Fe3(BF)3(CO)8 structures with face-semibridging fluoroborylene ligands and a bicapped tetrahedral Fe3B3 cluster isoelectronic with Os6(CO)18

Abstract

The report (2009) of an unusual and unexpected Fe3(μ3-BF)2(μ-BF)(CO)9 structure, an isoelectronic fluoroborylene analogue of the long known Fe3(CO)12, makes of interest the structures of the unsaturated Fe3(BF)3(CO)8 isoelectronic with Fe3(CO)11. In this connection, seven singlet and eight triplet structures are found for Fe3(BF)3(CO)8 within 30 kcal/mol in energy of the global minimum. Natural bond orbital (NBO) analysis aids in the understanding of the electronic structure of these systems. None of the predicted structures has even a single terminal BF group. In the three lowest energy singlet Fe3(BF)3(CO)8 structures two of the BF groups are face-semibridging ligands whereas the third BF group is a simple edge-bridging ligand. The face-semibridging BF groups in these structures are joined to the Fe3 triangle with two short Fe–B bonds of ∼2.0 A and one long Fe–B bond of 2.4 A. A fourth singlet Fe3(B3F3)(CO)7(η2-μ-CO) structure, lying only ∼7 kcal/mol above the lowest energy singlet structure, has a central Fe3B3 capped tetrahedron analogous to the central Os6 bicapped tetrahedron in the long-known isoelectronic Os6(CO)18. This last cluster suggests the potential accessibility of fluoroborane metal carbonyl clusters having deltahedral structures related to those found in the deltahedral boranes.