Binuclear fluoroborylene (BF) cobalt carbonyls: Comparison with homoleptic cobalt carbonyls

The recent synthesis by Vidović and Aldridge of the complex (η5-C5H5)2Ru2(CO)4(μ-BF) containing the fluoroborylene ligand (BF), isoelectronic with CO, suggests the possibility of introducing the BF ligand into binuclear cobalt carbonyl derivatives of the type Co2(BF)2(CO)n (n = 7, 6, 5, 4, 3). This has now been explored using density functional theory. Structures in which both BF groups bridge a cobalt–cobalt bond are energetically preferred for the Co2(BF)2(CO)n (n = 6, 5, 4, 3) derivatives over other types of structures. Thus the Co2(μ-BF)2(CO)6 structure with two bridging BF groups lies more than 18 kcal/mol below the next lowest Co2(BF)2(CO)6 structure and appears to be a reasonable synthetic objective. This differs from the well-known isoelectronic Co2(CO)8 for which the structures with two bridging CO groups and with all terminal CO groups are so close in energy that they are found experimentally to exist in equilibrium. Examples of Co2(B2F2)(CO)n (n = 7, 4, 3) derivatives are found in which the two BF groups have coupled to form a bridging difluorodiborene (B2F2) ligand with a B–B distance of ∼1.9 Å. However, the Co2(B2F2)(CO)7 structures do not appear to be viable since loss of a CO group to give Co2(BF)2(CO)6 is predicted to be an exothermic process.

The lowest energy Co2(BF)2(CO)n (n = 6, 5, 4, 3) structures are predicted to have two bridging BF ligands. Interesting examples of bridging B2F2 ligands are found in the carbonyl-rich system Co2(B2F2)(CO)7.Figure optionsDownload as PowerPoint slideHighlights
► The lowest energy Co2(BF)2(CO)n structures have two bridging BF ligands.
► The unbridged Co2(BF)2(CO)6 structure is relatively unfavorable.
► A bridging B2F2 ligand is found in Co2(B2F2)(CO)7.