Pendent group effects, PGEs, in P-donor ligandsElectronic supplementary information (ESI) available: Tables of data used in analyses. See DOI: 10.1039/c3nj00263b

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) 3 L complexes coupled with p K 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. Properties of P-donor ligands are dependent on the pendent groups attached to the phosphorus atom and can be expressed in terms of the equation shown where the term highlighted in red is quite common but has no current theoretical explanation.