Reactions of L-cysteine with cobalt(II) and nickel(II)

Abstract

The kinetics of complexation of Co(II) and Ni(II) with L-cysteine have been studied at 20°C and µ= 0.1 M using the temperature jump method. Both anionic forms of this ligand are attacking forms, and the rate constants for the reactions [graphic omitted] have been determined for n= 1 and 2. For Co(II) they are k₁= 5.6 × 10⁶M^–1 sec^–1; k′₁= 5.6 × 10⁵ M^–1 sec^–1; k_–1= 2.8 × 10^–3 sec^–1; k′_–1= 3.6 × 10⁶ M^–1 sec^–1; k₂=k′₂= 1.5 × 10⁶ M^–1 sec^–1; k_–2= 3.8 × 10^–2 sec^–1; k′_–2= 4.9 × 10⁸ M^–1 sec^–1; and for Ni(II) they are k₁= 1.5 × 10⁵ M^–1 sec^–1; k′₁= 1.5 × 10⁴ M^–1 sec^–1; k_–1= 3.4 × 10^–5sec^–1; k′_–1= 4.4 × 10⁴ M^–1 sec^–1; k₂=k′₂= 4.4 × 10⁴ M^–1 sec^–1; k_–2= 1.1 × 10^–5 sec^–1; k′_–2= 1.4 × 10⁵ M^–1 sec^–1. Differences in forward rate constants for attack of the different ligand species on a given metal ion are related to variations in equilibrium constants for ion-pair formation. The complexes of Co(II) and Ni(II) are probably octahedral and square-planar, respectively. The presence of L-cysteine in the inner co-ordination sphere of these cations seems to cause a greater increase in the rate of loss of remaining water molecules than has been observed with similar ligands. The characteristics of L-cysteine which might account for this effect are discussed.