Metal−Metal Quintuple and Sextuple Bonding in Bent Dimetallocenes of the Third Row Transition Metals

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(z²) 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 Re₂Cl₈²⁻ in that it is a 2σ + 2π bond with no δ components but only σ and π components.