The resultant coordination polymer has a zig-zag topology as shown in Figure 4b.To a first approximation, a similar coordination polymer is found in the crystal of {Zn[S2P(OiPr)2]2(bpy)}n ( 9) [34].The asymmetric unit of 9 comprises four independent repeat units but, each zinc atom exists within a distorted tetrahedral N2S2 donor set.The dihedral angles for the bpy ligands range from 26 to 45°.The third structure has bulky cyclohexyl substituents and therefore, owing to steric hindrance, only binuclear {Zn[S2P(OCy2)2]2}2(bpy) (10) [35] could be isolated.Here, the dithiophosphate ligands  (19).Additional colour code: phosphorus, pink.
There is a single example of a structure with a bridging trans-1,2-bis(4-pyridyl)propane (bpp) ligand, namely {Zn[S 2 P(OiPr) 2 ] 2 (bpp)} n (19) [32].Each of the two independent zinc atoms in the structure have distorted tetrahedral coordination geometries and, owing to the curved nature of the n-propyl bridge between the 4-pyridyl residues, the resulting coordination polymer has a helical topology as shown in Figure 4e.The following structures to be summarised contain less frequently studied bipyridyl-type molecules.
The ligand, bis(4-pyridyl)disulphide (bpS 2 ), is bridging in {Zn[S 2 P(OiPr) 2 ] 2 (bpS 2 )} n (20) [32].In 20, one dithiophosphate ligand is S-monodentate and the other is S,O-chelating but with the 2n-O separation about 0.9 Å longer than the Zn-S bond length.The N 2 OS 2 donor set defines a coordination geometry tending towards square-pyramidal with the value of τ being 0.38.The resultant coordination polymer has a zig-zag topology (glide symmetry) and is illustrated in Figure 5a.A zig-zag coordination polymer is also found in the crystal of {Zn[S 2 P(OiPr) 2 ] 2 (bpa)} n (21) [32], where bpa is bis(4-pyridyl)amine; the zinc atom exists in a distorted tetrahedral N 2 S 2 donor set.The notable characteristic of the bpa ligand is the presence of hydrogen bonding functionality.Indeed, amine-N-H . . .O(alkoxy) hydrogen bonds are formed which serve to link chains into supramolecular layers as shown in Figure 5b.Thus far, all bipyridyl-type ligands described have had the 4-pyridyl isomer; in the three remaining dithiophosphate structures, isomeric 2-pyridyl and 3-pyridyl species connect zinc atoms.Unlike the polymeric bpe analogue, 11, the structure of Zn[S2P(OiPr)2]2(2-bpe)}2 ( 23) [32], where 2-bpe is the 2-pyridyl isomer of bpe, that is trans-1,2-bis(4-pyridyl)ethylene, is dimeric, no doubt owing to steric congestion, Figure 6b.In the centrosymmetric molecule, the NS4 donor set approximates a trigonal-bipyramidal geometry (τ = 0.80) with the less tightly bound sulphur atoms The 1,2-bis(4-pyridylmethylene)hydrazine (4-pyald; 4-pyridylaldazine) ligand is the first example of a bipyridyl-type ligand in this survey with heteroatoms in the bridge linking the 4-pyridyl residues.With four atoms in the link-that is, an all-trans CN 2 C sequence, separating the 4-pyridyl residues-there is sufficient space to accommodate the bulky cyclohexyl groups in {Zn[S 2 P(OCy) 2 ] 2 (4-pyald)} n (22) [40], and a zig-zag coordination polymer ensues, Figure 6a.The curious feature of this structure relates to the relative disposition of the sulphur atoms.In the related preceding structures, trans-N 2 S 4 donor sets were noted but, here the donor set is cis-N 2 S 4 .

Zinc Dithiocarbamate Structures
There are 30 dithiocarbamate structures, that is, 28-57 , to be described in this section, Table 1.While the majority of the structures conform to already-observed motifs, there are a number of unexpected modes of coordination for the bipyridyl-type ligands, often in the presence of dithiocarbamate ligands functionalised with groups capable of forming hydrogen bonding interactions.Hence, this section is divided into discussions of structures not capable of forming hydrogen bonds and those that are.
There are single examples each of dithiophosphate analogues where the alkoxy groups are systematically substituted by alkyl groups, that is, giving dithiophosphonate and dithiophosphinate, Figure 1.In {Zn[S 2 P(4-MePh)OMe] 2 (bpeh)} n (26) [42], a tetrahedral N 2 S 2 donor set and zig-zag coordination polymer is seen as in 15, 16 and 18.Finally, in {Zn[S 2 P(iBu) 2 ] 2 (bpy)} n (27) [43], Crystals 2018, 8, 18 10 of 29 an octahedral N 2 S 4 donor set and linear coordination polymer is observed.This is in contrast to the zig-zag polymers observed in the bpy analogues 8 and 9 but, in accord with the linear polymers seen in the bpe (12) and bpeh (17) analogues perhaps suggesting a propensity for this type of polymer topology for isobutyl substituents.

Zinc Dithiocarbamate Structures
There are 30 dithiocarbamate structures, that is, 28-57 , to be described in this section, Table 1.While the majority of the structures conform to already-observed motifs, there are a number of unexpected modes of coordination for the bipyridyl-type ligands, often in the presence of dithiocarbamate ligands functionalised with groups capable of forming hydrogen bonding interactions.Hence, this section is divided into discussions of structures not capable of forming hydrogen bonds and those that are.

Zinc Dithiocarbamate Structures Not Capable of Forming Hydrogen Bonds
The most unexpected structure to be described in this section is monomeric Zn(S 2 CNEt 2 ) 2 (bpy) (28) [44] as, rather than bridging, the bpy molecule coordinates in the monodentate mode: see Figure 7a.With a value of τ = 0.51, the NS 4 donor set is nearly perfectly intermediate between square-pyramidal and trigonal-pyramidal.The remaining structures involving bpy, 29-36 [45][46][47][48][49][50][51][52] have this ligand in the expected bridging mode: see Table 1.The prototype structure of the majority of these [Zn(S 2 CNR 2 ) 2 ] 2 (bpy) structures is the R = Me derivative, 29 [45], shown in Figure 7b.The molecule in 29 is located about a centre of inversion indicating the dihedral angle between the pyridyl rings of bpy is 0 • .Identical symmetry is found in the R = nPr (31) [44], iPr (32) [48], iBu (33) [49] and CH 2 Ph (34) [50] structures.There are three variations.In {Zn[S 2 CN(CH 2 ) 4 ] 2 } 2 (bpy) (35) [51], the molecule is situated about a site of symmetry 222.In [Zn(S 2 CNEt 2 ) 2 ] 2 (bpy) (30) [46,47], there are two independent molecules in the asymmetric unit: one where the ZnNNZn vector is coincident with the two-fold axis, and the other where the two-fold axis is perpendicular to the ZnNNZn vector.In a sense, the structure of {Zn[S 2 CN(CH 2 ) 5 ] 2 } 2 (bpy) (36) [52] represents a synthesis of the above in that there are two independent molecules, one situated about a centre of inversion and the other molecule having 2-fold symmetry coincident with the ZnNNZn axis.In the non-centrosymmetric molecules, the range of dihedral angles between the pyridyl rings of bpy is 25 to 39 The four zinc dithiocarbamate species with bpe, that is 37-40 [53][54][55][56], Table 1, also adopt the same dimeric motif as illustrated in Figure 7a; the bpe is planar or close to planar in each case.The NS4 donor sets tend to be closer to square-pyramidal in three examples, that is τ ranges from 0.13 for the R = Et (40) to 0.39 for R = Me (37) derivatives, while NS4 is closer to trigonal-bipyramidal for the R = Et (38) species, with τ = 0.56.While no molecular symmetry is event for 37-39, the binuclear molecule in 40 is disposed about a centre of inversion.The structure of 40 is of particular interest as it was isolated as a 1:1 lattice adduct with a non-coordinating molecule of bpe during unsuccessful attempts to encourage polymer formation by having an excess of bpe in solution during crystallisation [56].The final structure in this section also follows the motif shown in Figure 7b, that is, binuclear {Zn[S2CN(nPr)2]2}2(bpeh) (41) [57].The molecule has no symmetry and the NS4 donor set approaches trigonal-bipyramidal with τ values of 0.64 and 0.72 for the independent zinc atoms.

Zinc Dithiocarbamate Structures Capable of Forming Hydrogen Bonds
Many of the structures described in this section conform to the motifs established earlier in this review.However, with hydrogen bonding functionality, supramolecular aggregation between the aggregates usually occurs leading to supramolecular architectures of higher dimensionality.
The first three structures to be discussed in this category have dialkyldithiocarbamate ligands coupled with bipyridyl-type ligands that have hydrogen bonding potential, for example N,N′bis(pyridin-4-ylmethyl)ethanediamide (4-pdaO).In [Zn(S2CNEt2)2]2(4-pdaO) (42) [58], the binuclear The four zinc dithiocarbamate species with bpe, that is 37-40 [53][54][55][56], Table 1, also adopt the same dimeric motif as illustrated in Figure 7a; the bpe is planar or close to planar in each case.The NS 4 donor Crystals 2018, 8, 18 11 of 29 sets tend to be closer to square-pyramidal in three examples, that is τ ranges from 0.13 for the R = Et (40) to 0.39 for R = Me (37) derivatives, while NS 4 is closer to trigonal-bipyramidal for the R = Et (38) species, with τ = 0.56.While no molecular symmetry is event for 37-39, the binuclear molecule in 40 is disposed about a centre of inversion.The structure of 40 is of particular interest as it was isolated as a 1:1 lattice adduct with a non-coordinating molecule of bpe during unsuccessful attempts to encourage polymer formation by having an excess of bpe in solution during crystallisation [56].The final structure in this section also follows the motif shown in Figure 7b, that is, binuclear {Zn[S 2 CN(nPr) 2 ] 2 } 2 (bpeh) (41) [57].The molecule has no symmetry and the NS 4 donor set approaches trigonal-bipyramidal with τ values of 0.64 and 0.72 for the independent zinc atoms.

Zinc Dithiocarbamate Structures Capable of Forming Hydrogen Bonds
Many of the structures described in this section conform to the motifs established earlier in this review.However, with hydrogen bonding functionality, supramolecular aggregation between the aggregates usually occurs leading to supramolecular architectures of higher dimensionality.
The first three structures to be discussed in this category have dialkyldithiocarbamate ligands coupled with bipyridyl-type ligands that have hydrogen bonding potential, for example N,N -bis(pyridin-4-ylmethyl)ethanediamide (4-pdaO).In [Zn(S 2 CNEt 2 ) 2 ] 2 (4-pdaO) (42) [58], the binuclear molecule is disposed about a centre of inversion with the crank-shaft shape of the 4-pdaO bridge leading to the distinctive kink in the molecule, Figure 8a.The NS 4 donor set approximates a square-pyramid as indicated by the value of τ = 0.20.The next two molecules contain the 3-pyridyl isomer of 4-pdaO, N,N -bis(pyridin-3-ylmethyl)ethanediamide (3-pdaO) and two distinctive conformations are observed.In [Zn(S 2 CNMe 2 ) 2 ] 2 (3-pdaO) (43) [59], isolated as a di-dimethylformamide (DMF) solvate, a similar centrosymmetric conformation for the 3-pdaO molecule is seen as for 42.Similarly, the value of τ = 18 indicates the NS 4 coordination geometry is based on a square-pyramid.The co-crystallised DMF molecules are associated with the binuclear molecule via amide-N-H . . .O(DMF) hydrogen bonds, Figure 8b, so no further supramolecular association is possible in the crystals, at least via hydrogen bonding.A major conformational change is evident for the 3-pdaO molecule in {Zn[S 2 CN(nPr 2 )] 2 } 2 (3-pdaO) (44) [59].As seen from Figure 8c, the bridging 3-pdaO molecule has a U-shape; the molecule has 2-fold symmetry.In contrast to 42, the NS 4 coordination geometry in 43 is based on a trigonal-pyramid with τ = 0.76.While such remarkable differences in conformation may seem incongruent, computational chemistry performed on 3-pdaO show that the energy difference between the extended form, where the 3-pyridyl rings are anti-periplanar, and the curved form, where the rings are syn-periplanar, is less than 1 kcal/mol [71].This observation highlights the conformational flexibility of this class of molecule and their related thioamide derivatives [72].A consequence of the U-shape of the molecule in 43 is that rows of alternating oppositely orientated molecules are linked by amide-N-H . . .O(amide) hydrogen bonds leading to a supramolecular tape, as in Figure 8d.
The structure of {Zn[S 2 CN(CH 2 CH 2 OH) 2 ] 2 } 2 (pyr) has been determined solvent-free (45), as well as its di-dioxane solvate (46) [60].The molecular structures resemble the binuclear aggregate shown in Figure 7b.In 45, the molecule is disposed about a centre of inversion and the same is true for each of the two independent molecules in 46.The values of τ value from 0.19 (45) to 0.26 and 0.40 (46) indicating a tendency towards square-pyramidal.In each of 45 and 46 there is extensive hydroxyl-O-H . . .O(hydroxyl) hydrogen bonding, involved all hydroxyl groups as acceptors and donors, leading to a three-dimensional architecture, Figure 9.As discussed in the original publication, there are no solvent accessible voids in 45 and the key crystal indicators, that is density (1.709 compared with 1.570 g/cm 3 ) and packing efficiency (74.9 compared with 71.6%), suggest that 46 represents a less efficient packing arrangement, owing to the presence of dioxane molecules.Consistent with this is the lack of strong intermolecular interactions between the host lattice and the dioxane molecules resident in the channels.The rationale for the formation of 46 rests with the observation that owing to the high boiling point of dioxane, the dioxane molecules were trapped in the crystal under the conditions of specific crystallisation experiment [60].The structure of {Zn[S2CN(CH2CH2OH)2]2}2(pyr) has been determined solvent-free (45), as well as its di-dioxane solvate (46) [60].The molecular structures resemble the binuclear aggregate shown in Figure 7b.In 45, the molecule is disposed about a centre of inversion and the same is true for each of the two independent molecules in 46.The values of τ value from 0.19 (45) to 0.26 and 0.40 (46) indicating a tendency towards square-pyramidal.In each of 45 and 46 there is extensive hydroxyl-O-H … O(hydroxyl) hydrogen bonding, involved all hydroxyl groups as acceptors and donors, leading to a three-dimensional architecture, Figure 9.As discussed in the original publication, there are no solvent accessible voids in 45 and the key crystal indicators, that is density (1.709 compared with 1.570 g/cm 3 ) and packing efficiency (74.9 compared with 71.6%), suggest that 46 represents a less efficient packing arrangement, owing to the presence of dioxane molecules.Consistent with this is the lack of strong intermolecular interactions between the host lattice and the dioxane molecules resident in the channels.The rationale for the formation of 46 rests with the observation that owing to the high boiling point of dioxane, the dioxane molecules were trapped in the crystal under the conditions of specific crystallisation experiment [60].[61], is non-symmetric with coordination geometries close to square-pyramidal (τ = 0.21) and intermediate between this and trigonal-bipyramidal (τ = 0.51) for the second zinc atom.However, the di-methanol solvate of 48, Zn[S 2 CN(Et)CH 2 CH 2 OH] 2 } 2 (bpy).2CH 3 OH (49) [61], is centrosymmetric with a coordination geometry approaching trigonal-bipyramidal (τ = 0.62).In the fourth structure of this series, with a symmetrically substituted dithiocarbamate ligand, {Zn[S 2 CN(CH 2 CH 2 OH) 2 ] 2 } 2 (bpy) (50) [61], there is no molecular symmetry and with τ values of 0.34 and 0.54, the coordination geometries follow the pattern as for 48.The molecular packing in each of the four crystals is quite distinct.In the crystal of 47, hydroxyl-O-H . . .O(hydroxyl) hydrogen bonding connects molecules into supramolecular layers via eight-membered { . . .H-O} 4 synthons.From Figure 10a, left, it is apparent that the layers have rather large voids and, indeed, perpendicularly inclined layers interpenetrate to form a three-dimensional architecture, shown in Figure 10a  The next structure to be described was isolated from frustrated experiments to force the formation of a coordination polymer by having an excess of bpy in the crystallisation.Thus, Zn[S2CN(iPr)CH2CH2OH]2(bpy).(bpy)0.5 (51) [62] has monodentate and uncoordinated bpy in the lattice.A view of the zinc-containing molecule is shown in Figure 11a.The familiar NS4 donor set is noted and this approaches a trigonal-bipyramidal geometry as judged by the value of τ = 0.64.There is a considerable twist in the coordinated bpy molecule of 28° which compares with 0° for the uncoordinated, centrosymmetric molecule.The next structure to be described was isolated from frustrated experiments to force the formation of a coordination polymer by having an excess of bpy in the crystallisation.Thus, Zn[S 2 CN(iPr)CH 2 CH 2 OH] 2 (bpy).(bpy)0.5 (51) [62] has monodentate and uncoordinated bpy in the lattice.A view of the zinc-containing molecule is shown in Figure 11a.The familiar NS 4 donor set is noted and this approaches a trigonal-bipyramidal geometry as judged by the value of τ = 0.64.There is a considerable twist in the coordinated bpy molecule of 28 • which compares with 0 • for the uncoordinated, centrosymmetric molecule.The next structure to be described was isolated from frustrated experiments to force the formation of a coordination polymer by having an excess of bpy in the crystallisation.Thus, Zn[S2CN(iPr)CH2CH2OH]2(bpy).(bpy)0.5 (51) [62] has monodentate and uncoordinated bpy in the lattice.A view of the zinc-containing molecule is shown in Figure 11a.The familiar NS4 donor set is noted and this approaches a trigonal-bipyramidal geometry as judged by the value of τ = 0.64.There is a considerable twist in the coordinated bpy molecule of 28° which compares with 0° for the uncoordinated, centrosymmetric molecule.The familiar distorted NS4 coordination geometries are found in each of centrosymmetric Zn[S2CN(Me)CH2CH2OH]2(3-pdaS) (55), Figure 14, isolated as a DMF di-solvate, and Zn[S2CN(Me)CH2CH2OH]2(3-pdaS) (56), whichwas co-crystallised serendipitously with two equivalents of S8 owing to partial desulphurisation during crystallisation [65].The values of τ are 0.46 and 0.08, respectively, suggesting the coordination geometry in 56 is relatively closely based on a square-pyramid.In the molecular packing of 55, half of the hydroxyl groups form hydroxyl-O-H … O(DMF) hydrogen bonds and the remaining hydroxyl groups form hydroxyl-O-H … O(hydroxyl) hydrogen bonds to form a supramolecular ladder, via the 28-membered {…HOC2NCSZnSCNC2O}2 synthons commented upon above, Figure 15a.Additional stabilisation to the supramolecular one-dimensional chain is provided by amide-N-H … O(DMF) and amide-N-H … S(thioamide) hydrogen bonds, not shown.The key feature of the supramolecular aggregation in 56 is the forming of planar {…HO}4 synthons contributed to by four separate molecules and which extend laterally to form a twodimensional array resembling that shown for 48 in Figure 10b.Layers are connected into a threedimensional architecture by amide-N-H … S(amide) hydrogen bonds to define channels in which reside the S8 molecules, Figure 15b.
The molecular structure of Zn[S2CN(Me)CH2CH2OH]2(triazine).1.5(dioxane)(57) [66] is noteworthy for being the only example having monodentate-coordinating dithiocarbamate ligands, as in Figure 16a.This arises as the   The final zinc-containing structure to be described is one containing the 2,4,6-tris (pyridin-2-yl)-1,3,5-triazine ligand abbreviated as triazine.The molecular structure of Zn[S 2 CN(Me) CH 2 CH 2 OH] 2 (triazine).1.5(dioxane)(57) [66] is noteworthy for being the only example having monodentate-coordinating dithiocarbamate ligands, as in Figure 16a.This arises as the triazine molecule is tridentate, forming bonds to zinc via a triazine-and two pyridyl-nitrogen atoms.The resulting N 3 S 2 donor set is very close to a square-pyramid with τ = 0.07 and with a sulphur atom occupying the apical position.In the molecular packing, molecules are arranged in rows and are held in place by hydroxyl-O-H . . .N(triazine), N(pyridyl) hydrogen bonds, and these are connected into a two-dimensional array via hydroxyl-O-H . . .S(dithiocarbamate) hydrogen bonds as shown in Figure 16b.

Cadmium Xanthate Structure
There is a sole example of a cadmium xanthate structure, [Cd(S2COiPr)2(bpy)]n (58) [74], making xanthates the least represented of the conventional 1,1-dithiolate ligands included in this review; data for all 31 cadmium structures, that is, 58-88 [62, [74][75][76][77][78][79][80][81][82][83][84][85][86][87][88][89][90][91][92], are listed in Table 2.There are two independent formula units in the crystal and each of these is disposed about a two-fold axis of asymmetry with the cadmium and nitrogen atoms lying on the axis.Each independent formula unit self-associates to form a linear chain.The conformations of the bridging bpy molecules distinguish the structures.In one repeat unit, the bpy is effectively planar, with the dihedral angle between the two rings being 3°, Figure 17, while, in the other residue, the rings are twisted forming a dihedral angle of 26°.This observation highlights the conformational flexibility of the bpy molecule as noted

Cadmium Dithiophosphate and Related Structures
By far the most numerous cadmium-containing structures covered in this survey are the dithiophosphates, which have been the subject of a number of systematic studies.There are four structures featuring bridging bpy ligands, {Cd[S2P(OR)2]2(bpy)}n for R = Me (59) [75], Et (60) [76], iPr

Cadmium Dithiophosphate and Related Structures
By far the most numerous cadmium-containing structures covered in this survey are the dithiophosphates, which have been the subject of a number of systematic studies.There are four structures featuring bridging bpy ligands, {Cd[S 2 P(OR) 2 ] 2 (bpy)} n for R = Me (59) [75], Et (60) [76], iPr (61) [77] and Cy (62) [77].All structures present different crystallographic symmetry.Thus, in 59 the bpy ligand is disposed about a centre of inversion, in 60 the cadmium atom is located on a centre of symmetry, for 61 both bpy and cadmium are disposed about centres of inversion whereas for 62, the cadmium and nitrogen atoms lie on a 2-fold axis of symmetry.In 59-61, the bpy molecules are planar or effectively planar but, in 63 the dihedral angle of 50 • between the rings indicates a significant twist.A common feature of the four structures is the N 2 S 4 donor set for cadmium, as the dithiophosphate ligands adopt a symmetrically cheating mode, but this has a cis-disposition in 59, leading to a zig-zag chain, Figure 18a, whereas trans-N 2 S 4 geometries are found in 60-62 leading to linear topologies for the resultant coordination polymers, illustrated for 62 in Figure 18b.Linear coordination polymers and trans-N 2 S 4 donor sets are also found in each of {Cd[S 2 P(OiPr) 2 ] 2 (bpe)} n (63) {Cd[S 2 P(OCy) 2 ] 2 (bpe)} n (64) and {Cd[S 2 P(OiPr) 2 ] 2 (bpeh)} n (65) [77].While neither of 63 and 64 have crystallographic symmetry, each of the two independent repeat units in 65 have the cadmium atom lying on a centre of symmetry and the bpy molecule lying about a centre of inversion; independent repeat unit self-associate to form independent chains. of symmetry and the bpy molecule lying about a centre of inversion; independent repeat unit selfassociate to form independent chains.
There are two structures available with the dpp ligand and the supramolecular aggregation in these are dictated by steric effects.In {Cd[S2P(OiPr)2]2(bpp)}n (66) [77], with the cadmium atom lying on a 2-fold axis of symmetry and with the dpp bisected by a 2-fold axis of symmetry, a cis-N2S4 donor set is found resulting in a linear coordination polymer with characteristic arches owing to the conformation of the dpp ligand, Figure 19a.A similar cis-N2S4 donor set and arched conformation is found in the structure of {Cd[S2P(OCy)2]2(bpp)}2 (67) [77], but not the formation of a coordination polymer owing to the steric bulk of the cyclohexyl groups.Instead, a centrosymmetric dimeric aggregate as shown in Figure 19b.In {Cd[S2P(OiPr)2]2(bpS2)}n (68) [77], which is devoid of crystallographic symmetry, a linear coordination polymer is formed.As seen in Figure 19c, this topology resembles that found for 66 illustrated in Figure 19a.The structures of {Cd[S2P(OiPr)2]2(4pyald)}n ( 69) and the 3-pyald analogue (70) [78] share common features in terms of symmetry (the cadmium atom lies on a centre of symmetry and the 4-pyald molecule lies about a centre of inversion), trans-N2S4 coordination geometry which defines an octahedron and linear topology for the resultant coordination polymer compared with Figure 18b.A similar description pertains for {Cd[S2P(OCy)2]2(4-pyald)}n (71) [79] but, in this case only the cadmium atom lies on a centre of inversion.A different motif is formed when reactions are conducted with the isomeric 2-pyald ligand.As shown in Figure 19d, all four nitrogen atoms of 2-pyald are employed in coordination to cadmium, which exists within a cis-N2S4 donor set in the centrosymmetric dimer, {Cd[S2P(OiPr)2]2(2-pyald)}2 ( 72) [78].The final two structures are also 2-pyridyl isomers of the more commonly explored bpe ligand.In both {Cd[S2P(OR)2]2(2-bpe)}n for R = iPr (73) [79], Figure 19e, and Cy (74) [77], the cadmium (within trans-N2S4 donor sets) and 2-bpe molecules are disposed on/about centres of inversion.Linear coordination polymers are formed as the chelation observed in 72 is no longer possible, thereby making available additional coordination sites for bridging.There are five dithiophosphonate structures and one example of a dithiophosphinate structure available in the CSD [2] and these uniformly are one-dimensional coordination polymers but, with distinct topologies.The structures of two bpy-containing species {Cd[S2P(OR)4-MeOPh]2(bpy)}n for There are two structures available with the dpp ligand and the supramolecular aggregation in these are dictated by steric effects.In {Cd[S 2 P(OiPr) 2 ] 2 (bpp)} n (66) [77], with the cadmium atom lying on a 2-fold axis of symmetry and with the dpp bisected by a 2-fold axis of symmetry, a cis-N 2 S 4 donor set is found resulting in a linear coordination polymer with characteristic arches owing to the conformation of the dpp ligand, Figure 19a.A similar cis-N 2 S 4 donor set and arched conformation is found in the structure of {Cd[S 2 P(OCy) 2 ] 2 (bpp)} 2 (67) [77], but not the formation of a coordination polymer owing to the steric bulk of the cyclohexyl groups.Instead, a centrosymmetric dimeric aggregate as shown in Figure 19b.In {Cd[S 2 P(OiPr) 2 ] 2 (bpS 2 )} n (68) [77], which is devoid of crystallographic symmetry, Crystals 2018, 8, 18 20 of 29 a linear coordination polymer is formed.As seen in Figure 19c, this topology resembles that found for 66 illustrated in Figure 19a.The structures of {Cd[S 2 P(OiPr) 2 ] 2 (4-pyald)} n (69) and the 3-pyald analogue (70) [78] share common features in terms of symmetry (the cadmium atom lies on a centre of symmetry and the 4-pyald molecule lies about a centre of inversion), trans-N 2 S 4 coordination geometry which defines an octahedron and linear topology for the resultant coordination polymer compared with Figure 18b.A similar description pertains for {Cd[S 2 P(OCy) 2 ] 2 (4-pyald)} n (71) [79] but, in this case only the cadmium atom lies on a centre of inversion.A different motif is formed when reactions are conducted with the isomeric 2-pyald ligand.As shown in Figure 19d, all four nitrogen atoms of 2-pyald are employed in coordination to cadmium, which exists within a cis-N 2 S 4 donor set in the centrosymmetric dimer, {Cd[S 2 P(OiPr) 2 ] 2 (2-pyald)} 2 (72) [78].The final two structures are also 2-pyridyl isomers of the more commonly explored bpe ligand.In both {Cd[S 2 P(OR) 2 ] 2 (2-bpe)} n for R = iPr (73) [79], Figure 19e, and Cy (74) [77], the cadmium (within trans-N 2 S 4 donor sets) and 2-bpe molecules are disposed on/about centres of inversion.Linear coordination polymers are formed as the chelation observed in 72 is no longer possible, thereby making available additional coordination sites for bridging.

Cadmium Dithiocarbamate Structures
As for the zinc dithiocarbamates, this section is divided into a discussion of structures incapable of hydrogen bonding and those that are capable of hydrogen bonding.In the latter category, a number of unprecedented motifs are apparent.There are five dithiophosphonate structures and one example of a dithiophosphinate structure available in the CSD [2] and these uniformly are one-dimensional coordination polymers but, with distinct topologies.The structures of two bpy-containing species {Cd[S 2 P(OR)4-MeOPh] 2 (bpy)} n for R = Me (75) [80] and Et (76) [81] are linear polymers, each with the trans-N 2 S 4 octahedral cadmium atom located on a centre of inversion and bpy situated about a centre.In the same way, the three structures of general formula {Cd[S 2 P(OMe)4-MeOPh] 2 (N∩N)} n for N∩N = bpe (77) [82], beph (78) [83] and dpp (79) [84] are very similar, all lacking symmetry, adopting zig-zag polymers and each with cis-N 2 S 4 octahedral cadmium.The sole example of a dithiophosphinate, namely {Cd[S 2 P(OiBu) 2 ] 2 (bpy)} n (80) [85], is distinct from the aforementioned structures in that the CdNNCd axis lies on a two-fold axis of symmetry implying the coordination polymer is linear; the cadmium atom is octahedrally coordinated within a trans-N 2 S 4 donor set.

Cadmium Dithiocarbamate Structures
As for the zinc dithiocarbamates, this section is divided into a discussion of structures incapable of hydrogen bonding and those that are capable of hydrogen bonding.In the latter category, a number of unprecedented motifs are apparent.

Cadmium Dithiocarbamate Structures Capable of Forming Hydrogen Bonds
The final four structures to be described in this bibliographic review are zero-dimensional and are perhaps not of great interest in terms of the generation of coordination polymers.However, the isolation of these species raises questions relating to the relative importance of coordinate-bond formation versus hydrogen bonding interactions and, therefore, these are of fundamental interest.The molecular structures of {Cd[S2CN(iPr)CH2CH2OH]2(bpy)}2 (85) [69], co-crystallised with half a bpy molecule, {Cd[S2CN(iPr)CH2CH2OH]2}2(bpe)3 (86) [90], {Cd[S2CN(nPr)CH2CH2OH]2}(4-pyald)2 (87) [91] and {Cd[S2CN(iPr)CH2CH2OH]2}2(3-pyald)2 (88) [92] are illustrated in Figure 21a-d.In 85, which features a monodentate bpy ligand, the cadmium atom is five-coordinate within a NS4 donor set that defines a geometry approaching trigonal-bipyramidal (τ = 0.67) with the less tightly bound sulphur atoms defining the axial positions.A distorted cis-N2S4 donor set is found in centrosymmetric and binuclear 86 which features a bridging bpe ligand and two terminally bound bpe molecules.An octahedral geometry is also found in 87 but, based on a trans-N2S4 donor set as the cadmium centre,

Cadmium Dithiocarbamate Structures Capable of Forming Hydrogen Bonds
The final four structures to be described in this bibliographic review are zero-dimensional and are perhaps not of great interest in terms of the generation of coordination polymers.However, the isolation of these species raises questions relating to the relative importance of coordinate-bond formation versus hydrogen bonding interactions and, therefore, these are of fundamental interest.(88) [92] are illustrated in Figure 21a-d.In 85, which features a monodentate bpy ligand, the cadmium atom is five-coordinate within a NS 4 donor set that defines a geometry approaching trigonal-bipyramidal (τ = 0.67) with the less tightly bound sulphur atoms defining the axial positions.A distorted cis-N 2 S 4 donor set is found in centrosymmetric and binuclear 86 which features a bridging bpe ligand and two terminally bound bpe molecules.An octahedral geometry is also found in 87 but, based on a trans-N 2 S 4 donor set as the cadmium centre, located on a centre on inversion, is coordinated by two monodentate 4-pyald molecules.Finally, and quite remarkably, the structure of centrosymmetric 88, features octahedral cadmium centres within NS 5 donor sets as the 3-pyald molecules are coordinating in the monodentate mode.In all of the previous 87 structures described in this review, the original parent structure, which is often aggregated [25], has been disrupted by the addition of base.In 88, the thermodynamically favoured dimeric structure, as opposed to the initially formed one-dimensional coordination polymer [93,94], is retained but, with the addition of base in the form of 3-pyald to increase the normally observed S 5 donor set to NS 5 .These four structures, again, point to the unpredictability of this chemistry and substantiate the need of systematic studies.It is probably of no coincidence that the aforementioned four structures each feature hydrogen bonding capability in the dithiocarbamate ligands.The structure of 85 is isostructural with the zinc analogue, 51, and therefore the description of the molecular packing is identical to that described above, see Figure 12a.In the crystal of 86, supramolecular layers are formed mediated by hydroxyl-O-H … O(hydroxyl) and hydroxyl-O-H … N(bpe) hydrogen bonding; that is, with the nitrogen acceptors derived from the monodentate bpe ligands, shown in Figure 22a.Layers interpenetrate to yield a three-dimensional structure, as shown in Figure 22b.To date, not much mention of the synthesis conditions has been made.Here, it is pertinent to note that the stoichiometry of the reactants in these reactions with functionalised dithiocarbamate ligands does not necessarily determine the synthetic outcome.For example, crystals of 86 can be isolated from solutions containing 2:1, 1:1 and 1:2 ratios of "Cd[S 2 N(iPr)CH 2 CH 2 OH] 2 " and bpe [90].
The structure of 85 is isostructural with the zinc analogue, 51, and therefore the description of the molecular packing is identical to that described above, see Figure 12a The structure of 85 is isostructural with the zinc analogue, 51, and therefore the description of the molecular packing is identical to that described above, see Figure 12a.In the crystal of 86, supramolecular layers are formed mediated by hydroxyl-O-H … O(hydroxyl) and hydroxyl-O-H … N(bpe) hydrogen bonding; that is, with the nitrogen acceptors derived from the monodentate bpe ligands, shown in Figure 22a.Layers interpenetrate to yield a three-dimensional structure, as shown in Figure 22b.

Overview and Conclusions
The foregoing overview of the structural chemistry of zinc and cadmium 1,1-dithiolates with potentially bridging bipyridyl-type ligands reveals a very real dependence upon aggregation patterns on the nature of the 1,1-dithiolate ligand as well as upon the metal centre.The maximum dimension for any coordination polymer in the structures is one, and for zinc, polymers are only formed for xanthates and dithiophosphates.For zinc dithiocarbamates, only zero-dimensional

Overview and Conclusions
The foregoing overview of the structural chemistry of zinc and cadmium 1,1-dithiolates with potentially bridging bipyridyl-type ligands reveals a very real dependence upon aggregation patterns on the nature of the 1,1-dithiolate ligand as well as upon the metal centre.The maximum dimension for any coordination polymer in the structures is one, and for zinc, polymers are only formed for xanthates and dithiophosphates.For zinc dithiocarbamates, only zero-dimensional aggregates are formed; an observation which is readily rationalised in terms of the relatively greater coordination potential of the dithiocarbamate ligand compared with the other 1,1-dithiolates.This greater chelating ability arises owing to the relative importance of the resonance form where there are two formal negative charges on the sulphur atoms, Figure 24.The situation changes for cadmium where one-dimensional coordination polymers are found for all 1,1-dithiolate ligands, an observation correlating with the larger size of cadmium versus zinc.

Overview and Conclusions
The foregoing overview of the structural chemistry of zinc and cadmium 1,1-dithiolates with potentially bridging bipyridyl-type ligands reveals a very real dependence upon aggregation patterns on the nature of the 1,1-dithiolate ligand as well as upon the metal centre.The maximum dimension for any coordination polymer in the structures is one, and for zinc, polymers are only formed for xanthates and dithiophosphates.For zinc dithiocarbamates, only zero-dimensional aggregates are formed; an observation which is readily rationalised in terms of the relatively greater coordination potential of the dithiocarbamate ligand compared with the other 1,1-dithiolates.This greater chelating ability arises owing to the relative importance of the resonance form where there are two formal negative charges on the sulphur atoms, Figure 24.The situation changes for cadmium where one-dimensional coordination polymers are found for all 1,1-dithiolate ligands, an observation correlating with the larger size of cadmium versus zinc.A wide array of coordination geometries are observed across the series, often arising from flexible coordination modes of xanthate and dithiophosphate ligands which range from monodentate to chelating and sometimes involve the alkoxy-O atoms in coordination.The steric role of 1,1-dithiolate-bound R groups was shown to be instrumental in controlling aggregation patterns in that larger groups, such as cyclohexyl, preclude the close approach of entities and thereby, exert an A wide array of coordination geometries are observed across the series, often arising from flexible coordination modes of xanthate and dithiophosphate ligands which range from monodentate to chelating and sometimes involve the alkoxy-O atoms in coordination.The steric role of 1,1-dithiolate-bound R groups was shown to be instrumental in controlling aggregation patterns in that larger groups, such as cyclohexyl, preclude the close approach of entities and thereby, exert an influence both over aggregation and polymer topology.A fascinating observation is made for dithiocarbamate ligands functionalised with hydrogen bonding potential, in that an apparent competition between the formation of M←N(pyridyl) coordinate-bonds and hydroxyl-O-H . . .N(pyridyl) hydrogen bonds exists leading to monodentate binding modes for the bipyridyl-type ligands.Such a competition might indicate the energy of stabilisation of these bonding modes are similar.Certainly, the unexpected structures demand further systematic studies as more insight will be gained into the fundamental coordination chemistry of metal 1,1-dithiolates.
upon a general description of the modes of coordination of the 1,1-dithiolate and bipyridyl-type ligands and describing the resultant aggregation patterns.Details of geometric parameters are available in the original publication and are not generally considered.Similarly, generally descriptions of molecular packing are avoided unless relevant to the putative competition between hydrogen bonding and M←N(pyridyl) coordinate-bonds.In all, there are 57 zinc-containing structures and 31 cadmium analogues.
the coordination chemistry of metal 1,1-thiolates is well documented[25,68,70].With τ = 0.51, the NS4 coordination geometry in 6 is almost exactly intermediate between the two ideal fivecoordinate geometries.The influence of aggregation patterns in 4-6-that is, polymeric versus dimeric-was shown to have a distinct influence upon the solid-state luminescence responses[31].
• for the molecule with two-fold axes coincident with the ZnNNZn vector in 30 and the molecule in 222-symmetric 35, respectively.In each of 29-36, the donor set about the zinc atom is defined by NS 4 atoms.The coordination geometries range from almost square-pyramidal for the R = nPr (31) derivative with τ = 0.11 to approaching trigonal-bipyramidal for the R = CH 2 Ph (34) derivative with τ = 0.70.range from almost square-pyramidal for the R = nPr (31) derivative with τ = 0.11 to approaching trigonal-bipyramidal for the R = CH2Ph (34) derivative with τ = 0.70.
by amide-N-H … O(amide) hydrogen bonds leading to a supramolecular tape, as in Figure8d.
, right.Supramolecular layers are also formed in the R = Et analogue, 48.The assembly is distinct, as in Figure 10b, left, from that in 47.Centrosymmetrically related layers are connected via further hydroxyl-O-H . . .O(hydroxyl) hydrogen bonding to form double layers, shown in Figure 10b, right.The key supramolecular synthon is a 12-membered { . . .H-O} 6 ring connected to two further exocyclic hydroxyl-O-H . . .O(hydroxyl) hydrogen bonds.The presence of solvent methanol in 49 effectively blocks off half of the hydrogen bonding capacity of the hydroxyethyl groups owing to the formation of methanol-O-H . . .O(hydroxyl) hydrogen bonds.The remaining hydroxyethyl groups form hydroxyl-O-H . . .O(hydroxyl) hydrogen bonds to generate the supramolecular ladder shown in Figure 10c.In 50, where both N-bound R group carry hydrogen bonding potential, a three-dimensional architecture ensues, Figure 10d.It is of interest that one of the hydroxyl groups in 50 does not form hydroxyl-O-H . . .O(hydroxyl) hydrogen bonds but hydroxyl-O-H . . .S hydrogen bonds instead.The propensity of such hydrogen bonds in -S 2 CNCH 2 CH 2 OH dithiocarbamate ligands has been summarised recently, a summary which suggested these are relatively common in the structural chemistry of these ligands [73].Crystals 2018, 8, 18 14 of 30 bonding potential, a three-dimensional architecture ensues, Figure 10d.It is of interest that one of the hydroxyl groups in 50 does not form hydroxyl-O-H … O(hydroxyl) hydrogen bonds but hydroxyl-O-H … S hydrogen bonds instead.The propensity of such hydrogen bonds in -S2CNCH2CH2OH dithiocarbamate ligands has been summarised recently, a summary which suggested these are relatively common in the structural chemistry of these ligands [73].
. In the crystal of 86, supramolecular layers are formed mediated by hydroxyl-O-H . . .O(hydroxyl) and hydroxyl-O-H . . .N(bpe) hydrogen bonding; that is, with the nitrogen acceptors derived from the monodentate bpe ligands, shown in Figure 22a.Layers interpenetrate to yield a three-dimensional structure, as shown in Figure 22b.In the crystal of 87, only hydroxyl-O-H . . .N(4-pyald) hydrogen bonds are formed and these lead to 40-membered [CdSCNC 2 OH . . .NC 4 N 2 C 4 N] 2 synthons and supramolecular ladders as shown in Figure 23a.In the crystal of 88, supramolecular layers sustained by hydroxyl-O-H . . .O(hydroxyl) and water-O-H . . .O(hydroxyl) hydrogen bonding are connected into a three-dimensional architecture via water-O-H . . .N(pyridyl) hydrogen bonding, as in Figure 23b.

Figure 24 .
Figure 24.Chemical diagram of a significant resonance structure for the dithiocarbamate ligand.R, R′ = alkyl, aryl.

Figure 24 .
Figure 24.Chemical diagram of a significant resonance structure for the dithiocarbamate ligand.R, R = alkyl, aryl.

Table 1 .
Summary of the general features of zinc structures 1-57.