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

The bonding nature of metallocene acetylene complexes Cp 2 M(η 2 -H 3 SiC 2 SiH 3 ) 1M and Cp 2 M(η 2 -HC 2 H) 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 Cp 2 M fragment and the H 3 SiC 2 SiH 3 (also HC 2 H) 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.