Reactivity of co-ordinated amino-acids. Part 2. The role of an uncharged intermediate in the carbon-14-labelled-ligand-exchange studies on the trans(fac)-bis(N-methyliminodiacetrato)chromate(III) anion

Abstract

The rate of exchange of trans(fac)-bis(N-methyliminodiacetato)chromate(III) with free ligand which has been labelled with ¹⁴C has been studied at 50 °C, and c_H+= 5.7 × 10^–5–l.59 × 10^–2 mol dm^–3, and shown to be dependent on the hydrogen-ion concentration. A mechanism involving partial aquation of one of the ligands followed by re-anation has been shown to account for essentially the whole of the observed ligand-exchange rate implying that direct bimolecular exchange is negligible. The contributing rate parameters at 50 °C have been calculated. Confirmation of the mechanism is provided by an anation study of triaqua(N-methyliminodiacetato)-chromium(III) perchlorate at 50 °C. An uncharged intermediate in the aquation–anation equilibrium has been isolated and equilibrium constants involving the three species in solution are evaluated at 25 °C.