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) 3 L 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 s-donation, p 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.

π Acceptor Abilities of Anionic Ligands: Comparisons Involving Anionic Ligands Incorporated into Linear d10 [(NH3)Pd(A)]−, Square Planar d8 [(NN2)Ru(A)]−, and Octahedral d6 [(AsN4)Tc(A)]− Complexes

“…The effects of π backbonding appear in aspects of transition-metal coordination chemistry ranging from evaluation of orbital gaps and the spectrochemical series to the thermodynamic trans influence and the kinetic trans effect. − A textbook example involves the trend in M­(CO) 6 q complexes where the extent of backbonding correlates with the carbon–oxygen vibrational stretching frequencies. Experimentally, evaluating relative amounts of π acceptor capacity for different ligand types has proved challenging, mostly because it is difficult to separate σ-bonding and π-bonding effects. − Carbonyl carbon–oxygen vibrational stretching frequencies have been used to assess the π backbonding capacities of other ligands bound to the same metal, but care must be taken in selecting ligands, metals, and geometries to obtain trustworthy results. − Lichtenberger and coworkers argued that gas-phase photoelectron spectroscopy is the only experimental technique that can quantitatively separate ligand π-bonding effects from σ-bonding and charge potential effects. , Using this technique, they found that CCH – acts as nearly a pure donor toward d 6 CpM­(CO) 2 (M = Fe and Ru) fragments rather than as an acceptor and that SiCl 3 – acts as a modest acceptor in these systems. Recently, Roithová, Rasika Dias, and coworkers used gas-phase bond dissociation to show that alkynes bind coinage metal complexes more strongly than alkenes do, a result they trace to greater backbonding in the former …”