Low-valent ditantalum complex Ta₂(μ-BH₃)(μ-dmpm)₃(η₂-BH₄)2: First dinuclear compound containing a bridging BH₃ group with direct Ta-B bonds

The reduction of TaCl₅ with LiBH₄ in the presence of the bidentate ligand bis(dimethylphosphino)-methane, dmpm, produces the novel BH₃-bridged ditantalum complex Ta₂(μ-BH₃)(μ-dmpm)₃(η₂-BH₄)₂ (1) and the acid- base adduct dmpm (BH₃)₂ (2). Two crystalline forms of 1 were obtained, and their structures have been determined by X-ray crystallography. The unique feature of 1 is that the bridging borane is bound only to the ditantalum metal center, forming a simple metallaborane cluster, Ta₂B, with averaged Ta-Ta and Ta-B bond distances of 2.7792[5] and 2.307[9]Å, respectively; the averaged Ta-B separation for the axially coordinated BH₄^- groups, which are bound to the Ta atoms through hydrogen atoms, is ~0.2 Å longer than that of the bound BH₃ group. The μ-BH₃ group has one exo-hydrogen atom with a B- h(exo) distance of 1.10-[8] Å. The distance between the endo-hydrogen atom and the boron atom is 1.23[8] Å. Each of the two endo-hydrogen atoms is also bound to a Ta atom with the Ta-H distance of 1.98[8] Å. Compound 1 represents the first example of a structurally characterized metallaborane complex containing a μ-BH₃ group bringing two metal centers. The structure of the air-stable compound 2 has also been determined; it shows a strong P to B dative bond with the P-B distance of 1.903(4) Å.