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 (or + 2 pi + 2 delta) in singlet Cp2Os2 and a formal Re-Re sextuple bond (2 sigma + 2 pi + 2 delta) 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 delta one-electron half bonds (M = Os) or a single delta two-electron bond (M = Re) and a second sigma 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 Re2Cl82- in that it is a 2 sigma + 2 pi bond with no delta components but only sigma and pi components.