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Download file# Complex Formation in the Ternary System Tl(III)−CN^{-}−Cl^{-} in Aqueous Solution.
A ^{205}Tl NMR Study

journal contribution

posted on 1996-11-20, 00:00 authored by Katja E. Berg, Johan Blixt, Julius GlaserThe existence of mixed complexes of the general formula
Tl(CN)

*Cl*_{m}_{n}^{3-}^{m}^{-}^{n}^{}(*m*+*n*≤ 4) in aqueous solution containing 3 M ionic medium {(H^{+}+Li^{+}),ClO_{4}^{-}} has been established by means of^{205}Tl NMR. All six ternary complexes have been identified, and their compositions, chemical shifts,^{205}Tl−^{13}C spin−spin coupling constants, and peak integrals were determined and used to calculate the stability constants, β = [Tl(CN)*Cl*_{m}_{n}^{3-}^{m}^{-}^{n}^{}]/{[Tl^{3+}][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)_{m}^{3-}^{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*K*_{CN}, show linear dependence on both the spin−spin coupling constants,^{1}*J*(^{205}Tl−^{13}C), and the chemical shifts, δ_{Tl}. 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^{1}*J*(^{205}Tl−^{13}C) 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.