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

Abstract

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 π-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) ₂ and Sr(cp) ₂ just as it is for the anomalous shapes of some of the dihalide molecules. The key orbital interaction, however, is quite different.