Pendent group effects, PGEs, in P-donor ligands

Abstract

Pendent groups or atoms attached to phosphorus atoms control the latters' behaviour in a variety of ways. Empirical quantitative determination of the extent of back-donation from a variety of metal centres to P-donor ligands is remarkably simple and can be accomplished quickly by comparison with readily available C–O stretching frequencies in Ni(CO)₃L complexes coupled with pK_a data that have been available for many years. However, these estimates are not the whole story and a fourth effect, in addition to σ-donation, π back-donation, and steric repulsions, can make a major contribution to metal–P bonding. This effect should be taken into account before any theoretical analysis of metal–P bonding is attempted; otherwise a profound disconnect between empirical evidence and theoretical descriptions of the bonding (i.e. between fact and “fiction”?) will hinder progress towards a better understanding of the behaviour of a wide variety of organometallic complexes.