Polymerization Isomerism in Co-M (M = Cu, Ag, Au) Carbonyl Clusters: Synthesis, Structures and Computational Investigation

The reaction of [Co(CO)4]− (1) with M(I) compounds (M = Cu, Ag, Au) was reinvestigated unraveling an unprecedented case of polymerization isomerism. Thus, as previously reported, the trinuclear clusters [M{Co(CO)4}2]− (M = Cu, 2; Ag, 3; Au, 4) were obtained by reacting 1 with M(I) in a 2:1 molar ratio. Their molecular structures were corroborated by single-crystal X-ray diffraction (SC-XRD) on isomorphous [NEt4][M{Co(CO)4}2] salts. [NEt4](3)represented the first structural characterization of 3. More interestingly, changing the crystallization conditions of solutions of 3, the hexanuclear cluster [Ag2{Co(CO)4}4]2− (5) was obtained in the solid state instead of 3. Its molecular structure was determined by SC-XRD as Na2(5)·C4H6O2, [PPN]2(5)·C5H12 (PPN = N(PPh3)2]+), [NBu4]2(5) and [NMe4]2(5) salts. 5 may be viewed as a dimer of 3 and, thus, it represents a rare case of polymerization isomerism (that is, two compounds having the same elemental composition but different molecular weights) in cluster chemistry. The phenomenon was further studied in solution by IR and ESI-MS measurements and theoretically investigated by computational methods. Both experimental evidence and density functional theory (DFT) calculations clearly pointed out that the dimerization process occurs in the solid state only in the case of Ag, whereas Cu and Au related species exist only as monomers.


Introduction
Isomerism in molecular metal clusters is attracting considerable and renewed interest in view of its relevance to atomically precise metal nanoparticles, metal nanoclusters, ultrasmall metal nanoparticles and nanomaterials in general [1][2][3][4][5]. In analogy to organic and coordination chemistry, both stereoisomerism and structural isomerism have been observed in the field of gold nanoclusters and related molecular nanoclusters. These advancements have been possible owing the total structural determination of molecular nanoclusters and atomically precise metal nanoclusters by single-crystal X-ray diffraction (SC-XRD) [6][7][8][9][10]. Enantiomerism is the main type of stereoisomerism found up to now in molecular nanoclusters which, therefore, can be chiral [11,12]. Different types of structural isomerism have been revealed, including core (kernel) isomerism, staple (shell) isomerism and complex isomerism [13]. These show some analogies with chain, positional and functional isomerisms, which are well known in organic chemistry.
Polymerization isomerism may be described as two compounds having the same elemental composition but different molecular weights [32,33]. This is a very rare phenomenon, both in coordination chemistry and molecular cluster chemistry. Probably, this term was applied for the first time to a metal carbonyl cluster in the case of the triangle and square polymerization isomers  [34]. Further examples of polymerization isomerism may be found in the literature concerning metal carbonyls, even if they were referred to differently. For instance, the Ru tetracarbonyl may exist as a Ru3(CO)12 triangular isomer (trimer), or as a [Ru(CO)4]∞ polymer [35]. A similar phenomenon has been observed in the case of the 1:1 adduct between Cu + and [Co(CO)4] − (1). Indeed, this may adopt the {CuCo(CO)4}4 square structure (tetramer) [36] or a {CuCo(CO)4}∞ polymeric structure (Figures 1 and 2) [37]. In contrast, in the case of Ag, only the square tetramer {AgCo(CO)4}4 has been reported [38], whereas no Au related species has been described up to now. It must be remarked that {AgCo(CO)4}4 is perfectly planar, whereas {CuCo(CO)4}4 is not planar.  In particular, three different isomers have been structurally characterized for 2 (see Results and Discussion) [39][40][41], whereas a single structure has been reported for 4 [42] and none for 3.
In view of these interesting results and in order to better rationalize the M-Co(CO)4 (M = Cu, Ag, Au) system, herein we report a detailed study on the reactions between 1 and M(I) salts.
In view of these interesting results and in order to better rationalize the M-Co(CO)4 (M = Cu, Ag, Au) system, herein we report a detailed study on the reactions between 1 and M(I) salts.

Results and Discussion
The reaction of Na[Co(CO) 4 ] (Na(1)) in CH 3 Table 1). The structures of 2 and 4 were previously reported as different salts [39][40][41][42], whereas the one herein reported represents the first structural determination of 3, even if its synthesis was previously reported [43]. The M(I) ion of 2-4 displays a linear coordination and the two Co-centers adopt both a trigonal bipyramidal (TBP) geometry as previously found in [Cu(IMes) 2 ](2) (IMes = C 3 N 2 H 2 (C 6 [42]. The two Co(CO) 4 groups of 2-4 adopt a staggered conformation in all the salts, including those described in this paper. An exception is represented by [Cu(dmpe) 2 ](2) [40], whose unit cell contains two independent anions, both containing two TBP-Co(CO) 4 groups, but one anion adopts a staggered conformation and the other an eclipsed conformation ( Figure 4). A similar phenomenon has been observed for Hg{Co(CO) 4 } 2 , in which the two TBP-Co(CO) 4 groups may adopt a staggered or eclipsed conformation, and both have been characterized by SC-XRD [44].

M-Co b -M h
---60.725 (7) 61.951 (12) 62.806 (4) 63.08(11) and 62.75(11) (7) 118.048 (12) 117.194 (5) 116.93 (11) and 117.25(11) (7) 59.51 (2) 57.031(5) and 60.163 (5) 58.20 (10)  A further isomer of 2 was reported as [PPN](2) salt [41], where a Co center is TBP and the second one displays a tetrahedral coordination of the four CO ligands, with Cu capping one edge of the Co(CO) 4 tetrahedron ( Figure 4). Thus, three different isomers of 2 have been reported up to now: (a) TBP-TBP staggered; (b) TBP-TBP eclipsed; (c) TBP-Td ( Figure 5). The Cu-Co distances in the structures reported are comprised in a very narrow range [2.326-2.411 Å] and are comparable in all the isomers. Notably, DFT computations indicate that the TBP-TBP staggered (a) is the most stable conformer of the TBP-TBP isomer in the gas phase (see Figure S0 in the Supporting Information) and in the presence of a solvent dielectric. The geometry optimizations of the (a), (b) and (c) isomers of 2, indeed, all converge into the TBP-TBP staggered (a) isomer. However, if the [PPN] + counterion is placed close by the Cu complex, a local minimum with TBP-Td geometry can be found (see Figure S0 in the Supporting Information), in agreement with the experimental evidence for [PPN](2) salt [41].  (7) 59.51 (2) 57.031(5) and 60.163 (5) 58.20(10) and 58.47(11) a As found in [NEt4](2). b As found in [NEt4](3). c As found in [NEt4](4). d As found in Na2 (5)    (2) and [PPN](4) suitable for SC-XRD were obtained and these displayed the same cell parameters and crystal structures previously reported in the literature for the 2 and 4 monoanions with the same cation [41,42]. Surprisingly, in the case of M = Ag, crystals of [PPN] 2 [Ag 2 {Co(CO) 4 } 4 ]·C 5 H 12 were obtained, which contained the [Ag 2 {Co(CO) 4 } 4 ] 2− dianion (5). We may view 5 as a dimer of 3, and all the experimental evidence (see below) points out that 5 is formed upon crystallization. In particular, the ESI-MS analyses of both crystals of 3 and 5 (see below) indicate that only the monomer 3 is detected in solution, regardless of the species present in the solid state. A potential equilibrium between 3 and 5 has been theoretically investigated by DFT computations (see Scheme S1 in the Supporting Information), showing that 3 is slightly more stable than 5. Nonetheless, there is no experimental evidence of the presence of 5 in solution, which is, therefore, formed in the solid state upon crystallization. This represents a further case of polymerization isomerism, since 3 and 5 have the same elemental compositions but different molecular weights.
In order to shed light on this point, the crystallization of the other salts of the product of the reaction of 1 and Ag + in a 2:1 molar ratio was attempted, following similar procedures to that described above (see Experimental for details). This resulted in the structural characterization by SC-XRD of four new salts, that is Na 2 (5) (5). All of them contain the dimeric dianion 5. Thus, it is possible to assume that depending on the crystallization conditions, either salts of 3 and 5 can be obtained. Conversely, in the case of Cu and Au, only the monomers 2 and 4 have been observed and structurally characterized up to now. DFT calculations (see Scheme S1 in the Supporting Information) suggest that these observations rely on a different thermodynamic profile of the Ag system compared to Cu and Au ones. Thus, the monomer and dimer display very similar energies for Ag, whereas the monomer is largely favored in the case of Cu and Au.
The  (5). The bands at 1938-1957 cm −1 feature a significant asymmetric broadening towards lower frequencies while the narrow bands at 2025-2027 cm −1 show a shoulder at higher frequencies around 2040 cm −1 . From the similarities in the experimental IR spectra, it can be concluded that the monomers 2-4 are present in solution, since 2 and 4 also exist only as monomers in the solid state. The spectra of 3 and 5 in solution are almost identical, indicating that mainly (or only) one species is actually present in solution. Indeed, it is likely that in the case of Ag, 3 is the main (or almost the only) species present in solution, whereas 5 is formed only upon crystallization. This point has been further corroborated by ESI-MS analyses.
DFT computations provided useful insights into the experimentally observed lineshapes of IR spectra. In particular, we monitored various effects that can shape the IR spectra of complex 3 (see Figures S6 and S7 in the Supporting Information). The simulated IR spectrum of 3 in the gas phase (see Figure S6) features the same two main bands observed experimentally but with an underestimated relative frequency gap (ca. 60 vs. 80 cm −1 ). The addition of an implicit solvent model, which takes into account the effect of the solvent dielectric, results into a larger frequency gap, with the modes at ca. 1938-1957 cm −1 being red-shifted and split so that the IR band is broadened. By including the explicit effect of the local interactions between 3 and the dichloromethane solvent (see Figure S7), the DFT simulated spectrum features a broadening of the band at ca. 1938-1957 cm −1 , in line with experimental spectral lineshape.
The IR spectra of [NEt 4 ](3) and [NMe 4 ] 2 (5) registered in the solid state by ATR mode are sensibly shifted to lower frequencies compared to those recorded in solution (Figures S8-S11 in the Supporting Information). In order to obtain information on the red-shift observed in solid state ATR spectra with respect to IR spectra in solution, we performed DFT simulations of the IR spectrum of 3 in the presence of an explicit molecule of the [NEt 4 ] + counterion (see Figure S12 in the Supporting Information), assuming a tight ionpair conformation as observed in the solid state structure. The tight ion-pair spectrum shows a significant broadening of the main band of 3 in solution (at ca. 1938-1957 cm −1 ), with a sizeable red-shift of the stretching modes for the CO groups point towards the [NEt 4 ] + counterion, which agrees well with the red-shifted band in the ATR spectrum. By including a nearby counterion molecule in the model, also the 2040 cm −1 shoulder of the experimental narrow band at 2025-2027 cm −1 is recovered. These results suggest that the experimental lineshape of 3 is dominated by the distortion of symmetry induced by local interactions with counterions (or, eventually, close by solvent molecules).
In order to further investigate the nature of the species present in solution, ESI-MS studies on CH 3 OH solutions of 2-5 have been carried out (Figures S13-S19 and Tables S1-S3 in the Supporting Information). All anions have been studied as [PPN] + salts, in order to avoid ion pairing in the gas phase. As expected, only the monomers [M{Co(CO) 4 5) is present within the crystals. Indeed, the very strong peak of the molecular ion at m/z 449 shows the typical isotopic pattern of an ion that contains a single Ag atom. Comparison of the experimental peak with the calculated ones for 3 and 5 ( Figure S18 in the Supporting Information), completely rules out the presence of even traces of the dimer in solution. This point is further corroborated by the presence of a peak at m/z 421 attributable to the loss of one CO ligand (28 amu) from the monoanionic molecular ion. It must be concluded that the dimer 5 is formed during crystallization.
It must be remarked that similar results have been obtained both by analyzing the crystals of compounds 2-5 by ESI-MS as well as by performing the ESI-MS analyses on the solutions obtained from the reactions of 1 and M + salts before crystallization. Thus, it may be concluded that the monomers [M{Co(CO) 4 } 2 ] − (M = Cu, 2; Ag, 3; Au, 4) are the only species present in solution (at least to the limit of detection of the employed analytical techniques), whereas the dimer 5 is observed only in the solid state. Its formation might be due to packing effects. We cannot rule out the presence in solution of an equilibrium between 3 and 5, where 3 is the prevalent species and 5 is present in a very small amount that escapes any available analytical techniques. Nonetheless, there is no clear experimental evidence for the presence of 5 in solution at the moment.
The structure of the new anion 5 ( Figure 6) has been determined as four different salts, that is Na 2 (5) (5), displaying very similar geometries and bonding parameters. We may view 5 as the dimer of 3 and, thus, 3/5 represents a further example of polymerization (monomer/dimer) isomerism in carbonyl clusters. The dimeric structure of 5 is also unprecedented for Cu and Au. It is composed by an Ag 2 unit bonded to two terminal (Co t ) and two edge bridging (Co b ) Co(CO) 4 (5)) and is indicative of an argentophilic interaction as found in other Ag clusters supported by organometallic carbonyl fragments [45][46][47]. As expected, the Co t -Ag contacts the fact that Ag displays coordination number two in 3, and three (four considering also the Ag-Ag contact) in 5. Indeed, the Ag-Co contact [2.75 Å] in the mononuclear complex Co(CO) 4 {AgAs 3 (CH 3 ) 5 (C 6 H 4 ) 2 }, which contains an Ag center strongly bonded to three As atom (Ag coordination number 4), is even longer than in 5 [48].
(a) (b) The crystal packing of Na2(5)·C4H6O2 contains an interesting network of isocarb linkages involving the Na + ions (Figure 7). Indeed, each Na + is coordinated to O-atoms of four CO ligands of four different 5 anions. The overall coordination nu of each Na + ion is seven, being coordinated to the endo-cyclic O-atom of one cocry lized γ-butyrolactone C4H6O2, the exo-cyclic O-atom of the same C4H6O2 molecule as as the exo-cyclic O-atom of a second C4H6O2 molecule. In turn, each C4H6O2 molec terminally bonded to one Na + via the endo-cyclic O-atom and μ-bridging to Na + through the exo-cyclic O-atom. This results in (Na + )2 dimers ( in 3, and three (four considering also the Ag-Ag contact) in 5. Indeed, the Ag-Co contact [2.75 Å] in the mononuclear complex Co(CO)4{AgAs3(CH3)5(C6H4)2}, which contains an Ag center strongly bonded to three As atom (Ag coordination number 4), is even longer than in 5 [48].
(a) (b) The crystal packing of Na2(5)·C4H6O2 contains an interesting network of isocarbonyl linkages involving the Na + ions (Figure 7). Indeed, each Na + is coordinated to the O-atoms of four CO ligands of four different 5 anions. The overall coordination number of each Na + ion is seven, being coordinated to the endo-cyclic O-atom of one cocrystallized γ-butyrolactone C4H6O2, the exo-cyclic O-atom of the same C4H6O2 molecule as well as the exo-cyclic O-atom of a second C4H6O2 molecule. In turn, each C4H6O2 molecule is terminally bonded to one Na + via the endo-cyclic O-atom and μ-bridging to Na + ions through the exo-cyclic O-atom. This results in (Na + )2 dimers (Figure 7) bridged by two C4H6O2 molecules and two 5 anions (through two isocarbonyl linkages each), with four further 5 anions acting as terminal isocarbonyl ligands. The so formed {Na2(5)6(C4H6O2)2} 4− units are bonded via isocarbonyl linkages to thirty further Na + ions, resulting in a 3-D network ( Figure S20 in the Supporting Information). In the attempt to prepare neutral Au-Co(CO)4 species related to M4{Co(CO)4}4 ( Cu, Ag) and {CuCo(CO)4}∞ [36][37][38], the reactions of 1 with increasing amounts of A salts was investigated. By employing a 1:1 molar ratio, the IR spectra clearly indic that the only species present in solution was still the 2:1 adduct 4. Even increasing amount of Au(I) salt, the only species detected by IR spectroscopy was 4 accompanied decomposition to Au metal. During all these attempts, among the decomposition p ucts of the reaction, crystals of Na2[Au{Co3(CO)9}2][Au{Co2(CO)7}]·6H2O (Na2(7)(6)·6H were obtained. This salt contains the unprecedented anions [Au{Co2(CO)7}] − (6) [Au{Co3(CO)9}2] − (7). We may view 6 as being composed by an Au(III) center coordina to two [Co2(CO)7] 2− anions (Figure 8), and its structure is reminiscent of [Au{Fe2(CO [49]. The structure of the free anion [Co2(CO)7] 2− has not been reported in the literat but several of its adducts with main group and transition metals have been structur characterized [50][51][52]. In agreement with the +3 oxidation state, the Au center is perfe square planar.
Within the crystal of Na 2 (7)(6)·6H 2 O, each Na + cation is octahedrally coordinated to two O-atoms of two CO ligands one belonging to 6 and one to 7, and four H 2 O molecules. The two isocarbonyls are in relative cis position, and two cis H 2 O molecules act as bridging ligands toward a second (equivalent) Na + ion. This results in the formation of {Na 2 (6) 2 (7) 2 (H 2 O) 6    Within the crystal of Na2(7)(6)·6H2O, each Na + cation is octahedrally coordinate two O-atoms of two CO ligands one belonging to 6 and one to 7, and four H2O molecu The two isocarbonyls are in relative cis position, and two cis H2O molecules ac bridging ligands toward a second (equivalent) Na + ion. This results in the formatio {Na2(6)2(7)2(H2O)6} 2− units ( Figure 10) which are bonded via isocarbonyl linkages to further Na + ions resulting in a 2-D network. H-bonds involving H2O molecules and ligands are present in the crystals ( Figure S21 and Table S4 in the Supporting In mation).
Aiming at preparing Ag compounds related to 6 and 7, the reactions of 1 with creasing amounts of Ag(I) salts were investigated. Unfortunately, these resulted onl decomposition products, among which a few crystals of [ [NMe3(CH2Ph)]2(8) contains the octahedral anion 8 ( Figure S22 in the Suppor Information) which has been previously described [58].   Within the crystal of Na2(7)(6)·6H2O, each Na + cation is octahedrally coordinated to two O-atoms of two CO ligands one belonging to 6 and one to 7, and four H2O molecules. The two isocarbonyls are in relative cis position, and two cis H2O molecules act as bridging ligands toward a second (equivalent) Na + ion. This results in the formation of {Na2(6)2(7)2(H2O)6} 2− units ( Figure 10) which are bonded via isocarbonyl linkages to four further Na + ions resulting in a 2-D network. H-bonds involving H2O molecules and CO ligands are present in the crystals ( Figure S21 and Table S4 in the Supporting Information).

General Experimental Procedures
All reactions and sample manipulations were carried out using standard Schlenk techniques under nitrogen and in dried solvents. All the reagents were commercial products

General Experimental Procedures
All reactions and sample manipulations were carried out using standard Schlenk techniques under nitrogen and in dried solvents. All the reagents were commercial products (
C 44

Synthesis of [PPN][Au{Co(CO) 4 } 2 ] ([PPN](4))
A solution of Au(Et 2 S)Cl (0.099 g, 0.307 mmol) in THF (5 mL) was added to a solution of [PPN](1) (0.290 g, 0.409 mmol) in THF (10 mL) over a period of 1 h at room temperature under nitrogen atmosphere. Then, the mixture was filtered through a celite pad and the solvent removed under vacuum. The residue was dissolved in CH 2 Cl 2 (10 mL) and layered with n-pentane (20 mL) affording crystals of [PPN](4) suitable for X-ray analyses (yield 0.134 g, 61% based on Co, 41% based on Au). The crystals have been identified by comparison of the unit cell with that reported in the literature [42].
C 44

Synthesis of [NEt 4 ][Au{Co(CO) 4 } 2 ] ([NEt 4 ](4))
Au(Et 2 S)Cl (0.535 g, 1.66 mmol) was added as a solid to a solution of Na(1) (0.650 g, 3.35 mmol) in CH 3 OH (10 mL). The mixture was stirred at room temperature under inert atmosphere and the reaction was monitored by FT-IR spectroscopy. After 1 h the mixture was filtered through a celite pad and then the product was precipitated by adding a saturated solution of [NEt 4 ]Br in H 2 O (20 mL). The solid was collected by filtration, washed with H 2 O (40 mL) and extracted in CH 3 CN (15 mL). The yellow solution was evaporated to dryness at reduced pressure, dissolved in CH 2 Cl 2 (10 mL) and layered with n-pentane (20 mL) affording crystals of [NEt 4 ](4) suitable for X-ray analyses (yield 0.551 g, 49% based on Co, 50% based on Au Solid AgCl (0.932 g, 6.45 mmol) was added in small portions to a THF (10 mL) solution of Na(1) (0.250 g, 1.29 mmol). The mixture was stirred at room temperature under inert atmosphere and the reaction was monitored by FT-IR spectroscopy. After 36h the mixture was filtered through a celite pad and then, the pale-yellow solution was evaporated to dryness at reduced pressure. The residue was dissolved in CH 2 Cl 2 (10 mL). Suitable crystal for X-ray diffraction of Na 2 (5)·C 4  A solution of Au(Et 2 S)Cl (0.112 g, 0.348 mmol) in CH 2 Cl 2 (5 mL) was added to a solution of Na(1) (0.270 g, 1.39 mmol) in CH 2 Cl 2 (10 mL) over a period of 1 h. The mixture was stirred at room temperature under inert atmosphere. At the end of the reaction, the mixture was filtered, and the dark green dichloromethane solution was layered with npentane (30 mL). Black crystals of Na 2 (7)(6)·6H 2 O were obtained from the CH 2 Cl 2 /pentane double layer as a decomposition product of the reaction(yield 0.062 g, 22% based on Co, 9% based on Au).  (8)) AgNO 3 (1.07 g, 6.36 mmol) was added as a solid to a solution of Na(1) (0.650 g, 3.35 mmol) in CH 3 OH (10 mL). The mixture was stirred at room temperature under inert atmosphere and the reaction was monitored by FT-IR spectroscopy. After 1 h the mixture was filtered through a celite pad and then the product was precipitated by adding a saturated solution of [NMe 3 (CH 2 Ph)]Cl in H 2 O (20 mL). The solid was collected by filtration, washed with H 2 O (40 mL) and extracted in CH 3 CN (15 mL). The yellow solution was evaporated to dryness at reduced pressure, dissolved in CH 2 Cl 2 (10 mL) and layered with n-pentane (20 mL). A few crystals of [NMe 3 (CH 2 Ph)] 2 (8) were isolated from the CH 2 Cl 2 /pentane double layer as a decomposition product of the reaction. These were analyzed by SC-XRD but, owing the very limited amount, no further analysis was carried out. AgBF 4 (0.231 g,1.18 mmol) was added as a solid, in small portions, to a solution of [PPN](1) (0.280 g, 0.395 mmol) in THF (10 mL).The mixture was stirred at room temperature under nitrogen for 2h. Then, the mixture was filtered through celite and the celite pad washed with THF (5 mL). The solution was evaporated to dryness under reduced pressure and the residue dissolved in CH 2 Cl 2 (10 mL). A few crystals of [PPN] 2 [Co(THF) 4 (BF 4 ) 2 ][BF 4 ] 2 ·4CH 2 Cl 2 were isolated from the CH 2 Cl 2 /pentane double layer as a decomposition product of the reaction. These were analyzed by SC-XRD but, owing the very limited amount, no further analysis was carried out.  Table S5 in the Supporting Information. The diffraction experiments were carried out on a Bruker APEX II diffractometer equipped with a PHO-TON2 detector using Mo-Kα radiation. Data were corrected for Lorentz polarization and absorption effects (empirical absorption correction SADABS) [63]. Structures were solved by direct methods and refined by full-matrix least-squares based on all data using F 2 [64]. Hydrogen atoms were fixed at calculated positions and refined by a riding model. All non-hydrogen atoms were refined with anisotropic displacement parameters.
Geometry optimizations were performed using LANL2DZ basis set with pseudpotential for transition metals [69], whereas 6-31G(d,p) basis set was used for all other atoms [70], confirming the character of the stationary points by vibrational analysis. IR frequencies have been computed analytically, as implemented in Gaussian 16, and rescaled using a 0.961 scaling factor [71].
For the thermodynamics of complexes' equilibria, the reported Gibbs free energies have been calculated using larger-basis-set (i.e., 6-311+G(2d,2p) for all atoms but transition metals) single-point computations and including Gibbs free energy corrections (at 298.15 K) and Grimme-D3 corrections for dispersions [72] and using the conductor like polarizable continuum model (C-PCM) [73,74] for solvation effects.

Conclusions
The M-Co(CO) 4 (M = Cu, Ag, Au) system has been reinvestigated unraveling a new example of polymerization isomerism in metal carbonyl clusters. Thus, depending on the crystallization conditions, the monomer 3 or the dimer 5 have been isolated in the solid state in the case of Ag. Conversely, only the monomers 2 and 4 have been obtained for Cu and Au, respectively. This difference relies on thermodynamic effects, as pointed out by DFT calculations. Several other examples of isomerism in metal carbonyl clusters, molecular clusters and nanoclusters have been described in the literature as summarized in the introduction. The scope of this field is rapidly expanding and gives new insights into isomerism, which for a longtime has mainly been discussed within the framework of organic and coordination chemistry.