KINETICS AND MECHANISM OF COMPLEX-FORMATION OF SOME BIVALENT AND TRIVALENT METAL-IONS WITH PENTAAMMINE(NITRILOTRIACETATO)COBALT(III) IN AQUEOUS-MEDIUM

Abstract

The kinetics of reversible complex formation of Ni-II, Co-II and Cu-II with the pentaammine(nitrilotriacetato)cobalt(III) ion, [Co(NH3)(5)(H(2)nta)](2+) (H(3)nta = nitrilotriacetic acid) have been investigated at 0.0025 less than or equal to [M(2+)] less than or equal to 0.04, 0.004 less than or equal to 0.05 mol dm(-3), 10.0 less than or equal to T less than or equal to 40.0 degrees C and I = 0.03 mol dm(-3). The rate constants for the formation of the binuclear species are at least 10(3) 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)(5)Co/(nta) acts at least as a tridenate ligand and the chelate ring closure/opening via N-M(II) bond formation/dissociation is rate limiting. Complex formation with Fe-III and Al-III has been investigated at 15-35 degrees C (I = 1.0 mol dm(-3)) and 25 degrees C (I = 0.03 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.