Fe3(BF)3(CO)8 structures with face-semibridging fluoroborylene ligands and a bicapped tetrahedral Fe3B3cluster 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 Å and one long Fe–B bond of 2.4 Å. 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.