Variable Borohydride Hapticity in Nickel(II) Scorpionate Complexes [(TpR,Me)Ni(ηn‐BH4)]: TpR,Me = hydrotris{3‐R‐5‐methyl‐1‐pyrazolyl}borate; R = Ph, n = 3 vs. R = Me, n = 4

A “second-generation” scorpionate ligand was utilized to prepare the nickel(II) borohydride complex [(TpPh,Me)Ni(η3-BH4)], wherein the borohydride was coordinated through two bridging B–H bonds, leaving two terminal B–H bonds uncoordinated as determined by X-ray crystallography. The distorted square-pyramidal complex is paramagnetic (S = 1), with 18 valence electrons, and contrasts with a previously reported 20-electron pseudo-octahedral “first-generation” analogue, [(TpMe,Me)Ni(η4-BH4)] (P. J. Desrochers, et al. Inorg. Chem. 2003, 42, 7945–7950). These distinct borohydride coordination modes were distinguished by FTIR spectroscopy, and rationalized by DFT calculations on simplified models, [(Tp)Ni(ηn-BH4)] (n = 3, 4). The difference in borohydride hapticities is attributed to the steric effect of the scorpionate 3-pyrazole phenyl substituents disposed proximally to the metal, thus demonstrating the subtle versatility of Trofimenko's scorpionate ligands in controlling ligand field geometries. The precursor complex [(TpPh,Me)Ni(κ2-NO3)] was also prepared and characterized.