Lone pair functionality in divalent lead compounds

The role of the lone pair of electrons of Pb(II) in determining the coordination geometry is analyzed from crystallographic studies and ab initio molecular orbital optimizations. Of particular interest are factors that contribute to the disposition of ligands around the lead with geometries that are (1) holodirected, in which the bonds to ligand atoms are distributed throughout the surface of an encompassing globe, and (2) hemidirected, in which the bonds to ligand atoms are directed throughout only part of an encompassing globe, i.e., there is an identifiable void in the distribution of bonds to the ligands. The preferred coordination numbers for lead were found to be 4 for Pb(IV) and 4 and 6 for Pb(II). All Pb(IV) structures in the CSD have a holodirected coordination geometry. Pb(II) compounds are hemidirected for low coordination numbers (2-5) and holodirected for high coordination numbers (9, 10), but for intermediate coordination numbers (6-8), examples of either type of stereochemistry are found. Ab initio molecular orbital studies of gas-phase Pb(II) complexes show that a hemidirected geometry is favored if the ligand coordination number is low, the ligands are hard, and there are attractive interactions between the ligands. In such complexes, the lone pair orbital has p character and fewer electrons are transferred from the ligands to the bonding orbitals of Pb(II), resulting in bonds that are more ionic. A holodirected geometry is favored when the coordination number is high and the ligands are soft and bulky or show strong interligand repulsion. The lone pair orbital has little or no p character when the geometry is holodirected, and the bonds are more covalent than in the hemidirected structures. The energy cost of converting a hemidirected to a constrained holodirected structure is of the order 8-12 kcal/mol in the absence of strong interligand interactions.