Cobalt−Cobalt Multiple Bonds in Homoleptic Carbonyls? Co₂(CO)_x (x = 5−8) Structures, Energetics, and Vibrational Spectra

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Abstract

Homoleptic binary cobalt carbonyls with multiple cobalt−cobalt bonds have been examined theoretically using established levels of density functional methodology. These species include 19 structures ranging from the experimentally well characterized dibridged (CO)3Co(CO)2Co(CO)3 to the proposed monobridged (CO)2Co(CO)Co(CO)2 structure with a formal quadruple bond. Consistent with experiment, three energetically low-lying equilibrium structures of Co2(CO)8 were found, of C2v (dibridged), D3d (unbridged), and D2d (unbridged) symmetry. For Co2(CO)8, the BP86 method predicts the dibridged structure to lie 3.7 kcal/mol below the D2d structure and 6.3 kcal/mol below the D3d structure. The D2d and D3d structures thus have the opposite energetic ordering of that deduced from experiment by Sweany and Brown. A satisfactory harmony between theoretical and experimental vibrational frequencies and IR intensities is found, although the D2d and D3d structures are essentially indistinguishable in this regard. For Co2(CO)7 the unbridged asymmetric structure suggested by Sweany and Brown is confirmed with the BP86 method, and with perhaps one exception the vibrational features agree well for theory and experiment. For Co2(CO)6 only one vibrational feature has been assigned from experiment, but this band (2011 cm-1) fits very well with BP86 predictions for the monobridged D2d symmetry structure with a formal Co⋮Co triple bond. For the Co2(CO)5 molecule, for which no experimental results exist, the most interesting structure is the monobridged closed-shell singlet with a very short (2.17 Å) cobalt−cobalt bond, to which we assign a formal bond order of four. Potential energy distributions have been analyzed to identify the principal vibrations with cobalt−cobalt stretching contributions. The condensed phase Raman analysis by Onaka and Shriver of the Co−Co stretches for the three known isomers of Co2(CO)8 is remarkably consistent with the present predictions for the gas-phase species. Prospects for the synthesis of these and related dicobalt compounds are discussed.

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