A stable monomeric nickel borohydride

A stable discrete nickel borohydride complex (Tp*NiBH4 or Tp*NiBD4) was prepared using the nitrogen-donor ligand hydrotris(3,5-dimethylpyrazolyl) borate (Tp*(-)). This complex represents one of the best characterized nickel(II) borohydrides to date. Tp*NiBH4 and Tp*NiBD4 are stable toward air, boiling water, and high temperatures (mp > 230 degreesC dec), X-ray crystallographic measurements for Tp*NiBH4 showed a six-coordinate geometry for the complex, with the nickel(II) center facially coordinated by three bridging hydrogen atoms from borohydride and a tridentate Tp*- ligand. For Tp*NiBH4, the empirical formula is C15H26B2N6Ni, a = 13.469(g) Angstrom, b = 7.740(l) A, c = 18.851(2) Angstrom, beta = 107.605(9)degrees, the space group is monoclinic P2(1)/c, and Z = 4. Infrared measurements confirmed the presence of bridging hydrogen atoms; both nu(B-H)terminal and nu(B-H)bridging are assignable and shifted relative to nu(B-D) of Tp*NiBD4 by amounts in agreement with theory. Despite their hydrolytic stability, Tp*NiBH4 and Tp*NiBD4 readily reduce halocarbon substrates, leading to the complete series of Tp*NiX complexes (X = Cl, Br, I). These reactions showed a pronounced hydrogen/deuterium rate dependence (kH/kD approximate to 3) and sharp isosbestic points in progressive electronic spectra. Nickel K-edge X-ray absorption spectroscopy (XAS) measurements of a hydride-rich nickel center were obtained for Tp*NiBH4, Tp*NiBD4, and Tp*NiCl. X-ray absorption near-edge spectroscopy results confirmed the similar six-coordinate geometries for Tp*NiBH4 and Tp*NiBD4. These contrasted with XAS results for the crystallographically characterized pseudotetrahedral Tp*NiCl complex. The stability of Tp*Ni-coordinated borohydride is significant given this ion's accelerated decomposition and hydrolysis in the presence of transition metals and simple metal salts.