Kinetics of the iron(II) reduction of glycinatobis(malonato)-, trans-bis(malonato)bis(pyridine)-, nitrilotriacetato(malonato)-, and nitrilotriacetato(oxalato)cobaltates(III).

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The kinetics of the iron(II) reduction of glycinatobis(malonato)-, trans-bis(malonato)bis(pyridine), nitrilotriacetato(malonato)-, and nitrilotriacetato(oxalato)cobaltates(III) have been studied in aqueous perchlorate medium at I=1.0 mol dm−3 (LiClO4) and 30 °C in the [H+] range 0.01–0.90 mol dm−3. The reductions are found to be second order. The reduction of [Co(gly)(mal)2]2− and [Co(py)2(mal)2]− is accelerated by H+, while the reduction of [Co(nta)(mal)]2− and [Co(nta)(ox)]2− is independent of [H+] in the range 0.1<[H+]<0.9 mol dm−3. The reduction of [Co(nta)(mal)]2− is, however, faster at [H+]<0.1 mol dm−3. The activation parameters for the reduction of [Co(gly)(mal)2]2−, [Co(py)2(mal)2]−, [Co(nta)(mal)]2−, and [Co(nta)(ox)]2− are respectively as follows: ΔH≠=49.8±4.8, 51.2±2.6, 41.1±3.2, and 41.4+3.6 kJ mol−1 ΔS≠=−98.3±8.3, −93.7±7.2, −119.2±9.7, and −114.6+9.6 J K−1 mol−1. The proposed mechanism invokes [Co(L)(H2O)(mal)(malH)] (L-py2 or gly) formed in a H+-assisted step as the reactive species for the bis(malonato) complexes. [Co(nta)(mal)]2− is proposed to be present as [Co(nta)(malH)(H2O)]− while [Co(nta)(ox)]2− remains unaffected by H+.
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