Refining the VSEPR Model: Limits on the Lone Pair Size as Described by the Electron Localization Function.

To investigate exceptions to the VSEPR principle that species with asymmetric lone pairs have bond angles smaller than those of their idealized molecular shapes, we computationally optimized the structures of a wide variety of small molecules and polyatomic anions, including as many experimentally characterized structures as possible, and applied a topological analysis of the electron localization function (ELF) as well as Natural Bond Orbital analysis (NBO). Species were chosen to represent each valence shell electron pair repulsion (VSEPR) shape with asymmetric lone pairs on the central atom. Results confirmed well-established precedents, including ligand close packing and Bent's rule, clarifying our understanding of lone pair size in terms of volume per electron around the central atom: (a) lone pairs exclude more volume around the surface of a main-group central atom than a single covalent bond from the central atom to a substituent of similar size; (b) lone pairs exclude more volume around central atoms in lower oxidation states than in higher oxidation states; and (c) substituents larger than the central atom, multiply bonded to the central atom, and/or less electronegative than the central atom compete more effectively with lone pairs to expand bond angles and limit steric demands of lone pairs.