Major Difference between the Isoelectronic Fluoroborylene and Carbonyl Ligands: Triply Bridging Fluoroborylene Ligands in Fe3(BF)3(CO)9 Isoelectronic with Fe3(CO)12

Abstract

The structure of Fe(3)(BF)(3)(CO)(9) is predicted to be very different than that of any of the isoelectronic homoleptic M(3)(CO)(12) derivatives (M = Fe, Ru, Os). Thus the lowest energy Fe(3)(BF)(3)(CO)(9) structure by approximately 19 kcal/mol has mu(3)-BF groups bridging the top and bottom of the Fe(3) triangle with a third edge-bridging BF group in addition to nine terminal carbonyl groups. No analogous M(3)(CO)(12) structures are found with mu(3)-CO groups bridging the M(3) triangle. Higher energy Fe(3)(BF)(3)(CO)(9) structures with two edge-bridging mu-BF groups and one terminal BF group are also found, analogous to the experimentally known Fe(3)(CO)(12) structure. However, these structures are transition states leading to local minima with one unsymmetrical face-bridging mu(3)-BF group, one edge-bridging mu-BF group, and one terminal BF group. No Fe(3)(BF)(3)(CO)(9) structures are found with exclusively terminal BF and CO groups analogous to the known structures of M(3)(CO)(12) (M = Ru, Os). These studies suggest that the BF group has a significantly greater tendency than the CO group to bridge two or three metal atoms, probably owing to the reluctance of the fluorine of the BF ligands to be a part of a formal double or triple bond.