Major difference between the isoelectronic fluoroborylene and carbonyl ligands: triply bridging fluoroborylene ligands in Fe3(BF)3(CO)9 isoelectronic with Fe3(CO)12.

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.