Hard to Handle: Extremely Labile Hydrido Nickel(II) Complexes of Cyclometalated C ^∧N ^∧N and N ^∧C ^∧N Ligands

From the two organonickel(II) scaffolds [Ni(Phbpy)] ⁺ and [Ni(PyPhPy)] ⁺ (HPhbpy = 6-phenyl-2,2′-bipyridine≡HC ^∧N ^∧N and Py(HPh)Py = 2,6-di-2-pyridyl-benzene≡N ^∧(HC) ^∧N) the hydrido complexes [Ni(C ^∧N ^∧N)H] and [Ni(N ^∧C ^∧N)H] were studied in a combined experimental/theoretical approach. The hydrido complexes were prepared via the reaction of the halido derivatives [Ni(C ^∧N ^∧N)X] and [Ni(N ^∧C ^∧N)X] (X = Cl, Br) with Li(Et ₃BH) (superhydride). The C ^∧N ^∧N complex undergoes rapid reductive elimination (RE) yielding HPhbpy and Ni particles even at the lowest temperatures, while the PyPhPy derivative is more stable. Low-temperature ¹H nuclear magnetic resonance (NMR) spectroscopy allowed detection of a signal at δ = −2.86 ppm assignable to the hydrido ligand. The different stabilities can be directly correlated to the cis (Phbpy) and trans (PyPhPy) orientations of the carbanionic phen-2-ide group with the hydrido ligand, and rapid RE occurs from the cis position which is also supported by the density functional theory (DFT) calculations which are presented. A further TD-DFT/UV-vis absorption study is also reported to rationalize and confirm the fleeting existence of the Ni-H moiety, and proposals are made on the route of its decomposition.