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[TlIII]/dt=k[TlIII]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 TlIII the observed rate law is as in equation (ii) where K1 is the complex-formation –d[TlIII]/dt=k[SCN–]T2/(1 +K1[TlIII])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.