The nitrile complexes trans-[PtCl2(RCN)2] (R=Et (NC1), tBu (NC2), Ph (NC3), p-BrC6H4 (NC4)) and cis-[PtCl2(RCN)2] (R=Et (NC5), tBu (NC6), Ph (NC7)) react with 1 equiv of the hydroxyguanidine OC4H8NC(=NOH)NH2 (HG) furnishing the mono-addition products trans- and cis-[PtCl2(RCN){NH=C(R)ON= C(NH2)NC4H8O}] (1–4 and 9–11; 7 examples; 54–74% yield). Treatment of any of the nitrile complexes NC1–NC7 with HG in a 1:2 molar ratio generated the bis-addition products transand cis-[PtCl2{NH=C(R)ON=C(NH2)NC4H8O}2] (5–8 and 12–14; 7 examples; 69–89% yield). The PtII-mediated coupling between nitrile ligands and HG proceeds substantially faster than the corresponding reactions involving amid- and ketoximes and gives redox stable products under normal conditions. Complexes 1, 6·4CH2Cl2, 7·4CH2Cl2, 8·2CH2Cl2, and NC4 were studied by X-ray crystallography. Platinum(II) species 1–3, 10, 11, and especially 9, efficiently catalyze the hydrosilylation cross-linking of vinyl-terminated poly(dimethylsiloxane) and trimethylsilyl- terminated poly(dimethylsiloxane-co-ethylhydrosiloxane) giving high-quality thermally stable silicon resins with no structural defects. The usage of these platinum species as the catalysts does not require any inhibitors and, moreover, the complexes and their mixtures with vinyl- and trimethylsilylterminated polysiloxanes are shelf-stable in air. ; This study is a combination of two projects focused on the reactivity of metal-activated nitriles (funded by Russian Science Foundation; grant 14-13-00060 for N.A.B.) and on novel polymer materials (funded by Russian Fund for Basic Research; grant 14-03- 00260-a for R.M.I.). M.Y.D. is much obliged to Saint Petersburg State University for a postdoctoral fellowship (12.50.1557.2013). Physicochemical studies were performed at the Center for Magnetic Resonance, Center for X-ray Diffraction Studies, Center for Chemical Analysis and Materials Research, and the Center for Thermogravimetric and Calorimetric Research (all belong to Saint Petersburg State University). The polymers for evaluation of catalytic activity were obtained from FSUE NIISK (Saint Petersburg, Russian Federation).