Determination of the composition and stability constants of the complexes of AgI with sulphur-containing aminopyridines

The stability of the complexes of Ag^I with sulphur-containing aminopyridines of general formula Py? (CH₂)_n?1? S? (CH₂)_m? NH₂ (where: n ? 1, m = 1,2; 1,3; 2,2; 2,3) have been determined at 25°C and for an ionic strength of 0.5 M (K)NO₃ by a combined pH–pM metric method. The study of complex formation and the determination of the approximative values for the constants proceeded by graphical means. The constants were refined by a least squares computer method. In acid medium (pH < 3) the protonated species AgLH₂³⁺ and AgL₂H₄⁵⁺ are formed and coordination occurs through the thioether group. At higher pH values (pH > 3) additional chelation occurs through the pyridine-nitrogen donor in the complexes AgLH²⁺, AgL₂H₃⁴⁺, and AgL₂H₂³⁺. The former complex extensively transforms into a dimer Ag₂L₂H₂⁴⁺, which may also arise on the addition of a protonated ligand to the dinuclear Ag₂LH³⁺ species. At still higher PH values (pH > 6) also the aminogroup participates in chelation in the species Ag₂L₂H³⁺, Ag₂L₂²⁺ and Ag₂L²⁺. With an excess of ligand however a white precipitate is formed.