Are Metallocene−Acetylene (M = Ti, Zr, Hf) Complexes Aromatic Metallacyclopropenes?

Abstract

The bonding nature of metallocene acetylene complexes Cp2M(η2-H3SiC2SiH3) 1M and Cp2M(η2-HC2H) 1M′ (M = Ti, Zr, Hf) was studied by density functional theory method. It is found that this acetylene complex has indeed a metallacyclopropene moiety with two in-plane M−C σ-bonds and one out-of-plane π-bond interacting with the metal center, resulting in the formation of a delocalized three-center and two-electron (3c-2e) system. Along with its delocalized out-of-plane bonding, this complex has been characterized as aromatic on the basis of the computed stabilizing energy and negative nucleus-independent chemical shifts (NICS). The aromatic stabilization increases from Ti to Zr and Hf, and this is because of the increased charge separation between the Cp2M fragment and the H3SiC2SiH3 (also HC2H) unit. The decrease of the M−C bond length from Zr to Hf is attributed to the increased s character of both M and C hybridization of the M−C σ-bonds.