Application of the E-C approach to understanding the bond energies thermodynamics of late-metal amido, aryloxo and alkoxo complexes: an alternative to p pi/d pi repulsion

The concept of p pi/d pi repulsion in alkoxo and amido complexes of the lat e transition metals has been critically examined, especially as it relates to recent work by the authors on a series of nickel complexes, The thermody namic preferences of late-metal M-X bonds (X = NR2, OR) can be explained by an alternative model which uses bond polarity as a starting point and E-C theory for quantification, In this model, the greater electrostatic charact er of the M-X bond (relative to the H-X bond) is sufficient to explain obse rved thermodynamic trends in ligand exchange reactions, without the inclusi on of pi-effects. The implications of adopting the polar bonding model have been examined, including discussion of metal alkyl complexes as well as ox o and imido complexes. In general, this polar model appears to lead to grea ter consistency with observation and deeper understanding, although more da ta are necessary to clearly determine the best way of describing bonds betw een late metals and X ligands.