Analysis of the unrestricted corresponding orbitals (UCOs) shows that the highest-lying doubly occupied orbital and the four singly-occupied orbitals are the dxy, dz 2 , dx 2 -y 2 , dyz orbitals of high-spin Fe1, respectively (Figure ). These appear very similar to the frontier orbitals in other three-coordinate b-diketiminate complexes that are high-spin iron(II). The only significant contribution from Fe2 is in the dz 2 orbital, which can be described as the σ*orbital of the Fe-Fe bond. Since the σ-orbital is doubly occupied and the σ*-orbital is singly occupied, the simplest valence-bond picture is of a bond order of 0.5. The Mayer bond order calculated for the Fe-Fe bond is 0.59, in agreement with this assessment. So, even though the Fe-Fe bond is very short, the covalent interaction between the metals is limited because there is an electron in an antibonding orbital. This implies that there is a significant electrostatic contribution to the bond. Overall, the predominance of Fe2 in the s-bonding orbital and predominance of Fe1 in the s*-bonding orbital strongly support the idea that the Fe-Fe bond is a Lewis base-Lewis acid interaction between a [Fp] -anion to a [LFe] + cation, i.e.