Oxidation of the Ni(II) species [Ni(NCN′)X] (NCN′ = C6H3(CH2NMe2)2-o,o′) by appropriate reagents [CuX2 (X = Br, Cl), I2 (X = I)] affords in high yield the unique five-coordinate Ni(III) aryl species [Ni(NCN′)X] (X: Br, 1a; Cl, 1b, I, 1c) that are precursors to the NO3 and NO2 analogues 1d and 1e, respectively. ESR data for 1a-e indicate a d7 electronic configuration with a single unpaired electron in the dz2 orbital; a low-spin configuration is also consistent with the magnetic susceptibility of 1.989 (3) μB found for 1c. The iodo analogue 1c has also been the subject of an X-ray crystallographic study that established a square-pyramidal coordination geometry for the Ni(III) center. Crystals of 1c, C12H19NiN2I2, are monoclinic, space group P21/c, with a = 13.9696 (9) Å, b = 7.8683 (9) Å, c = 15.0510 (17) Å, β = 108.769 (7)°, V = 1566.4 Å3, Z = 4, dcalcd = 2.136 cm3, F(000) = 956 e, μ(Mo ̅α) = 48.6 cm-1. On the basis of 3365 unique reflections with Fo > 2.0σFo, the structure has been refined to R = 0.0499 and Rw = 0.0603. Within the nickel coordination sphere one iodine atom I2, the aryl C1 atom, and the mutually trans N-donor atoms of NCN′define the square base, while the second iodine atom I1 is at the pyramid apex; the Ni atom lies ca. 0.325 Å above the plane of the basal atoms. Conductivity measurements on 1b and 1d show full dissociation of both X anions in aqueous solution. In acetone solution, where dissociation is not significant, [NiII(NCN′)X] and [NiIII(NCN′)X2] are electrochemically interconvertible when additional X anion is present. The E1/2 value for the redox couple [Ni(NCN′)H2O]+/[Ni-(NCN′)(H2O)n]2+ in H2O is +0.14 V (vs SCE). The importance of organometallic nickel(III) species is discussed, with particular respect being paid to their intermediacy in oxidative-addition reactions.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry