Kinetics and mechanism of the reaction between thallium(III) and thiocyanate ion in aqueous perchloric acid solutions

Abstract

A study of the reaction between thallium(III) and thiocyanate ion in aqueous HClO₄(0.2–2.0 mol dm^–3) solutions has shown the complex [Tl(SCN)]²⁺ to be reactive, yielding final products possibly through a dimer. Two other complexes, [Tl₂(SCN)]⁵⁺ and [HTl(SCN)₂]²⁺ are also formed but are not reactive. In the presence of excess of SCN^– the observed rate law is as in equation (i) where –d[Tl^III]/dt=k[Tl^III]²/(1 +K₂[H⁺][SCN^–])²(i)K₂ is the complex-formation constant for [HTl(SCN)₂]²⁺; k and K₂ were found to be 2.2 ± 0.2 dm³ mol^–1 s^–1 and 24 ± 1 dm⁶ mol^–2 respectively at 20 °C and I= 2.0 mol dm^–3. In the presence of excess of Tl^III the observed rate law is as in equation (ii) where K₁ is the complex-formation –d[Tl^III]/dt=k[SCN^–]_T²/(1 +K₁[Tl^III])²(ii) constant for [Tl₂(SCN)]⁵⁺;k and K₁, were found to be 2.1 ± 0.2 dm³ mol ^–1s^–1 and 45 ± 5 dm³ mol^–1.