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.