Spin Changes in Iron Complexes

Abstract

As discussed in Chapter 2, the normal ground state of a free ion is the one of maximum multiplicity (Hund’s First Rule). This configuration minimizes the interelectronic repulsion. It was also shown there that a ligand field of octahedral symmetry partially removes the degeneracy, giving a lower lying triplet of π(t_2g) symmetry and a higher energy doublet of σ(e_g) symmetry. We showed in Chapter 6 that for high spin complexes the ligand field increases significantly with pressure, and that the interelectronic repulsion (Racah) parameters decrease. Thus, on the one hand, with pressure it takes an increasing amount of potential energy to occupy all the orbitals; on the other hand, the decreasing Racah parameters decrease the spin pairing energy. Both of these factors increase the probability of a high spin to low spin conversion. (The increase in field is probably the major effect.) Since Griffith [1] has shown that an intermediate spin configuration is never the most stable in octahedral symmetry, we need only deal with high and low spin states at this point.