Abstract
Theoretical studies on the dimetallocenes Cp2M2 (M = Os, Re, W, Ta) predict bent structures with short metal-metal distances suggesting high-order metal-metal multiple bonds. Analysis of the frontier bonding molecular orbitals indicates a formal Os-Os quintuple bond (σ + 2π + 2δ) in singlet Cp2Os2 and a formal Re-Re sextuple bond (2σ + 2π + 2δ) in singlet Cp2Re2, thereby giving the metals in both molecules the favored 18-electron metal configurations. Predicted low-energy triplet structures for Cp2M2 (M = Os, Re) have formal quintuple bonds but with only two δ one-electron "half" bonds (M = Os) or a single δ two-electron bond (M = Re) and a second σ component derived from overlap of the d(z(2)) orbitals. A quintuple bond similar to that found in triplet Cp2Re2 is found in singlet Cp2W2, giving both tungsten atoms a 16-electron configuration. The formal Ta-Ta quadruple bond in the lowest energy singlet Cp2Ta2 structure is different from that in the original Re2Cl8(2-) in that it is a 2σ + 2π bond with no δ components but only σ and π components.
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