Abstract
The nature of the carbon–phosphorus bond in trivalent and pentavalent carbon phosphorus compounds is currently a matter of some dispute. These compounds contain unusual bonds, unusual in the sense that the behavior of electrons in the compounds does not conform to that expected solely on the basis of carbon–carbon and carbon–hydrogen bonds. Quantitative measures of how a single electron, as a wave, is shared between any two spatial points are given by means of the sharing amplitude and the sharing index.1 These quantities are orbital independent, rooted in the single particle density matrix, and do not depend on arbitrary localization procedures. The quantitative characterization of the PC bonds in molecules such as PHCH~2~, PH~3~CH~2~, PF~3~CH~2~, and PH~3~CHF was carried out by means of the sharing indices. On the basis of interbasin sharing indices, the PC bonds are: (mostly) double in PHCH~2~, and single in PH~3~CH~2~, PF~3~CH~2~, and PH~3~CHF. On the basis of the group basin charges and intergroup sharing indices between the CH~2~ (or CHF) groups and the PH~1,3~ (or PF~3~) groups, the molecules are ionic with double bonds between the groups (in line with an ylene form). The sharing indices between atoms, which are not directly linked (secondary sharing indices), indicate that the electrons are quite delocalized over the basins of the following groups: CPH in PHCH~2~, CPH~3~ in PH~3~CH~2~, CPF~3~ in PF~3~CH~2~, and CPH~3~ in PH~3~CHF. The sharing indices and the sharing amplitude offer the needed tools to put our understanding of the chemistry of a large number of compounds on a rigorous basis by elucidating the behavior of a single electron in a many electron system. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 1387–1395, 2001