A Series of 4- and 5‑Coordinate Ni(II) Complexes: Synthesis, Characterization, Spectroscopic, and DFT Studies

A series of four- and five-coordinate Ni­(II) complexes Cz^tBu(Pyr^iPr)₂NiX (1–3 and 1·THF–3·THF), where X = Cl, Br, and I, were synthesized and fully characterized by NMR and UV–vis spectroscopy, X-ray crystallography, cyclic voltammetry, and density functional theory calculations. The solid-state structures of 1–3 reveal rare examples of seesaw Ni­(II) complexes. In solution, 1–3 bind reversibly to a THF molecule to form five-coordinate adducts. The electronic transitions in the visible region (630–680 nm), attributed to LMCT bands, for 1 → 3 exhibit a bathochromic shift. The thermochromic tendency of the five-coordinate complexes implies the loss of THF coordination at elevated temperatures. Finally, the electronic properties of all Ni­(II) complexes were studied by time-dependent density functional theory calculations to characterize the nature of the excited states.