Three Co(III) complexes of the type [Co(chel)(PBu3)]ClO4.H2O, (chel = 5-BrSalen, 5-MeOSalen and 4-MeOSalen), were synthesised and characterised by elemental analysis, IR, UV-Vis and 1H NMR spectroscopy. In their electronic spectra, the absorptions between 550 and 750 nm of these complexes are attributable to the lowest d–d transition. The axial ligands affect this transition through a σ-intraction with the dz2 orbital and the equatorial ligands affect it by π-interaction with populated d-orbitals. On the basis of an electronic structural model, in which the dz2 orbital is populated in forming cobalt(II), it is suggested that equatorial ligand substitution affects the reduction potentials less than axial ligand substitution.
