The reaction of Na­[Co­(CO)4] with M­(IPr)Cl (M = Cu, Ag, and Au; IPr = C3N2H2(C6H3 iPr2)2) affords the neutral heterometallic complexes [Co­(CO)4{M­(IPr)}] (M = Cu, 1; Ag, 2; and Au, 3). Formation of 2 is accompanied by traces of [Ag­(IPr)2]­[Ag­{Co­(CO)4}2] (4). The reaction of Na­[Co­(CO)4] with M­(IMes)Cl (IMes = C3N2H2(C6H2Me3)2) results in mixtures of [Co­(CO)4{M­(IMes)}] (M = Cu, 5; Ag, 6; and Au, 7) and [M­(IMes)2]­[M­{Co­(CO)4}2] (M = Cu, 8; Ag, 9; and Au, 10). In the cases of Cu and Ag, ionic complexes 8 and 9 are the major products, whereas neutral species 7 is the major product for Au. All species 1–10 have been spectroscopically characterized by IR and 1H and 13C­{1H} NMR spectroscopy. Moreover, the molecular structures of 2, 3, and 8 have been determined by single-crystal X-ray diffraction (SC-XRD). Bimetallic Co–M–NHC complexes 1–3 and 7–9 have been tested as catalysts for the dehydrogenation of ammonia–borane (AB) in THF as solvent, and their performances compared to [Fe­(CO)4{M­(NHC)}2], M­(NHC)­Cl, and Na­[Co­(CO)4]. DFT computations have been performed to provide information on the structure, IR spectroscopy, and the thermodynamics of Co–M carbonyl clusters.

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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.

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