Kinetics and mechanism of anation ofcis-diaquo-bis-ethylenediamine cobalt(III) ion by L-serine in ethanol-water mixtures of varying dielectric constant

Summary

We propose the following rate law for the title anation reaction

$$\begin{gathered} d[Co(en)_2 (Ser)^{2 + } ]/dt \hfill \\ = k_a K_E [cis{\text{ - Co(en)}}_{\text{2}} ({\text{H}}_{\text{2}} {\text{O)}}_{\text{2}}^{{\text{3 + }}} ]_{{\text{total}}} \hfill \\ \cdot [{\text{Ser}}{\text{.H]/\{ 1 + }}k_{\text{E}} [{\text{Ser}}{\text{.H]\} }} \hfill \\ \end{gathered}$$

within the 3.2 to 4.5 pH range, wherek _a is the anation rate constant, andK _E the ion-pair equilibrium constant between the substrate complex ion [Co(en)₂(H₂O)₂]³⁺ and incoming zwitterionic ligand serine (Ser.H). Reaction rates in different ethanol-water mixtures reach a limit at high ligand concentrations and theK _E increases as the concentration of the organic component in the medium increases, whereas reverse is true fork _a. The reaction is catalysed by NO ⁻₃ ion and the rate increases with the increase of pH within the quoted range. ΔH ^± and ΔS ^± values were obtained from an Eyring plot and are compared with data for the isotopic water exchange process. A mechanism involving ion-pair formation between two reacting species, followed by dissociative interchange of the outer-sphere complex with the product is proposed.