The contribution to bonding by lone pairs

The terms lone pair and non-bonding pair are not generally equivalent to each other. Regardless of the particular basin decomposition of the molecular space, orbitals of any type cannot be completely localized and will exhibit a presence in bonding regions to varying degrees. Using the degree of polarization (ionicity) of orbitals in the localization procedure of Cioslowski [J. Math. Chem. 8 (1991) 169; Int. J. Quant. Chem. Quant. Chem. Symp. 24 (1990) 15] at the B3LYP/6311 +G(d, p) level we can characterize orbitals as core, lone pair, or conventional bonding orbitals. This allows us to investigate the relative contributions of the bonding and lone pair orbitals to the overall covalent bond order involving atoms described chemically as possessing lone pair electrons. We find that even in what would be deemed chemically conventional systems lone pairs can contribute approximately 10-15% to the covalent bond order. In the classic back bonding PO bond of the phosphine oxides it is some 44% of the total covalent bond order. A similar presence exists in the analogous nitrogen oxides and is sufficiently large in the NO bonds in (HO)(3)NO and F3NO to characterize them as triple. (C) 2003 Elsevier Science B.V. All rights reserved.