Ab initio study of the change from η⁵- to η¹-coordination in group 12 dimetallocenes MM′(C₅H₅)₂ with M, M′ = Zn, Cd, Hg

For group 12 dimetallocenes, ab initio plane wave density functional theory (DFT) quantum electronic structure calculations predict that the metal atom can move from center η⁵-C₅H₅ to edge η¹-C₅H₅ coordination with increase in atomic number. Systems studied include homo- and hetero-dimetallocene systems M₂Cp₂ and ZnMCp₂ (where Cp = C₅H₅) in staggered and eclipsed initial configurations, and M is a group 12 atom Zn, Cd or Hg. The electronic change that drives the geometrical change is explored in calculations of the total charge density, partial charge densities of Kohn-Sham levels and the electron localization function (ELF). The zinc atoms prefer central η⁵-C₅H₅ coordination, cadmium an off center position and mercury the edge displaced η¹-C₅H₅ coordination. In this latter configuration the nearest carbon atom shows signs of sp³-hybridization and the other four carbons adopt a cis-butadiene-like structure. In the hetero-systems we find distinct geometries with each M-Cp unit adopting the coordination of the homo-dimetallocene.