Structure and bonding in the gaseous dihalides and dialkyl derivatives of the Group 2 and Group 12 metals

Abstract

This chapter opens with a discussion of the difficulties of establishing unequivocally whether a monomeric, gaseous Group 2 metal dihalide has a linear or angular equilibrium structure. The accumulated experimental evidence shows that most of these triatomic molecules are linear, but gaseous CaF2, SrF2, SrCl2, and all the barium dihalides are angular. These angular structures are not in accord with the Valence Shell Electron Pair Repulsion (VSEPR) model, but may be rationalized in terms of the polarizable ion model or in terms of sd hybridization on the metal atom. Bond distances and mean bond energies are listed. The bonding may be described in terms of two delocalized (i.e., three-center) or two localized (i.e., two-center) bonding molecular orbitals. The gaseous dihalides of the Group 12 metal atoms are all linear. The bonding radius of Hg is found to be two or three pm smaller than that of Cd, an anomaly which may be due to a combination of the f-block contraction and relativistic effects.