[(Me3P)3Co(Bcat)3]: Equilibrium Oxidative Addition of a B–B Bond and Interconversion between the fac-Tris-Boryl and the mer-Tris-Boryl Complex

The tris-boryl complex [(Me3P)3Co­(Bcat)3], obtained by oxidative addition of B2cat2 to [(Me3P)4Co-Me] via the mono-boryl complex [(Me3P)4Co-Bcat], exhibits unique structural and electronic properties. While it is formed in an equilibrium reaction (ΔG ≈ 0 kJ mol–1) from the intermediate mono-boryl complex [(Me3P)4Co-Bcat], it is isolated in the solid state, depending on the temperature of crystallization, as the fac or mer isomer. Further studies reveal that the fac and the mer isomers of [(Me3P)3Co­(Bcat)3] are comparable in Gibbs energy and their interconversion is thermodynamically feasible. The mer isomer, comprising an unexpected trans B–Co–B motif, was further studied by means of an AIM analysis of the computed (DFT) and experimental electron densities, giving evidence for appreciable B···B interactions between two of the boryl ligands. The complex mer-[(Me3P)3Co­(Bcat)3], featuring three distinct boryl ligands, may, as a limiting case, be described as a diborane(4) complex of [(Me3P)3Co-Bcat]. DFT computations suggest that those B···B interactions are crucial for the stability of the trans B–Co–B complex mer-[(Me3P)3Co­(Bcat)3].