Metal Ligand Bond Strengths and Bonding Theory

Abstract

Metal-ligand mean bond dissociation energies have been calculated in those transition-metal complexes for which we have values of ΔH°_f(complex, cryst.) and a measured or reliably estimated value for AH(sublimation). The complexes are of two general types, ML_nX₂, where L is a ligand molecule and X is a halogen, and ML_n, where L is a ligand radical, such as the pentanedionato group. For the octahedral complexes, ML₄C1₂, the values of the gas-phase mean bond dissociation energies, $\bar D$ (M-L), follow the trend Mn<Fe<Co<Ni>Cu>Zn, with a maximum value at Ni. For the tetrahedral complexes of Group II metal, ML₂X₂, the trend of $\bar D$ values is Zn>Cd>Hg, whilst for the square planar pentanedionato complexes, ML_2, the order $\bar D$(M-L) values is Mn>Co>Ni>Cu>Zn. A simple molecular orbital description (the Angular Overlap Model) of the complexes is consistent with, but not predictive of these trends in $\bar D$(M-L) values. A comment is made on the variation of $\bar D$(Co-pyridine) values, with substitution on the pyridine ring, in terms of the bonding theory.