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.