The complex trans-[PtCl4(EtCN)(2)] (14) reacts smoothly at 25 degrees C with the stable cyclic azomethine imines (RCH)-C-1=(NNC)-N-a(O)CH(NHC(O)C6H4R3)(CH)-H-b(C6H4R2)((a-b)) [R-1/R-2/R-3 = p-Me/H/H (8); p-Me/p-Me/H (9); p-Me/p-MeO/H (10); p-Me/p-Cl/p-Cl (11); p-MeO/p-Me/H (12); p-MeO/p-Cl/m-Me (13)], and the reaction proceeds as stereo-selective 1,3-dipolar cycloaddition to one of the EtCN ligands accomplishing the monocycloadducts trans-[PtCl4(EtCN){N-a=C(Et)(NC)-C-b(O)CH(NHC(O)C6H4R3)CH(C6H4R2)(NCHR1)-C-c-H-d}])((a-d;b-c)) [R-1/R-2/R-3 = p-Me/H/H (15); p-Me/p-Me/H (16); p-Me/p-MeO/H (17); p-Me/p-Cl/p-Cl (18); p-MeO/p-Me/H (19); p-MeO/p-Cl/m-Me (20)]. Inspection of the obtained and literature data indicate that the cycloaddition of the azomethine imines to the C N bonds of HCN and of Pt-IV-bound EtCN has different regioselectivity leading to Delta(2)-1,2,3-triazolines and Delta(2)-1,2,4-triazolines, respectively. Platinum(II) species trans-[PtCl2(EtCN){N-a-C(Et)(NC)-C-b(O)CH(NHC(O)-C6H4R3)CH(C6H4R2)(NCHR1)-C-c-H-d}]((a-d;b-c)) [R-1/R-2/R-3 = p-Me/H/H (27); p-Me/p-Me/H (22); p-Me/p-MeO/H (23); p-Me/p-Cl/p-Cl (24); p-MeO/p-Me/H (25); p-MeO/p-Cl/m-Me (26)] were obtained by a one-pot procedure from 14 and 8-13 followed by addition of the phosphorus ylide Ph3P=CHCO2Me. Delta(2)-1,2,4-Triazolines N-a=C(Et)(NC)-C-b(O)CH(NHC(O)-C6H4R3)CH(C6H4R2)(NCHR1(a-d;b-c))-C-c-H-d [R-1/R-2/R-3 = p-Me/H/H (27); p-Me/p-Me/H (28); p-Me/p-MeO/H (29); p-Me/p-Cl/p-Cl (30); p-MeO/p-Me/H (31); p-MeO/p-Cl/m-Me (32)] were liberated from 21-26 by the treatment with bis(diphenylphosphyno)ethane (dppe). Platinum(II) complexes 21-26 were characterized by elemental analyses (C, H, N), high-resolution electrospray ionization mass spectrometry (ESI-MS), and IR and H-1 and C-13{H-1} NMR spectroscopies and single crystal X-ray diffraction in the solid state for 25 center dot CH3OH, 26 center dot(CHCl3)(0.84). The structure of 26 was also determined by COSY-90 and NOESY NMR methods in solution. Quantitative evaluation of several pairs of interproton distances obtained by NMR and X-ray diffraction agrees well with each other and with those obtained by the MM+ calculation method. Platinum(IV) complexes 15-20 were characterized by H-1 NMR spectroscopy. Metal-free 6,7-dihydropyrazolo[1,2-a][1,2,4]triazoles (27-32) were characterized by high-resolution ESI-MS and IR and H-1 and C-13{H-1} NMR spectroscopies and single crystal X-ray diffraction for 29 center dot CDCl3. Theoretical density functional theory calculations were carried out for the investigation of the reaction mechanism, interpretation of the reactivity of Pt-bound and free nitriles toward azomethine imines and analysis of the regio- and stereoselectivity origin.