On the origin of the hemidirected geometry of tetracoordinated lead(II) compounds

In order to explain the difference between high-symmetric holodirected and less-symmetric hemidirected Pb(II) compounds, the structures of [PbF4]2−, [PbCl4]2− and [Pb(OH)4]2− have been optimized without any symmetry restrictions and/or within various symmetry groups using all-electron relativistic B3LYP treatment and alternatively using B3LYP and MP2 treatments with effective core potential for Pb atoms. Optimal geometries of both [PbF4]2− and [PbCl4]2− are of Td symmetry (holodirected structures). The [Pb(OH)4]2− optimal geometry of C2 symmetry cannot be explained by pseudo-Jahn-Teller effect due to energy and symmetry reasons. The less-symmetric geometry of tetracoordinated Pb(II) complexes with non-halogen ligands may be explained by the mutual interactions of their less-symmetric ligands. This symmetry decrease is subsequently supported by the increasing p electrons content in the Pb(II) lone electron pair via influencing the hybridization scheme of the Pb-ligands bond orbitals (i.e. not due to the repulsion between the ligands and the Pb(II) lone electron pair).