Gautam Neogi

Kinetics of oxidation of hydrazine and of t-butylhydrazine using tris(dimethylglyoximato)nickelate(IV) in the presence of added Cu2+(aq)

The kinetics of the oxidation of hydrazine and t-butylhydrazine using tris(dimethylglyoximato)-nickelate(IV), [Ni(dmg)3]2–, in the presence of added Cu2+(aq), and in the pH range 5.0–7.0 at 35 °C and I= 0.25 mol dm–3in aqueous medium, follow pseudo-first-order and pseudo-zero-order disappearance of the NiIVcomplex, respectively. Results of the Cu2+(aq)-promoted oxidation of hydrazine by [Ni(dmg)3]2–, are consistent with a probable scheme involving pH-dependent equilibrium formation of intermediate adducts between the NiIV and CuII–hydrazine complex species present in the solution and subsequent rate-determining electron transfer(s) to the adduct(s) from the hydrazine species in the presence of H+. Results of the Cu2+(aq)-catalyzed oxidation of t-butylhydrazine are interpreted in terms of a probable mechanism involving a rate-determining decomposition of the 1 : 1 intermediate complex between the CuII and t-butylhydrazine species in the solution, with a concomitant electron transfer. While the oxidation of hydrazine leads to nitrogen, the main products of the t-butylhydrazine oxidation are nitrogen and t-butyl alcohol.

Studies in nickel(IV) chemistry. Part 4. Kinetics of the electron transfer from L-ascorbic acid to tris(dimethylglyoximato)nickelate(IV) in aqueous medium

The kinetics of electron transfer from L-ascorbic acid (H2A) to tris(dimethylglyoximato)nickelate(IV), [Ni(dmg)3]2–, follow a pseudo-first-order disappearance of the NiIVcomplex in aqueous medium at 35 °C and I= 0.25 mol dm–3. The pseudo-first-order rate constant (kobs.) conforms to kobs.=kad+kred, where kad refers to the pseudo-first-order rate constant for the independently determined proton-assisted decomposition of the NiIV complex and Kred, to that for the intermolecular reduction of NiIV by H2A. The rate constants for the proton-assisted decomposition range from 4.6 to 2.6 × 10–7 s–1(pH range 1.6–12.3) and those for the reduction reaction range from 5.1 × 102 to 8 dm3 mol–1 s–1(pH range 1.6–11.1). The results are interpreted in terms of a probable mechanism involving a rate-determining outer-sphere one-electron transfer from the reductant to the oxidant NiIVspecies present in the solution, followed by a subsequent and kineticaliy indistinguishable rapid reaction of the NiIII intermediate.

Studies in nickel(IV) chemistry. Part 2. Kinetics of proton-assisted decomposition of tris(dimethylglyoximato)nickelate(IV) with concomitant intramolecular electron transfer in aqueous media

The kinetics of proton-assisted decomposition of tris(dimethylglyoximato)nickelate(IV), [Ni(dmg)3]2–, with concomitant Intramolecular electron transfer forming bis(dimethylglyoximato)nickel(II) and/or hexa-aquanickel (II) have been studied. The values of pseudo-first-order rate constants (kad) corresponding to the solvent-assisted and proton-assisted decomposition satisfy the expression (1)kad=kod+k1dK1H[H+]+k2dK1HK2H[H+]2//1 +K1H[H+]+K1HK2H[H+]2(1) at I= 0.57 and 0.25 mol dm–3, where kod, k1d, and k2d, respectively, refer to the pseudo-first-order rate constants for the solvent-assisted, first protonation-assisted, and second protonation-assisted decomposition paths, and K1H and K2H are the first and second protonation equilibrium constants of [Ni(dmg)3]2–. Results are interpreted in terms of a mechanism involving formation of protonated nickel(IV) species prior to axial ring-opening and concomitant Intramolecular electron transfer.

Studies in nickel(IV) chemistry. Part 3. Kinetics of metal(II) ionmediated acid decomposition of tris(dimethylglyoximato)nickelate(IV) in aqueous medium

The kinetics of metal(II) ion-mediated acid decomposition of tris(dimethylglyoximato)nickelate(IV), [Ni(dmg)3]2–, with concomitant intramolecular electron transfer to produce [M(Hdmg)2] and Ni2+(aq) have been studied at 35 °C and ionic strength 0.57 mol dm–3 in aqueous medium in the range 3.6 ≲ pH ≲ 6.8. The reactions proceed via equilibrium formation of 1 : 1 adducts involving the protonated NiIV complex and MII(aq)(M = Cu, Zn, or Ni) present in the medium. While the rate-determining steps in the reactions with CuII are envisaged to involve acid decomposition of the adduct formed in addition to proton-assisted decomposition of the NiIV complex, those in the reactions with ZnII and NiII proceed essentially by routes parallel to the proton-assisted decomposition. U.v.–visible spectral changes in the NiIV–CuII reaction indicate the formation of the 1 : 1 adduct.

Studies in nickel(IV) chemistry. Part 1. Preparation, characterization, and kinetics of formation of tris(dimethylglyoximato)nickelate(IV) in aqueous alkaline media

Potassium tris(dimethylglyoximato)nickelate(IV), K2[Ni(dmg)3], has been prepared in aqueous alkaline medium from nickel(II) and dimethylglyoximato (dmg2–) in the presence of an excess of two-electron oxidant. Electronic, i.r., and 1H n.m.r. spectra are in conformity with an octahedral (D3) geometry for this compound, with dmg2– units bonded through nitrogen donors. The lack of any significant e.s.r. signal establishes that the central nickel is in the oxidation state IV. The formation of [Ni(dmg)3]2–, from NiII and dmg2– in the presence of hypoiodite generated in situ from tri-iodide and hydroxide, exhibits pseudo-first-order kinetics. The rate constants of formation, kobs, show linear dependence on [oxidant]O in the concentration range employed. At [OH–]= 4.8 × 10–2 mol dm–3, the kobs. values show a linear increase with increasing [dmg2–]o, tending to limiting values at higher relative [dmg2–]o. The kobs, values remain almost constant with decreasing [OH–] in the range 0.105 [OH–] 1.1 × 10–2 mol dm–3, but exhibit a significant decrease with decrease in [OH–] in the range 1.1 × 10–2[OH–] 6 × 10–4 mol dm–3. The kinetic results are interpreted in mechanistic terms.