Equilibrium Dynamics in the Thallium(III)-Cyanide System in Aqueous Solution

I. Bányai, Julius Glaser, Judit Losonczi

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Abstract

Ligand exchange reactions of thallium(III) cyano complexes, Tl(CN)n 3-n, have been systematically studied in aqueous solution containing 4 M ionic medium {[ClO4 -]tot = 4 M, [Na+]tot = 1 M, [Li+]tot + [H+]tot = 3 M}, at 25 °C, using 205Tl and 13C NMR one-dimensional inversion transfer techniques. Rate constants for all dominating exchange pathways were determined and compared to the previously studied thallium(III) halide complexes. Also in the case of cyanide ligands the ligand exchange is dominated by the rare type of reactions occurring via a direct encounter of two complexes (self-exchange reactions), e.g. Tl(CN)3 + Tl(CN)2 + = Tl(CN)2 + + Tl(CN)3 (k32, k23) or Tl(CN)2 + + Tl(CN)4 - = 2Tl(CN)3 (k24, k33). The determined cyanide exchange rate constants between complexes Tl(CN)m 3-m and Tl(CN)n 3-n, kmn rds for the rate-determining step have all similar values, about 100-1000 s-1, that are 5 orders of magnitude smaller than for the corresponding halide exchange processes. This indicates the presence of a common rate-determining step for the self-exchange reactions of the cyanide ligand, proposed to be the breaking of the thermodynamically very stable Tl-CN bond. This is in contrast to the Tl-(III)-halide systems, where the breaking of the Tl-OH2bond was proposed to determine the reaction rate. The second type of cyanide exchange, namely anation, has been found only in two cases: Tl(CN)2 + + CN- + Tl(CN)3 (k'23, k'32); and Tl(CN)3 + CN- = Tl(CN)4 - (k'34, k'43). These reactions are very fast, k'mn ∼ 109 M-1 s-1, and are proposed to proceed through an associative interchange mechanism, where the rate-determining step is a water dissociation mediated by the incoming ligand, i.e. similarly as for the corresponding halide complexes. The third type of cyanide exchange reactions was possible to study due to the presence of an NMR-active nucleus (13C) in the ligand. Only the following ligand substitution reaction was observed: Tl(*CN)2 + + HCN = Tl(CN)2 + + H*CN (k2,HCN *, kHCN,2 *). The reason for the dominant role of the self-exchange reactions is the very low concentration of free CN- and the inertness of the HCN species in the ligand exchange reactions. The obtained dynamic information is discussed and compared to the corresponding data for the thallium(III) halide complexes.

Original languageEnglish
Pages (from-to)5900-5908
Number of pages9
JournalInorganic Chemistry
Volume36
Issue number25
Publication statusPublished - 1997

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Thallium
thallium
Cyanides
cyanides
Ligands
aqueous solutions
ligands
halides
Rate constants
Nuclear magnetic resonance
nuclear magnetic resonance
Interchanges
Reaction rates
encounters
Ion exchange
Substitution reactions
low concentrations
reaction kinetics
dissociation
substitutes

ASJC Scopus subject areas

  • Inorganic Chemistry

Cite this

Equilibrium Dynamics in the Thallium(III)-Cyanide System in Aqueous Solution. / Bányai, I.; Glaser, Julius; Losonczi, Judit.

In: Inorganic Chemistry, Vol. 36, No. 25, 1997, p. 5900-5908.

Research output: Contribution to journalArticle

Bányai, I. ; Glaser, Julius ; Losonczi, Judit. / Equilibrium Dynamics in the Thallium(III)-Cyanide System in Aqueous Solution. In: Inorganic Chemistry. 1997 ; Vol. 36, No. 25. pp. 5900-5908.
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N2 - Ligand exchange reactions of thallium(III) cyano complexes, Tl(CN)n 3-n, have been systematically studied in aqueous solution containing 4 M ionic medium {[ClO4 -]tot = 4 M, [Na+]tot = 1 M, [Li+]tot + [H+]tot = 3 M}, at 25 °C, using 205Tl and 13C NMR one-dimensional inversion transfer techniques. Rate constants for all dominating exchange pathways were determined and compared to the previously studied thallium(III) halide complexes. Also in the case of cyanide ligands the ligand exchange is dominated by the rare type of reactions occurring via a direct encounter of two complexes (self-exchange reactions), e.g. Tl(CN)3 + Tl(CN)2 + = Tl(CN)2 + + Tl(CN)3 (k32, k23) or Tl(CN)2 + + Tl(CN)4 - = 2Tl(CN)3 (k24, k33). The determined cyanide exchange rate constants between complexes Tl(CN)m 3-m and Tl(CN)n 3-n, kmn rds for the rate-determining step have all similar values, about 100-1000 s-1, that are 5 orders of magnitude smaller than for the corresponding halide exchange processes. This indicates the presence of a common rate-determining step for the self-exchange reactions of the cyanide ligand, proposed to be the breaking of the thermodynamically very stable Tl-CN bond. This is in contrast to the Tl-(III)-halide systems, where the breaking of the Tl-OH2bond was proposed to determine the reaction rate. The second type of cyanide exchange, namely anation, has been found only in two cases: Tl(CN)2 + + CN- + Tl(CN)3 (k'23, k'32); and Tl(CN)3 + CN- = Tl(CN)4 - (k'34, k'43). These reactions are very fast, k'mn ∼ 109 M-1 s-1, and are proposed to proceed through an associative interchange mechanism, where the rate-determining step is a water dissociation mediated by the incoming ligand, i.e. similarly as for the corresponding halide complexes. The third type of cyanide exchange reactions was possible to study due to the presence of an NMR-active nucleus (13C) in the ligand. Only the following ligand substitution reaction was observed: Tl(*CN)2 + + HCN = Tl(CN)2 + + H*CN (k2,HCN *, kHCN,2 *). The reason for the dominant role of the self-exchange reactions is the very low concentration of free CN- and the inertness of the HCN species in the ligand exchange reactions. The obtained dynamic information is discussed and compared to the corresponding data for the thallium(III) halide complexes.

AB - Ligand exchange reactions of thallium(III) cyano complexes, Tl(CN)n 3-n, have been systematically studied in aqueous solution containing 4 M ionic medium {[ClO4 -]tot = 4 M, [Na+]tot = 1 M, [Li+]tot + [H+]tot = 3 M}, at 25 °C, using 205Tl and 13C NMR one-dimensional inversion transfer techniques. Rate constants for all dominating exchange pathways were determined and compared to the previously studied thallium(III) halide complexes. Also in the case of cyanide ligands the ligand exchange is dominated by the rare type of reactions occurring via a direct encounter of two complexes (self-exchange reactions), e.g. Tl(CN)3 + Tl(CN)2 + = Tl(CN)2 + + Tl(CN)3 (k32, k23) or Tl(CN)2 + + Tl(CN)4 - = 2Tl(CN)3 (k24, k33). The determined cyanide exchange rate constants between complexes Tl(CN)m 3-m and Tl(CN)n 3-n, kmn rds for the rate-determining step have all similar values, about 100-1000 s-1, that are 5 orders of magnitude smaller than for the corresponding halide exchange processes. This indicates the presence of a common rate-determining step for the self-exchange reactions of the cyanide ligand, proposed to be the breaking of the thermodynamically very stable Tl-CN bond. This is in contrast to the Tl-(III)-halide systems, where the breaking of the Tl-OH2bond was proposed to determine the reaction rate. The second type of cyanide exchange, namely anation, has been found only in two cases: Tl(CN)2 + + CN- + Tl(CN)3 (k'23, k'32); and Tl(CN)3 + CN- = Tl(CN)4 - (k'34, k'43). These reactions are very fast, k'mn ∼ 109 M-1 s-1, and are proposed to proceed through an associative interchange mechanism, where the rate-determining step is a water dissociation mediated by the incoming ligand, i.e. similarly as for the corresponding halide complexes. The third type of cyanide exchange reactions was possible to study due to the presence of an NMR-active nucleus (13C) in the ligand. Only the following ligand substitution reaction was observed: Tl(*CN)2 + + HCN = Tl(CN)2 + + H*CN (k2,HCN *, kHCN,2 *). The reason for the dominant role of the self-exchange reactions is the very low concentration of free CN- and the inertness of the HCN species in the ligand exchange reactions. The obtained dynamic information is discussed and compared to the corresponding data for the thallium(III) halide complexes.

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