A Theoretical Study of Free and Fe(CO)₄-Complexed Borylenes (Boranediyls) and Heavier Congeners:  The Nature of the Iron−Group 13 Element Bonding

The singlet and lowest-lying triplet states of the univalent group 13 ligands MeM, (η 5 -C 5 H 5 )M, (η 5 -C 5 Me 5 )M, and (H 3 Si) 2 NM (M = B, Al, Ga, In) have been investigated by DFT methods. Each ligand possesses a singlet ground state. Four models were considered for the interaction of these ligands with the Fe(CO) 4 fragment: a purely M → Fe σ-bonded model ( A ) supplemented by one back-bonding interaction from Fe to M ( B ), or a M → Fe σ-bonded model supplemented by two back-bonding interactions from Fe to M ( C ), and a M Fe double-bonded model ( D ). In general, the DFT calculations indicated that the RM ligands behave as two-electron donors (i.e. bonding model A ). The RM ligands with non π-bonding substitutents, R, were found to have some π-acceptor capability that would be appropriate for iron → ligand back-bonding. However, evidence for such an interaction was only found in the case of MeBFe(CO) 4 .