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 °C 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(9) Å, b = 7.740(1) Å, c = 18.851(2)
Å, β = 107.605(9)°, the space group is monoclinic P21/c, and Z = 4. Infrared measurements confirmed the
presence of bridging hydrogen atoms; both ν(B−H)terminal and ν(B−H)bridging are assignable and shifted relative to
ν(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 ≈ 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.