Evidence for strong tantalum-to-boron dative interactions in (silox) ₃Ta(BH₃) and (silox)₃Ta(η²-B,Cl- BCl₂Ph) (silox = ^tBu₃SiO)¹

Treatment of (silox)₃Ta (1, silox = ^tBu ₃SiO) with BH₃·THF and BCl₂Ph afforded (silox)₃Ta(BH₃) (2) and (silox)₃Ta- (η²-B,Cl-BCl₂Ph) (3), which are both remarkably stable Ta(III) compounds. NMe₃ and ethylene failed to remove BH₃ from 2, and no indication of BH₃ exchange with BH ₃·THF-d₈ was noted via variable-temperature ¹H NMR studies. Addition of BH₃·THF to (silox) ₃TaH₂ provided the borohydride-hydride (silox) ₃HTa(η³-BH₄) (5), and its thermolysis released H₂ to generate 2. Exposure of 2 to D₂ enabled the preparation of isotopologues (silox)₃Ta(BH_3-nD _n) (n = 0, 2; 1, 2-D; 2, 2-D₂; 3, 2-D₃) for isotopic perturbation of chemical shift studies, but these failed to distinguish between "inverse adduct" (i.e., (silox)₃Ta→BH ₃) or (silox)₃Ta(η²-B,H-BH₃) forms of 2. Computational models (RO)₃Ta(BH₃) (R = H, 2′; SiH₃,2^SiH SiMe_3,2^SiMe, and Si^tBu₃, 2^SiBu) were investigated to assess the relative importance of steric and electronic effects on structure and bonding. With small R, η²-B,H structures were favored, but for 2 ^SiMe and 2^SiBu, the dative structure proved to be similar in energy. The electonic iand vibrational features of both structure types were probed. The IR spectrum of 2 was best matched by the η²-B,H conformer of 2^SiBu. In related computations pertaining to 3, small R models favored the oxidative addition of a BCl bond, while with R = Si ^tBu₃ (3^SiBu), an excellent match with its X-ray crystal structure revealed the critical steric influence of the silox ligands.