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. [(TpPh,Me)Ni(BH4)] exhibits η3‐BH4 coordination with two bridging and two terminal hydrides on boron. This contrasts with [(TpMe,Me)Ni(BH4)], previously shown to have an η4‐BH4 ligand. The borohydride hapticities diverge as a result of the different 3‐pyrazole substituents on the scorpionate co‐ligands, and can be distinguished by FTIR spectroscopy.