Kinetics of the reactions of the nickel(II) ion with methyl- and n-butylmalonic acids

Abstract

The kinetics of the reactions of nickel(II) with methyl- and n-butyl-malonic acids have been studied in aqueous solution by the temperature-jump method, at ionic strength 0·10M and 25 °C, over the acidity range from 2 × 10^–6 to 5 × 10^–4M. The kinetic data indicate that two competitive reaction paths lead to 1 : 1 chelate formation, i.e. the nickel ion reacts with dianionic and monoanionic forms of the ligands. The results obtained permit us to establish that the rate constant for ring closure is larger than that for dissociation of the monodentate complex (H₂O)₅Ni(O₂C·CHR·CO₂)(R = Me or Buⁿ; O₂C·CHR·CO₂^2–= dianionic form of the ligand).

At low acidities, the loss of the first water molecule co-ordinated to nickel(II) appears to be the rate-determining step in the overall chelate-formation process. Values are assigned to rate and equilibrium constants for each of the steps involved in this process.