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

The concept of p pi/d pi repulsion in alkoxo and amido complexes of the late transition metals has been critically examined, especially as it relates to recent work by the authors on a series of nickel complexes, The thermodynamic 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 character of the M-X bond (relative to the H-X bond) is sufficient to explain observed thermodynamic trends in ligand exchange reactions, without the inclusion of pi-effects. The implications of adopting the polar bonding model have been examined, including discussion of metal alkyl complexes as well as oxo and imido complexes. In general, this polar model appears to lead to greater consistency with observation and deeper understanding, although more data are necessary to clearly determine the best way of describing bonds between late metals and X ligands.