Kinetics of the reduction of 4-amino and 4-cyanopyridinechlorocobaloximes by iron(II)

Cobaloximes such as trans[Co(dmgH) 2 (Py-NH 2 )Cl] and trans-[Co(dmgH)2(Py-CN)Cl], where, dmgH = dimethylglyoximato anion, Py-NH 2 = 4-aminopyiridne and Py-CN = 4-cyanopyridine, were prepared and characterized by elemental analysis, UV-VIS, IR and NMR spectroscopy. The kinetics of iron(II) reduction of the complexes were studied spectrophotometrically at 300 nm in 2 % (v/v) DMSO-H 2 O medium at 27±0.1 °C and I = 0,25 M (LiClO 4 ) at various hydrogen ion concentrations in the range 2.5 x 10 -4 to 5.0 x 10 2 M under pseudo-first-order conditions using an excess of the reductant. The inverse dependence of rate on [H + ] suggests an equilibrium between the protonated and unportonated forms of the complexes, the protonated form reacting slower than the unprotonated form. Computation of the data enabled the evaluation of the rate constants for the protonated and unprotonated from of the complexes, leading to an evaluation of the protonation constant for the complexes.