To further understand the metal-metal bonding interaction, intrinsic bonding orbital (IAOIBO) calculations were performed (Figure ). The spin-down orbital 54β is the principal bonding orbital and is best described as a σ-bonding orbital formed primarily from Fe2 dorbitals (78.3%) with a small contribution from Fe1 (7.8%). The corresponding spin-up orbital, 61α, is also primarily Fe2 d-orbital based (84.7%), but has a much smaller contribution from Fe1 (3.4%). The discrepancy in the amount of contribution in the α and β orbitals indicates the presence of significant spin polarization.