Kinetics and mechanism of acid dissociation of cobalt(II) complex of 1,10-phenanthroline tetradentate diamine ligands

Abstract

The dissociation kinetics of cobalt(II) complx of 1,10- phenanthroline tetradentate diamine ligands have been studied using a stopped- flow spectrophotometer in 6.6×10^-3 ~0.5 mol^. Dm^-3 HCl (<i>I</i>=0.5 mol^. Dm^-3 HCl+NaCl) over a range of temperature. The dissociation rate follows the law

<i>k</i>_obs<i>=kK</i>₂[H⁺]/(1<i>+K₂ </i>[H⁺]). The acid-assisted dissociation can be rationalized in terms of a four-step kinetic process involving two rapid pre-equilibrium protonations and rate-determining cleavage of the Co(II)-N(phenanthroline) bond. The temperature dependence of the rate and equilibrium constants have been determined and the appropriate thermodynamic parameters

obtained. Steric effects in the dissociation kinetics have been discussed and the linear relationship between Δ<i>H</i>^≠<i> </i>and Δ<i>S</i>^≠has been found.