Abstract

The electronic structures of the neutral complexes [Ni(bpy)2]0, [Ni(tpy)2]0, and [Ni(phen)2]0 (bpy = 2,2-bipyridine; tpy = 2,2':6',2''-terpyridine; phen = 1,10-phenanthroline) have been investigated by a combination of UV/Vis spectroscopy, magnetochemistry, and X-ray crystallography {the structures of [Ni(bpy)2]0, [Ni(tpy)Cl]0, [Cu(bpy)2](PF6)2 (reinvestigation), and [(bpy)2Cu(μ2-NO3)Cu(bpy)2](PF6)3 are reported herein}. To further clarify the electronic structures of the title compounds and elucidate those of the electron transfer series produced by their reduction and oxidation, broken-symmetry (BS) density functional theory (DFT) calculations were performed for [Ni(bpy)2]m (m= 2+, 1+, 0, 1–,2–), [Ni(bpy)3]m (m= 3+, 2+, 1+, 0, 1–, 2–), [Ni(tpy)2]m (m=3+, 2+, 1+, 0) and, for calibration purposes, [Zn(phen)(NH3)xCl(2-x)]0 (x = 2, 1, 0). Through the aforementioned studies, it was shown that none of the so called "low-valent" Ni complexes contain zero-valent Ni. Instead, they each contain at least one (bpy·)1–, (tpy·)1–, or (phen·)1– π-radical monoanionic ligand. More specifically, they are all four-coordinate and possess the following electronic structures: [NiI(bpy·)(bpy0)] (delocalized), [NiI(tpy·)(tpy0)] (delocalized), and [NiII(Mephen·)2] (Mephen = 2,9-dimethyl-1,10-phenanthroline). In addition, all members of the series [Ni(bpy)2]m with m ≤ 1+ contain a NiI ion (d9, SNi = 1/2), and the redox processes that link them are ligand-centered. A similar finding was made for six-coordinate [Ni(bpy)3]m and [Ni(tpy)2]m, but in this case complexes with m ≤ 2+ contain Ni ions possessing an invariant +II oxidation state (d8, SNi = 1).