Metal−Metal Quintuple and Sextuple Bonding in Bent Dimetallocenes of the Third Row Transition Metals
journal contributionposted on 2010-03-09, 00:00 authored by Bing Xu, Qian-Shu Li, Yaoming Xie, R. Bruce King, Henry F. Schaefer
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.