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

The electronic structures or Lhe neutral complexes [Ni(bpy)(2)](0), [Ni(tpy)(2)](0), and [Ni(phen)(2)](0) (bpy 2,2'-bipyridine; tpy = 2,2':6',2 ''-terpyricline; phen = 1,10-phenanthroline) have been investigated by a combination of UV/Vis spedroscopy, magnetochemistry, and X-ray crystallography [the structures of [Ni(bpy)(2)](0), [Ni(tpy)Cl](0), [Cu(bpY)(2)](PF6)(2) (reinvestigation), and (bPY)(2)Cu(mu(2)-NO3)Cu(bPY)(2)](PF6)(3) are reported herein). To further clarify the ehi. ctronic 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) (in = 2+, 1+, 0, 1-, 2-), I Ni(bpy)(3)](m) (m = 3+, 2+, 1+, 0, 1-, 2-), iNi(tpy)(2)](m) (m = 3+, 2+, 1+, 0), and, for calibration purposes, [Zn(phen)(NH3)(x)- Cl(2-x)](0) (X = 2, 1, 0). Through the atoremenitionod 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-) pi-radical monoanionic ligand. More specifically, they are all four-coordinate. and possess the following electronic structures: [Nil(bpy)(bpy(0))] (delocalized), [Ni-I(tpy)(tpy(0))] (delocalized), and (Ni-II(Me-phen)(2)] ((Me)phen = 2,9-dimethy1-1,10-phenanthroline). In addition, all members of the series Ni(bpy)(2)](m) with m <= 1(+) contain a Ni' ion (d(9), S-Ni = 1/2), and Lhe 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).