Since the first reports of the thiocyanate ion linkage isomerization that were isolated in 1963, the numbers of the known thiocyanate linkage isomeric pairs have increased at a significant rate. It was discovered that, for palladium(II) and platinum(II) complexes, linkage preference, depends on the electronic effects of the other ligands. In this work we investigated the linkage isomerization of [Pt(SCN)2(4,7- diphenylphenanthroline)] complex in solid-state by differential scanning calorimetry (DSC) at several heating rates. The conversion of the bis-thiocyanato isomer (—SCN)2 to the bis-isothiocyanato isomer (—NCS)2 was accompanied by the appearance of an exothermic peak. However, no DSC peak was obtained for isomerization of the (—NCS)2. The results imply that the (—SCN)2 isomer is metastable which can convert to the stable isomer (—NCS)? at elevated temperatures. Assuming a two-stage irreversible isomerization, the enthalpy changes of the first stage, AH'SCN and also second stage, AH SCN isomerization were obtained using the mathematical resolution of the observed DSC peaks. The thermokinetic parameters of this conversion were determined by using Kissinger method. The activation energy values for the first and second stages of isomerization are evaluated (29.4±4.7) and (35.7±7.5) kJ moT , respectively. The low values of the activation enthalpy, AH# and the high negative activation entropy AS#, obtained by Eyring equation, supported an associative mechanism. A DFT study was employed to detect the electronic structures and the thermodynamic stabilities of the linkage isomers and the transition states.
