1,3-Dipolar cycloaddition of nitrones [H(R)C=N(CH3)O] to nitriles- free and ligated to Pt-II and Pt-IV in the complexes trans-[PtCl2(NCCH3)(2)] (1) and trans- [PtCl4(NCCH3)(2)] (2)-was investigated by theoretical methods at different levels of theory, and the substituent and solvent effects have been studied. The activation of the nitriles upon coordination and the enhancement of their reactivity from the Pt-II to the Pt-IV complexes are interpreted in terms of kinetic (dramatic decrease of the activation barrier) and thermodynamic (increase of the absolute values of the reaction energies) viewpoints as well as of orbital (lowering of the pi*(CN) MO energy) and charge (increase of the charge on the beta-C atom of RCN) control. The cycloaddition of nitrones occurs via a concerted mechanism for both free and coordinated nitriles rather than a stepwise mechanism. However, although the interaction with the uncoordinated RCN is nearly synchronous, the reaction with the complexes 1 and 2 is not only asynchronous but may be considered as of an intermediate type between the "conventional" synchronous cycloaddition (established for the free nitriles) and the nucleophilic addition to the beta-C atom. The C-substituted nitrones are significantly less reactive than the nonsubstituted H2C=N(CH3)O one, and the activation barriers of the aryl-substituted nitrones are higher than those of the C-alkyl nitrones. Replacement of the methyl by the phenyl group in the nitrile molecule RCN leads to a further promotion of its reactivity. The activation parameters and reaction energies have been calculated at different basis sets and levels of theory, up to MP4(SDTQ), CCSD(T), and CBS-Q. The activation energies are weakly sensitive to a change of the correlated methods. The consideration of the solvent effects results in the increase of the activation barriers, but the general trend of the change of the activation parameters in solution from free CH3CN to 1 and 2 remains the same as that for the gas phase.