Kinetics and mechanism of complex formation of some bivalent and trivalent metal ions with pentaammine-(nitrilotriacetato)cobalt(III) in aqueous medium
The kinetics of reversible complex formation of NiII, CoIIand CuII with the pentaammine-(nitrilotriacetato)cobalt(III) ion, [Co(NH3)5(H2nta)]2+(H3nta = nitrilotriacetic acid) have been investigated at 0.0025 ⩽[M2+]⩽ 0.04, 0.004 ⩽[H+]⩽ 0.05 mol dm–3, 10.0 ⩽T⩽ 40.0 °C and I= 0.3 mol dm–3. The rate constants for the formation of the binuclear species are at least 103 times less than the water exchange rate constants of [M(OH2)6]2+ under comparable conditions. General base catalysis indicated that proton transfer from the NH+ site of the co-ordinated ligand (nta) is involved in the rate determining step. The binuclear species undergo dissociation via spontaneous and acid-catalysed paths. The low values of spontaneous dissociation rate constants also support the chelate nature of the binuclear species. It is likely that the nta moiety of (NH3)5Co(nta) acts at least as a tridentate ligand and the chelate ring closure/opening via N–MII bond formation/dissociation is rate limiting. Complex formation with FeIII and AlIII has been investigated at 15–35 °C (I= 1.0 mol dm–3) and 25 °C (I= 0.3 mol dm–3), respectively. General base catalysis was not observed for these trivalent metal ions. The [M(OH2)5(OH)]2+ species reacted faster than [M(OH2)6]3+. The reaction of [M(OH2)6]3+ may involve an associative interchange mechanism while that for [M(OH2)5(OH)]2+ involves dissociative interchange.