Synthesis and theoretical analysis of the structures and bonding in five new neutral square planar bis[2,4-di(aryl)-1,3,5-triazapentadienato]nickel(II) complexes

Solvothermal reactions in methanol of nickel acetate tetrahydrate, Ni(OAc)2 · 4H2O, with benzonitrile derivatives NC(C6H4)X, where X is one of the electron withdrawing substituents –CN, –NO2, or –CF3, located at the m- or p-positions relative to –CN, yield complexes of the general formula Ni{HNdouble bond; length as m-dashC(R)–Ndouble bond; length as m-dashC(R)–NH}2. More specifically, 3-nitrobenzonitrile, 4-nitrobenzonitrile, 1,3-dicyanobenzene, 1,4-dicyanobenzene, and ααα-trifluoro-p-toluonitrile are found to react with Ni(OAc)2 · 4H2O to yield Ni{HNdouble bond; length as m-dashC(R)–Ndouble bond; length as m-dashC(R)–NH}2, where R = 3-(NO2)C6H4, 4-(NO2)C6H4, 3-(CN)C6H4, 4-(CN)C6H4, or 4-(CF3)C6H4, respectively. Analogous reactions of nitriles lacking electron withdrawing groups do not occur under similar conditions. Solid-state structures have been determined for the complexes with p-NO2, p-CN, and p-CF3 substituents on the phenyl rings. In addition, we describe density functional theory (DFT) and natural bonding orbital theory (NBO) studies on a simplified analog of these compounds, aimed at understanding their molecular bonding. It is shown that the new compounds for which solid-state structures have been determined are model examples of coordination compounds containing robust ω-bonds.