Thermodynamics of thiocyanate complexes of trivalent actinides and lanthanides

The thermodynamic parameters ΔF, ΔH and ΔS of the complexes of Cm(III), C(III) and Tm(III) with the SCN^? ion have been determined at 30°C in ammonium ion medium of unit ionic strength by the temperature variation method. It has been concluded that both the thiocyanate complexes of trivalent actinides and lanthanides are predominantly inner-sphere type. The higher stability of the second complexes of trivalent actinides is reflected either in the enthalpy or the entropy change depending on the degree of hydration of the trivalent actinide ions. The implications of the greater free energy change for PuSCN²⁺ as compared with other trivalent actinide or lanthanide first thiocyanate complexes are discussed.