Kinetics and mechanism of complex formation: the reaction of nickel(II) with 1,4,7-triazacyclononane- N, N′, N″-triacetic acid

• Published in: Inorganica Chimica Acta Vol. 251; no. 1; pp. 347 - 354
• Main Authors: Größ, Stefan, Elias, Horst
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.10.1996
• Subjects: Azamacrocyclic ligand complexes; Kinetics and mechanism; Nickel(II) complexes; Kinetics and mechanism; Nickel(II) complexes; Azamacrocyclic ligand complexes
• ISSN: 0020-1693; 1873-3255
• DOI: 10.1016/S0020-1693(96)05289-9

Conventional and multi wavelength stopped-flow spectrophotometry was used to study the kinetics of the reaction of Ni 2+ ions with the hexadentate macrocyclic ligand TCTA (1,4,7- triazacyclononane- N, N′, N″-triacetic acid) to yield the complex Ni(TCTA) −. The experiments were carried out at variable pH (4.4–6.8) and variable nickel concentration in buffered aqueous solution at I=0.8 M (NaClO 4) and 288 K. In the presence of an excess of nickel, the formation of the complex Ni(TCTA) − is first order in [TCTA]. The experimental rate constant, k abs, of the rate law obtained, rate = k abs[TCTA], depends on the nickel concentration and on the proton concentration according to k abs= kK[Ni 2+]/(1 + K[Ni 2+]) and k= k H/(1 + [H +]/ K H), with K = 13±1 M −1, k H=154 ±15 s −1 and p K H=5.77±0.04. The value obtained for p K H corresponds to the p K a of the ligand species H 2TCTA −. The results are in agreement with the mechanistic interpretation that; (i) nickel ions and the ligand species HTCTA 2− equilibrate rapidly (equilibrium constant K) to form the intermediate Ni(HTCTA), which is subject to protonation ( K H); (ii) Ni(TCTA) − is formed by slow first-order rearrangement ( k H) of the intermediate Ni(HTCTA) under proton release. The experimental evidence describing the state of coordination of the intermediate Ni(HTCTA) is discussed.