Theory predicts that the weaker π-accepting ligand diaminoborylene occupies the equatorial position in (OC)4Fe–B(NH2): theoretical study of (OC)4Fe–B(NH2) and (OC)4Fe–BH †

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Abstract
Quantum chemical calculations at the NL-DFT (BP86, B3LYP) and CCSD(T) levels of theory predicted that the borylene ligand in (OC)4Fe–B(NH2) occupies the equatorial position, while the axial and equatorial forms of the parent compound (OC)4Fe–BH are energetically nearly degenerate. The axial isomer (OC)4Fe–B(NH2) is a transition state on the potential energy surface. Charge and energy analysis of the bonding situation suggests that the borylene ligands are rather strong π acceptors. The strengths of the Fe→BR (R = NH2 or H) π-back donation in the axial and equatorial plane are very different from each other which yields very different bond lengths and bond angles of the axial and equatorial CO ligands. The calculations show that B(NH2) is a weaker π-accepting ligand than BH, which contradicts the qualitative rule that the equatorial position is occupied by the better π acceptor.