Metal-exchange reactions between cobalt(II) and lead(II) complexes of nitrilotriacetic acid and copper(II)

Abstract

The reactions between the cobalt(II) and lead(II) complexes of nitrilotriacetic acid (H₃nta) and the copper(II) ion have been studied using the stopped-flow technique over the concentration ranges [M(nta)]_o= 5 × 10^–4,[Cu^II]_o=(2.5–20)× 10^–3, and [H⁺]=(3–100)× 10^–6 mol dm^–3. The kinetics are found to agree with rate law (i). Values of k^M(nta)= 1.1 × 10^–2 and 4.0 × 10^–2 s^–1, k_H^M(nta)= 7.8 × 10² and 2.6 × 10³ dm³ mol^–1 s^–1, d[Cu(nta)^–]/dt={k^M(nta)+k_H^M(nta)[H⁺]+k_Cu^M(nta)[Cu²⁺]}[M(nta)^–](i) and k_Cu^M(nta)= 2.6 and 35 dm³ mol^1–s^–1 are found for M = Co^II and Pb^II respectively, at 25 °C. The mechanism of these exchange reactions is discussed with reference to the relative stability of the intermediate binuclear mixed complexes, the labilities of the metal ions investigated, and the stabilities of the metal–ligand complexes involved in each of the different reaction pathways.