A REMARKABLE LEWIS ACID-BASE ADDUCT - PREPARATION AND STRUCTURE OF [TAH5(DMPE)(2)][LIHBET3] (DMPE=1,2-BIS(DIMETHYLPHOSPHINO)ETHANE)

Treatment of [TaCl4(dmpe)(2)] with Li[HBEt3] yielded [TaH5(dmpe)(2)][L iHBEt3]. The structure of this molecule was determined at 130 K by a s ingle crystal X-ray diffraction analysis. The molecule crystallised in the space group Pbca with a=16.430(1), b=12.153(1) and c=28.124(2) An gstrom, The structure was solved by Patterson methods for 4927 unique observed reflections with I greater than or equal to 2.5 sigma(I). The hydride ligands on tantalum were located from this difference Fourier and refined satisfactorily. Refinement on F-o by full-matrix block-di agonal least-squares techniques with anisotropic thermal displacement parameters for the non-hydrogen atoms and one common isotropic thermal displacement parameter for the hydrogen atoms converged at R-F=0.047. The lithium is located between the tantalum and boron atoms (Li-Ta 2. 735(13), Li-B 2.176(14) Angstrom). It is bonded to the hydride of the triethylborohydride (Li-H 1.70(7) Angstrom) and also interacts with tw o of the tantalum hydrides and two hydrogens of the triethylborohydrid e (one alpha-H each of two of the ethyl groups). The NMR spectroscopic data show chemical shifts significantly different from the pentahydri de [TaH5(dmpe)(2)] indicating that the adduct does not dissociate in t oluene solution. The H-1 NMR spectrum at room temperature shows a pent et for the tantalum hydrides. At -40 degrees C this is split into two pentets at delta -0.71 and -1.50 ppm of relative intensity 1:4 and coo ling to -90 degrees C produced three hydride signals of relative inten sity 1:2:2.