A Correlation between Bond Length and Ionic Bond Order. Application to the Ylide–Ylene Problem

Abstract

The C—O and C—S bond lengths of the cations, radicals, and anions CH 3 O, CH 3 S, CH 2 OH, and CH 2 SH have been found not to correlate with the overlap populations of the C—X bonds. On the other hand, very satisfactory linear relations are observed with the ionic bond orders of the C—X bonds. It is suggested that, in certain molecular systems, it may be more meaningful to associate shortening of a bond A—B with greater coulombic attraction (or smaller coulombic repulsion) between the two point charges represented by the net atomic charges on the atoms A and B than with an increase in the overlap population between these atoms. It is noted that such an interpretation can account for the short C—P bond in a phosphonium ylide without resort to (p → d) π conjugation.