The nitrile complexes trans-[PtCl2(RCN)2] (R = Et, tBu, Ph, p-BrC6H4) and cis-[PtCl2(RCN)2] (R = Et, tBu, Ph) react with 1 equiv of the hydroxyguanidine OC4H8NC(=NOH)NH2 (HG) in dichloromethane solutions at RT for 5–10 min furnishing the mono-addition products cis- and trans-[PtCl2(RCN){NH=C(R)ON=C(NH2)NC4H8O}] (1–4 and 9–11; 54–74%). Treatment any one of trans- or cis-[PtCl2(RCN)2] with HG in a 1:2 molar ratio of the reactants in CH2Cl2 at RT for 2–6 h brings about generation of the bis-addition products cis- and trans-[PtCl2{NH=C(R)ON=C(NH2)NC4H8O}2] (5–8 and 12–14; 69–89%). The PtII-mediated coupling between nitrile ligands and HG proceeds substantially faster than the relevant reactions involving amid- and ketoximes. Platinum(II) species 1–3, 10–11, and especially 9, efficiently catalyze 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 de