KINETICS AND MECHANISM OF TETRACYANONICKELATE FORMATION .2. FROM MONO(TRIMETHYLENEDIAMINETETRAACETATO)NICKEL(II) COMPLEX

The kinetics of the ligand substitution reaction of nickel(II) complexed to trimethylenediaminetetraacetic acid with CN- has been studied spectrophotometrically in the pH range 7.0-11.5, at μ = 0.1 M (NaClO4) and t = 25°C. The formation of mixed ligand complexes of the type NiL(CN)x2-n-x has been verified. The transition between NiL2-n and Ni(CN)42- is kinetically controlled by the presence of three cyanide ions around one nickel ion in the rate-determining step. The reaction is first order in NiL2-n. A variable order dependence in cyanide is observed. The first-, second-, and third-order rate constants in cyanide dependence are k3 = (4.38 ± 0.35) × 10-1 M-1 s-1, K2k3 = (2.85 ± 0.31) × 101 M-2 s-1, and K1K2k3 = 5.27 × 105 M-3 s-1, respectively. The reverse reaction rates are first order in Ni(CN)42- and TMDTA4- each and inverse first order in free cyanide. The reverse rate constant kr (i.e., K4k3) = (1.96 ± 0.21) × 10-7 s-1 when NaClO4 was used for maintaining ionic strength and kr = (7.36 ± 0.92) × 10-7 s-1 when KNO3 was used. From the investigation of pH dependence on forward reaction rate it was found that one molecule of HCN is a reactant in addition to two molecules of cyanide in the pH range 7.0-9.0. The values of kf(CN) and kf(HCN) were resolved from kinetic data and are 2.85 × 101 and 1.43 × 101 M-2 s-1, respectively. From the pH dependence of the reverse reaction it is inferred that HL3- is less reactive in comparison to L4-. The stability constant of the NaTMDTA3- complex is 11.20 at μ = 0.5 M and 25°C as determined from kinetic data. Kinetic behavior of structurally similar ligands, viz., 1,2-PDTA4-, TMDTA4-, and EDTA4- has been compared. © 1979 American Chemical Society.