American Chemical Society
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Complex Formation in the Ternary System Tl(III)−CN-−Cl- in Aqueous Solution. A 205Tl NMR Study

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journal contribution
posted on 1996-11-20, 00:00 authored by Katja E. Berg, Johan Blixt, Julius Glaser
The existence of mixed complexes of the general formula Tl(CN)mCln3-m-n (m + n ≤ 4) in aqueous solution containing 3 M ionic medium {(H++Li+),ClO4-} has been established by means of 205Tl NMR. All six ternary complexes have been identified, and their compositions, chemical shifts, 205Tl−13C spin−spin coupling constants, and peak integrals were determined and used to calculate the stability constants, β = [Tl(CN)mCln3-m-n]/{[Tl3+][CN-]m[Cl-]n}. Very good agreement was obtained between the equilibrium constants determined in this work and those estimated by a theoretical formula using the stability constants for the binary complexes and a statistical factor. Specific interaction coefficients have been calculated for the Tl(CN)m3-m (1 ≤ m ≤ 4) complexes. Some interesting correlations were found for the obtained NMR parameters. The stepwise formation constants for addition of one cyanide ligand, log KCN, show linear dependence on both the spin−spin coupling constants, 1J(205Tl−13C), and the chemical shifts, δTl. Also the interatomic distance, d(Tl−C), is linearly correlated to the spin−spin coupling constant. The correlations are discussed in terms of the Ramsey equation, involving bond properties, stereochemistry, and stability of the complexes. Since 1J(205Tl−13C) also shows linear dependence on the Tl−CN force constant, it is concluded that the above correlations reflect the Tl−CN bond strength. Thus, the most important factor contributing to the thermodynamic stability of the complexes is the enthalpy term, dominated by formation of very strong σ-bonds between cyanide and thallium. These trends may prove useful for spectral/structural assignments but also for estimation of metal-to-ligand bond distances and stability constants for complexes which exist only in low concentration.