Kinetics and mechanism of tetracyanonickelate formation. 2. From mono(trimethylenediaminetetraacetato)nickel(II) complex

Abstract

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.

Kinetics of ligand substitution reactions. 1. A reinvestigation of the reaction of nickel(II) triethylenetetraminehexaacetate with cyanide ion

“…The apparatus and instruments used in the investigation have been described in our earlier communications. 3,4 The HeL solution was prepared from triethylenetetraminehexaacetic acid (Sigma Chemicals) and standardized by visual titration with standard zinc12 solution and also by potentiometric titrations. Nickel perchlorate and sodium cyanide solutions were standardized as in our earlier work.3,4 Na2Ni(CN)4 was prepared for use in reverse reaction by adding stoichiometric amounts of Ni(C104)2-6H20 and sodium cyanide in aqueous solution.…”

“…Introduction Stara and Kopanica1 have reported their results on the rates of the following substitution reaction at pH 10.6, 24 °C, and µ = 0.1 M (NaC104) been described in our earlier papers. 3,4 The rates of forward and reverse reactions were measured by spectrophotometric determination of the tetracyanonickelate concentration at 267 and 285 nm. The forward reactions were run in the presence of excess cyanide and gave excellent pseudo-first-order plots (log (A" -At) vs. time, where A denotes absorbance).…”

Kinetics and Mechanism of the Reaction Between Mono ((Ethylenedioxy) Diethylenedinitrilo)-Tetraacetatodinickelate(ii) and Cyanide Ion

“…T h s is due to the formation of Ni(CN)T and mixed complexes of the type NiYCN), according to equation (4). This, however, overweighs the slow formation of Ni(CN)i-according to equation (2). There is a region where order changes from zero to one correspondingly ( Fig.…”

“…The same trend was also found in the case of the reaction of cyanide with NizTTHA and Ni,DTPA in our earlier reported work. "^'* The zero-order dependence on cyanide indicates the slow dissociation of Niz L complex according to (2).…”

Kinetics of ligand substitution reactions. Mechanism of tetracyanonickelate formation from binuclear nickel(II) complexes

“…A computer calculation carried out by a program due to Perrin and Sayce29 shows that starting with a metal-to-ligand ratio of 2:1 the binuclear complex was the important chemical species in the working pH range. Both reactions are thermodynamically favored (log |8ní2dtpa = 25.70,30 log !® 2 = 32.2,21 and log d4,Ni(CN)4z-= 30.532). The forward reactions were run in the presence of excess cyanide under pseudo-first-order conditions.…”