Theory of the Spin Hamiltonian Parameters for Low Spin Cobalt(II) Complexes

The theory for the spin Hamiltonian parameters of low spin cobalt(II), d7, complexes has been developed to third order in perturbation theory for all reasonable ground states. Two extensions beyond previous treatments have been made: (1) the spin–orbit mixing of excited quartet states into the doublet ground state has been considered and (2) configuration interaction involving mixing of [Formula: see text]orbitals in lower symmetry has been included. In the case of axial complexes a more complete calculation has been made to assess the errors incurred in only going to third order in perturbation theory. The theory has been applied to porphyrin and phthalocyanine complexes and is shown to adequately account for the observed data on these complexes. The excited quartet states are much higher in energy than the ground state for the four coordinate porphyrin complexes but become much lower in energy when basic adducts are complexed along the z axis. Further there is mixing of the 4s orbital with the [Formula: see text]orbital for the four coordinate complexes which is completely removed when basic adducts are complexed to the porphyrin system. Application of equations to lower symmetry complexes is considered and it is shown that is is difficult to make an unambiguous assignment of ground state on the basis of g and 59Co hyperfine splitting values alone, although the relative values of gy and gz do seem to provide some clue as to the correct ground state.