A density functional investigation of the Bursten ligand additivity model for the d⁶ octahedral complexes [Mn(CO)_n(CNMe)_(6-n)]⁺ (M = Mn(I), Cr(0))

The Bursten ligand additivity model provides a description of orbital energies of complexes of the type [M(L)_n(L′)_6-n]^x+, where L and L′ are ligands of different π-acceptor abilities. The model allows for the prediction molecular orbital energies, oxidation potentials and valence photoelectron spectra of octahedral complexes. The model was derived by Bursten from the results of Fenske-Hall approximate molecular orbital calculations. In this study non-local density functional calculations are used to determine optimized geometries, molecular orbital energies and charge distributions for the complexes [M(CO)_n(CNMe)_(6-n)]⁺ (M = Mn(I), Cr(0)). The calculated DFT molecular orbital energies are used to derive ligand additivity relationships. Variations in C-O and M-C bond lengths and charge distributions are used to study the effect of ligand substitution on the degree of π-back-bonding in each complex. It is found that the results of DFT calculations show excellent agreement with the Bursten ligand additivity model, despite the considerable difference in computational methods and structural changes that occur on ligand substitution.