Kinetics and mechanism of the interaction of pentaammine(pyridine-2-carboxylato)cobalt(III) ions with hexaaquanickel(II) and nickel(II) complexes: the role of chelating ligands

Abstract

The reversible complexation of the pentaammine(pyridine-2-carboxylato)cobalt(III) ion [N₅Co{O₂C-(2)-C₅H₄ N}]²⁺ [N5=5HN₃ and tetraethylenepentaammine (tetren)] with Ni^IIL(OH₂)_6-n [L=H₂O (N₅=tetren); L=bipy, ida^2-(iminodiacetate) and nta^3- (nitrilotriacetate), N₅=5NH₃ and tetren] has been investigated by the stopped-flow technique at 20-40 °C, and I= 0.3mol dm^-3 . At 25°C, the rate constants, k_f(dm³ mol^-1 s^-1), ΔH^‡ (J K^-1-1 ) and Δ^‡(kJ^-1mol^-1 ) for the formation of the ternary complexes [(tetren)-Co^III{O₂C-(2)-C₅H₄N} Ni^IIL(OH₂)_6-n] are as follows: L=H₂O, 530±9, 53±2, -15±7, respectively; L = bipy, 640±30, 37±3, -65±9; L = ida^2- , 3900±100, 47±3, -18±11; L = nta^-3, 10200±400, 49±1, -2±2. Nickel(II), in the ternary complexes, is chelated by the free pyridyl-N and the carboxylato moiety of the pyridine-2-carboxylate bound to the cobalt centre. The formation rate constant (kf) and the associated activation parameters are relatively insensitive to the N₅ moieties for a given ligand L; k_f increased in the order: Ni(OH₂)₆²⁺ < Ni(bipy)(OH₂)₄²⁺ < Ni(ida)(OH₂)₃ < Ni(nta)(OH₂)₂^-. Data analysis indicated that the mechanism shifted from the dissociative interchange (I_d) to the chelation-controlled one, with the decrease of the available sites for coordination in Ni^IIL(OH₂)_6-n. The rate constants (kr) for the dissociation of [N₅Co^III{O₂C-(2)-C₅ H₄N}Ni^IIL(OH₂)_6-(n+2)] to the parent reactants indicated steric acceleration [k_r^L(5NH₃) < k_r^L(tetren)] and followed the trend: k_r^Ni(nta)- > kr Ni(ida) >k_r^Ni(bipy)2+ for both pentaammine substrates. The chelate ring opening rate constants for the ternary complexes were estimated, from which it was apparent that the tetren envelope of cobalt(III) exerted relatively greater steric pressure as compared with 5NH₃ in favouring opening up of the chelate ring.