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 (NaC104) and t = 25 °C. The formation of mixed ligand complexes of the type NiL(CN),2"'1"1 has been verified. The transition between NiL2-ri 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"'*. 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) X 10"1 M"1 s"1, K2k3 = (2.85 ± 0.31) X 101 M"2 s~\ and K4K2k3 = 5.27 X 10® 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) X 10"7 s'1 when NaC104 was used for maintaining ionic strength and kr = (7.36 ± 0.92) X 10"7 s"1 when KN03 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 fef(CN) and & ( ) were resolved from kinetic data and are 2.85 X 101 and 1.43 X 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.
The kinetics and mechanism of the system NiL2"" + 4CN~" Ni(CN)42" + L"", where L = DTPA (diethylenetriaminepentaacetic acid) and PDTA (1,2-diaminopropanetetraacetic acid), have been investigated. The reaction conditions are pH 11.0 ± 0.2, µ = 0.1 M and t = 25 ± 0.1 °C. As in the reaction of Ni(II)-EDTA2ã nd Ni(II) complexes of other aminocarboxylates reported earlier, the formation of mixed ligands of the type NiLCCN)*2""'1'"* was verified. The transition between NiL2"" and Ni(CN)42" is kinetically controlled by the presence of three cyanide ions around one nickel atom in the rate-determining step. The reaction rates are convenient to follow spectrophotometrically. Both reactions are first order in NiL2"". A variable dependence on cyanide ion concentration (always present in large excess) is observed. The reverse reaction rates are first order in Ni(CN)42" concentration, first order in L"", and inverse first order in cyanide ion concentration. The first-order forward rate constants (in cyanide dependence) k3 are (3.27 ± 0.22) X 10"1 and (5.70 ± 0.8) X 10"3 M"1 s"1; second-order rate constants K2k3 are (5.53 ± 0.77) and (1.77 ± 0.12) X 10"2 M"2 s"1 for DTPA and 1,2-PDTA, respectively. The reverse rate constants kT = (A4"1/z_3) for these two reactions are (4.78 ± 0.6) X 10"8 and (3.03 ± 0.24) X 10"9 s"1, respectively. From the investigation of the pH dependence of reaction rate over the pH range 7-11, it is inferred that one molecule of HCN is involved in the reaction in addition of two CN" upto the rate-determining step between pH 7.0 and 9.0.
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