The Bursten model of ligand additivity applied to orbital energetics of octahedral d⁶ phosphine complexes

Density functional theoretical calculations are used to determine structures and molecular orbital energies in the series of complexes [M(CO)_n(PX₃)_6−n]^x+ where M = Mn(I) and Cr(0) and X = H, F, Cl, Me, OMe and Ph. The resultant properties are analyzed in terms of additive behavior and studied within the context of the Bursten model of ligand additivity. It is shown that additive behavior previously described for highly symmetric ligands such as CO and CNCH₃ is also displayed in these phosphine complexes. Derivation of Bursten model equations from the calculated orbital energies leads to interesting insights into the nature of donor and acceptor ligand behavior of the selected phosphines in complexes of different metal oxidation states.