Are metallocene-acetylene (M = Ti, Zr, Hf) complexes aromatic metallacyclopropenes ?

The bonding nature of metallocene acetylene complexes Cp<SUB>2</SUB>M(η<SUP>2</SUP>-H<SUB>3</SUB>SiC<SUB>2</SUB>SiH<SUB>3</SUB>) 1M and Cp<SUB>2</SUB>M(η<SUP>2</SUP>−HC<SUB>2</SUB>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<SUB>2</SUB>M fragment and the H<SUB>3</SUB>SiC<SUB>2</SUB>SiH<SUB>3</SUB> (also HC<SUB>2</SUB>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