Mechanism of the S -> N isomerization and aquation of the thiocyanato pentaammine cobalt(III) ion

All of the stationary points on the potential energy surface of the S --> N isomerization and aquation of the Co(NH3)(5)SCN2+ ion have been investigat ed with ab initio quantum chemical methods. Also the corresponding anations of the Co(NH3)(5)OH23+ ion by the N and S ends of SCN- and the substitutio n of thiocyanate via the D mechanism have been studied, All calculations ha ve been performed by taking into account hydration. The most favorable reac tion of Co(NH3)(5)SCN2+ is the isomerization. It is concerted, follows the I or Id mechanism, depending on the applied criteria, and proceeds via a T- shaped transition state. The equations of Co(NH3)(5)SCN2+ and Co(NH3)(5)SCN 2+ and the corresponding inverse reactions, the anations, all proceed via t he Id mechanism. The activation energies, calculated for the isomerization and aquation, agree with experiment, and so does the difference of the acti vation energies for the anations by the two donors of SCN-. This energy dif ference reflects the disparate nucleophilicities of the N and S ends of SCN - and shows that bond making in the transition state is significant for the Id mechanism. Isomerization and aquation are two parallel reactions which proceed via two disparate transition states. The computed activation energy for the SCN- substitution via the D mechanism is the highest, and therefor e, this pathway is unlikely to operate for the isomerization and aquation o f Co(NH3)(5)SCN2+. The S --> N isomerization and the SCN- substitution via the D mechanism were furthermore computed for the free ions in the gas phas e: the isomerization would require a higher activation energy and follow th e I, mechanism. The activation energy for the SCN- substitution via the D m echanism would be very high, because of the large electrostatic work which is required for the removal of an anion from a (formally) 3+ charged cation .