| Abstract
| - The role of the lone pair of electrons of Pb(II) indetermining the coordination geometry is analyzed fromcrystallographic studies and ab initio molecular orbital optimizations.Of particular interest are factors that contributeto the disposition of ligands around the lead with geometries that are(1) holodirected, in which the bonds toligand atoms are distributed throughout the surface of an encompassingglobe, and (2) hemidirected, in whichthebonds to ligand atoms are directed throughout only part of anencompassing globe, i.e., there is an identifiablevoid in the distribution of bonds to the ligands. The preferredcoordination numbers for lead were found to be4 for Pb(IV) and 4 and 6 for Pb(II). AllPb(IV) structures in the CSD have a holodirected coordinationgeometry.Pb(II) compounds are hemidirected for low coordinationnumbers (2−5) and holodirected for high coordinationnumbers (9, 10), but for intermediate coordination numbers (6−8),examples of either type of stereochemistryare found. Ab initio molecular orbital studies of gas-phasePb(II) complexes show that a hemidirected geometryis favored if the ligand coordination number is low, the ligands arehard, and there are attractive interactionsbetween the ligands. In such complexes, the lone pair orbital hasp character and fewer electrons are transferredfrom the ligands to the bonding orbitals of Pb(II), resulting inbonds that are more ionic. A holodirected geometryis favored when the coordination number is high and the ligands aresoft and bulky or show strong interligandrepulsion. The lone pair orbital has little or no p character whenthe geometry is holodirected, and the bonds aremore covalent than in the hemidirected structures. The energy costof converting a hemidirected to a constrainedholodirected structure is of the order 8−12 kcal/mol in the absenceof strong interligand interactions.
- The role of the lone pair of electrons ofPb(II) in determining the coordination geometry has been analyzedfrom crystallographic studies and ab initio molecular orbitaloptimizations. Factors are discussed that contribute to thedisposition of ligands around the lead with geometries that areholodirected, wherein the bonds to ligand atoms aredistributed throughout the surface of an encompassing globe, andhemidirected, wherein the bonds to ligand atoms are directedthroughout only part of an encompassingglobe.
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