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 TaCl 5 with LiBH 4 in the presence of the bidentate ligand bis(dimethylphosphino)methane, dmpm, produces the novel BH 3 -bridged ditantalum complex Ta 2 (μ-BH 3 )(μ-dmpm) 3 (η 2 -BH 4 ) 2 ( 1 ) and the acid−base adduct dmpm(BH 3 ) 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, Ta 2 B, 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 BH 4 - groups, which are bound to the Ta atoms through hydrogen atoms, is ∼0.2 Å longer than that of the bound BH 3 group. The μ-BH 3 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 1.97[8] Å. Compound 1 represents the first example of a structurally characterized metallaborane complex containing a μ-BH 3 group bridging 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.