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, (eta(5)-C5H5)M, (eta(5)-C5Me5)M, and (H3Si)(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 sigma-bonded model (A) supplemented by one back-bonding interaction from Fe to M (B), or a M --> Fe sigma-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 pi-bonding substitutents, R, were found to have some pi-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).