The electroreduction of azides bound to nickel(II) ions in weak acidic media

Studies on the Formation and Kinetics of Self-Decomposition of Ni(IV)-N3 − Complexes in Aqueous Media

Both electrochemical (at Pt electrode) and homogeneous (with Cl2(aq)) oxidation of the aqueous solution of Ni(II)-N3 − yields the solution of a brown product that was found to be the azide complex of Ni(IV). Its UV/Vis spectrum, consisting of a single band of high intensity (ε = 26500 cm−1 dm3 mol−1 at λmax = 471 nm), has been ascribed to the ligand-to-metal charge-transfer mechanism (LMCT). The Ni(IV)-N3 − complex undergoes spontaneous decomposition in aqueous solution, with Ni(IV) reduced back to Ni(II) and N3 − oxidized to molecular nitrogen. The first-order kinetics of this process was studied spectrophotometrically and the following parameters were found: rate constant k = 1.96×10−2 s−1, Arrhenius activation energy E a = 82±7 kJ mol−1, standard activation enthalpy ΔH ≠0 = 79±7 kJ mol−1, and small standard activation entropy ΔS ≠0 = 10±20 J mol−1 K−1. The outline mechanism for the intramolecular redox Ni(IV)-N3 − decomposition has been suggested. Nickel ions at various oxidation states were found to be powerful catalysts for the redox processes of azide ions.