| Abstract
| - All of the stationary points on the potential energy surface of the S → N isomerization and aquation of theCo(NH3)5SCN2+ ion have been investigated with ab initio quantum chemical methods. Also the correspondinganations of the Co(NH3)5OH23+ ion by the N and S ends of SCN- and the substitution of thiocyanate via the Dmechanism have been studied. All calculations have been performed by taking into account hydration. The mostfavorable reaction of Co(NH3)5SCN2+ is the isomerization. It is concerted, follows the I or Id mechanism, dependingon the applied criteria, and proceeds via a T-shaped transition state. The aquations of Co(NH3)5SCN2+ and Co(NH3)5NCS2+ and the corresponding inverse reactions, the anations, all proceed via the Id mechanism. The activationenergies, calculated for the isomerization and aquation, agree with experiment, and so does the difference of theactivation energies for the anations by the two donors of SCN-. This energy difference reflects the disparatenucleophilicities of the N and S ends of SCN- and shows that bond making in the transition state is significantfor the Id mechanism. Isomerization and aquation are two parallel reactions which proceed via two disparatetransition states. The computed activation energy for the SCN- substitution via the D mechanism is the highest,and therefore, this pathway is unlikely to operate for the isomerization and aquation of Co(NH3)5SCN2+. The S→ N isomerization and the SCN- substitution via the D mechanism were furthermore computed for the free ionsin the gas phase: the isomerization would require a higher activation energy and follow the Ia mechanism. Theactivation energy for the SCN- substitution via the D mechanism would be very high, because of the largeelectrostatic work which is required for the removal of an anion from a (formally) 3+ charged cation.
- S → N isomerization and aquation of Co(NH3)5SCN2+ are two parallel reactions which proceed via interchange mechanisms. They involve two disparate transition states. A higher activation energy is required for the above-mentioned reactions via the D mechanism.
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