Complex Formation in the Ternary System Tl(III)-CN(-)-Cl(-) in Aqueous Solution. A (205)Tl NMR Study.
The existence of mixed complexes of the general formula Tl(CN)(m)()Cl(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, beta = [Tl(CN)(m)()Cl(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, delta(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 sigma-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.
Berg
Katja E.
KE
Department of Chemistry, Inorganic Chemistry, The Royal Institute of Technology (KTH), 100 44 Stockholm, Sweden.
Blixt
Johan
J
Glaser
Julius
J