ECE mechanisms as a tool for the investigation of unstable metal complexes. 3. Polarographic studies of ligand dissociation of dibromocobalt(II) and dibromocobalt(III) complexes

Article abstract of DOI:10.1021/ic50221a030

The electrode reaction of trans-[CoBr₂(N)₄]⁺ ((N)₄ = (en)₂, (pn)₂, (tn)₂, and (NH₃)₄; en = ethylenediamine, pn = propylenediamine, tn = trimethylenediamine) has been examined in 0.1 M sodium acetate buffer solutions at mercury electrodes. All complexes exhibit the complicated electrode processes in which both the parallel ECE reaction and the Br^- dissociation of cobalt(III) complexes take place. The dissociation rates of Br^- from the Co(III) and Co(II) complexes are determined. The Br^--dissociation rate constant (s^-1) at 25°C, ΔH^≠ (kcal mol^-1), and ΔS^≠ (cal K^-1 mol^-1) for trans-[Co^IIIBr₂(N)₄]⁺ complexes are as follows: en, 1.51 × 10^-4, 27.2, +15; pn, 3.30 × 10^-4, 19.1, -10; NH₃, 4.20 × 10^-3, 26.8, +20; tn, 2.12 × 10^-2, 15.2, -15. Also, the Br^--dissociation rate constant (s^-1) at 25°C, ΔH^≠ (kcal mol^-1), and ΔS^≠ (cal K^-1 mol^-1) for trans-[Co^IIBr₂(N)₄] complexes are as follows: en, 0.508, 1.3, -55; pn, 0.538, 2.9, -50; NH₃, 0.610, 1.3, -55; tn, 0.571, 6.5, -38.