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

Quantum chemical calculations at the NL-DFT (BP86, B3LYP) and CCSD(T) level s of theory predicted that the borylene ligand in (OC)(4)Fe-B(NH2) occupies the equatorial position, while the axial and equatorial forms of the paren t compound (OC)(4)Fe-BH are energetically nearly degenerate. The axial isom er (OC)(4)Fe-B(NH2) is a transition state on the potential energy surface. Charge and energy analysis of the bonding situation suggests that the boryl ene ligands are rather strong pi acceptors. The strengths of the Fe --> BR (R epsilon=epsilon NH2 or H) pi -back donation in the axial and equatorial plane are very different from each other which yields very different bond l engths and bond angles of the axial and equatorial CO ligands. The calculat ions show that B(NH2) is a weaker pi -accepting ligand than BH, which contr adicts the qualitative rule that the equatorial position is occupied by the better pi acceptor.