Kinetics and mechanism of complex formation between (oxalato)pentaamminecobalt(III) and Ni(II) in aqueous medium

Abstract

The kinetics of the reversible complexation of oxalatopentaamminecobalt(III) with Ni²⁺ has been investigated at 15-30° C and <i>I </i> = 0.30 mol dm^-3] At 25°C, <i>k_f, </i> = (2.98±0.31) 10³ dm³mol^-1s^-1, ∆<i>H</i>^{^{<img src='http://www.niscair.res.in/jinfo/equation40.gif' border=0>}} = 54.5 ±3.7 kJmol^-1, ∆<i>S </i>^{^{<img src='http://www.niscair.res.in/jinfo/equation40.gif' border=0>}} = +3.3 ± 12.3 JK^-1 mol^-1 , and K_r, = (57.9 ± <i>2.2)s^-1,</i> ∆<i>H </i>^{^{<img src='http://www.niscair.res.in/jinfo/equation40.gif' border=0>}} = (60.4 ± 1.5) kJ mol^-1, ∆<i>S </i>^{^{<img src='http://www.niscair.res.in/jinfo/equation40.gif' border=0>}} = - 8.8 ± 5.0 JK ^-1 mol^-1, where <i>k</i>_f and <i>k</i>_r denote the rate constants for the formation and dissociation of the binuclear species,(NH₃)₅CoC₂O₄Ni³⁺ respectively. It is likely that the binuclear species exists in equilibrium between its monodentate and chelated forms, the half bonded oxalate moiety of the cobalt (III) substrate acting as a chelating ligand. This dissociation of Ni²⁺ from themonodentate form of the binuclear species is rate-limiting while the formation of such a species from the reactants is predominantly governed by the rate-limiting water dissociation from Ni(OH₂)<img src='http://www.niscair.res.in/jinfo/equation27.gif' border=0>.