Electronic Structures of “Low‐Valent” Neutral Complexes [NiL₂]⁰ (S = 0; L = bpy, phen, tpy) – An Experimental and DFT Computational Study

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 ](PF 6 ) 2 (reinvestigation), and [(bpy) 2 Cu(μ 2 ‐NO 3 )Cu(bpy) 2 ](PF 6 ) 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)(NH 3 ) x Cl (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: [Ni I (bpy · )(bpy 0 )] (delocalized), [Ni I (tpy · )(tpy 0 )] (delocalized), and [Ni II ( Me phen · ) 2 ] ( Me phen = 2,9‐dimethyl‐1,10‐phenanthroline). In addition, all members of the series [Ni(bpy) 2 ] m with m ≤ 1 + contain a Ni I ion (d 9 , S Ni = 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 posessing an invariant +II oxidation state (d 8 , S Ni = 1).