ABINITIO STUDY OF STRUCTURES AND STABILITIES OF SUBSTITUTED LEAD COMPOUNDS - WHY IS INORGANIC LEAD CHEMISTRY DOMINATED BY PB(II) BUT ORGANOLEAD CHEMISTRY BY PB(IV)

The influence of electronegative substituents on the structures and relative stabilities of lead(IV) versus lead(II) species has been assessed by ab initio pseudopotential computations for a series of halogenated lead hydrides and methyllead compounds RnPbX4-n, (R = H, CH3; X = F, Cl; n = 0-4) and RnPbX2-n (n = 0-2). The calculated energies of various model reactions reveal the drastic degree of destabilization of tetravalent lead compounds by electronegative substituents X (X = F, Cl). The bond angles in compounds with different groups, RnPbX4-n (n = 1-3), deviate widely from 109.5-degrees. In agreement with Bent's rule, the angles between the most electronegative substituents (F or Cl) are smaller, while those between more electropositive groups (H, CH3, SiH3) are considerably larger than tetrahedral. The deformations computed for R2PbX2 and R3PbX are related to those observed experimentally in various organolead and organotin structures. The destabilization of the tetravalent species by relativistic effects is larger when electronegative substituents are present. There also are large relativistic effects on bond angles (up to ca. 14-degrees). All Pb-R and Pb-X bonds shorten upon successive substitution by electronegative groups; i.e., the weakening of the bonds is accompanied by a decrease of their lengths! A simple bonding model is proposed to explain both the thermodynamic and the structural observations: The increase of the positive metal charge upon halogen substitution results in greater contraction of the 6s-orbitals than the 6p-orbitals. Hence, the 6p-orbitals are less effective in sp(n) hybridization, and electronegatively substituted Pb(IV) compounds are destabilized. While the traditional term ''inert pair effect'' implies the energetic unavailability of a 6s-pair of electrons for bonding, the proposed concept emphasizes the size differences between s- and p-orbitals. This supports Kutzelnigg's analysis of the bonding in compounds of the heavy main group elements. A novel type of hyperconjugation, ''geminal sigmaPbH --> sigma*(PbH, PbF) hyperconjugation'' also influences the relative stabilities of substituted lead(IV) compounds.