Effect of Polar States on Superexchange Interaction. Application to Some Dimeric Copper (II) Complexes and to CuCl

The effect of polar states on the superexchange interaction between two identical spin 1/2 magnetic ions is examined. The one-body potential contributions to the effective exchange constant Jeff are written in terms of the splitting 2AQ between the bonding and antibonding single particle lower-states of the dimeric complex. Moreover, the energies of the basis triplet and singlet states are obtained in an approximated form as functions of Ao and of two-body molecular integrals. The resulting Jeff> that is the splitting between the lowest singlet and the triplet, differs from that of the pure Heitler-London treatment owing to the ionic contributions to the singlet wavefunction. The theory is applied to some dimeric copper (II) complexes and to CuCl2 • 2H2O linear chain. It is shown that the polar state corrections are generally small, but that they may become important in the case of copper chlorine, where the various contributions to the Heitler-London exchange constant tend to balance each other to give a very small value. Moreover, antiferromagnetic coupling for CuCl2 • 2H2O is accounted for on the basis of a cx-ground state for the single unit, rather than a n one. The conditions to fit the experimental value of Jeff are also discussed.