Role of Cation Polarization in holo- and hemi-Directed [Pb(H2O)n](2+) Complexes and Development of a Pb(2+) Polarizable Force Field.

Reduced Variational Space (RVS) calculations are reported that afford insight into the energetic origins of the hemi- and holo-directing behavior of [Pb(H2O)n](2+) complexes. It is shown that the distribution of ligands around the Pb(2+) center arises from a delicate balance between the first-order Coulomb plus exchange-repulsion energy that favors holo-directionality, and the second-order charge transfer plus polarization term that favors hemi-directionality. It is additionally demonstrated that the pseudopotential/basis set combination used to study such complexes should be carefully selected, as artifacts can arise when using large-core pseudopotentials. Finally, based on these findings, we introduce a new SIBFA force field parametrization for Pb(2+). Results yield close agreement with ab initio complexation energies in a series of [Pb(H2O)n](2+) complexes and successfully encapsulate the hemi- and holo-directing properties. SIBFA thus appears to be the first classical force field to be able to model the holo-/hemi-directed transition within Pb complexes, avoiding the need for explicit wave function treatment and consequently providing the opportunity to deal with large leaded systems of biological interest.