The complex trans-[PtCl₄(EtCN)₂] (14) reacts smoothly at 25°C with the stable cyclic azomethine imines R¹CH=N^aNC(O)CH(NHC(O)C₆H₄R³)C^bH(C₆H₄R²)^(a-b) [R¹/R²/R³ = 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 stereoselective 1,3-dipolar cycloaddition to one of the EtCN ligands accomplishing the monocycloadducts trans-[PtCl₄(EtCN){N^a=C(Et)N^bC(O)CH(NHC(O)C₆H₄R³)CH(C₆H₄R²)N^cC^dHR¹}])^(a-d;b-c) [R¹/R²/R³ = 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 Δ²-1,2,3-triazolines and Δ²-1,2,4-triazolines, respectively. Platinum(II) species trans-[PtCl₂(EtCN){N^a=C(Et)N^bC(O)CH(NHC(O)C₆H₄R³)CH(C₆H₄R²)N^cC^dHR¹}]^(a-d;b-c) [R¹/R²/R³ = p-Me/H/H (21); 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. Δ²-1,2,4-Triazolines N^a=C(Et)N^bC(O)CH(NHC(O)C₆H₄R³)CH(C₆H₄R²)N^cC^dHR^1(a-d;b-c) [R¹/R²/R³ = 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 ¹H and ¹³C{¹H} NMR spectroscopies and single crystal X-ray diffraction in the solid state for 25·CH₃OH, 26·(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 ¹H 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 ¹H and ¹³C{¹H} NMR spectroscopies and single crystal X-ray diffraction for 29·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.