Mono and dinuclear hydrotris(3,5-dimethylpyrazolyl)borato tantalum complexes

In an effort to obtain suitable starting materials, the synthesis and crystal structure of hydrotris(3,5-dimethylpyrazolyl)borato (Tp^Me2) tantalum(III) complexes are reported. Reaction of KTp^Me2 with TaCl₂(tht)]₂(μ-Cl)₂(μ-tht) (1) (tht=tetrahydrothiophene) gives the X-ray characterized unsymmetrical (Tp^Me2Ta)(TaCl₃)(μ-Cl₂)(μ-tht) (2) in fair yield. This doubly bonded TaTa complex 2.6791(5) Å, diamagnetic, 18e per Ta] is a rare example of an unsymmetrical Ta₂X₆L₃ complex. The X-ray structure of Tp^Me2TaCl₂(PhCCMe) (3) is also reported. It has the (4e)-alkyne in the symmetry plane of the molecule. The trans effects of tht and phenylpropyne are discussed. 1 is inert towards the reaction with a second equivalent of KTp^Me2.