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^IIand Cu^II 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–M^II bond formation/dissociation is rate limiting. Complex formation with Fe^III and Al^III 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.