The shapes of bis(cyclopentadienyl) complexes of the s-block metals

The structures and bonding of the bis(cyclopentadienyl) complexes of the alkaline-earth metals and the alkali metals have been studied using density functional calculations within the linear combination of Gaussian-type orbitals framework. The compounds Ca(cp)(2) and Sr(cp)(2) are predicted to have bent equilibrium structures in agreement with the experimentally determined shapes but in contrast to previous theoretical investigations; Mg(cp)(2) and the isoelectronic [M(cp)(2)](-) (M = Li, Na, K or Rb) ions are predicted to have coparallel, staggered rings. Analysis of the bonding shows that x-type interactions dominate for the complexes of the heavier metals of both groups as a result of the increased influence of the metal d functions. Their role is key to the driving force for the bent geometries of Ca(cp), and Sr(cp),just as it is for the-anomalous shapes of some of the dihalide molecules. The key orbital interaction, however, is quite different.