All models have a total charge of zero and contain iron in the formal oxidation state +II with an intermediate spin ground state, S = 1. The Fe 3d orbital occupation pattern is (xy) 2 (xz,yz) 3 (z 2 ) 1 (x 2 -y 2 ) 0 for 1 and 2, whereas for 3, the z 2 orbital is sufficiently stabilised to lead to (xy) 2 (z 2 ) 2 (xz) 1 (yz) 1 (x 2y 2 ) 0 . These electronic configurations are reflected in Mulliken spin populations of 1.95-2.29 at the iron ion, see Table .