Reactions of ruthenium silylene complexes of the type [Cp*(PMe3)(2)Ru=SiR2](+) with unsaturated compounds were investigated. Nonpolar, unsaturated substrates such as ethylene, acetylene, and 2-butyne do not react with [Cp*(PMe3)(2)Ru=SiR2]B(C6F5)(4) (1a, R = Me; 1b, R = Ph). However, methyl isocyanate inserts into an Si-S bond of the silylene complex [Cp*(PMe3)(2)Ru=Si(STol)(2)][BPh4] (5) to give the 1,2-dipolar addition product {Cp*(PMe3)(2)-RuSi(STol)[eta(2)-O(MeN)C(STol)]}[BPh4] (6a) in 87% yield. This product war characterized by X-ray crystallography as possessing a base-stabilized silylene ligand with thiolate and thiocarbamate substituents. Compound 1a reacts with methyl and phenyl isocyanate to give the 2 + 2 cycloaddition products [Cp*(PMe3)(2)RuSiMe2NRC=O][B(C6F5)(4)] (7, R = Me; 8, R = Ph). The analogous triflate complexes [Cp*(PMe3)(2)RuSiR2NMeC=O][OTf] (9, R = Ph; 10, R = Me) were prepared by reaction of the appropriate ruthenium silyls Cp*(PMe3)(2)RuSiR2OTf with methyl isocyanate. Heating 9 to 100 degrees C in toluene resulted in dissociation of the isocyanate and regeneration of the triflate Cp*(PMe3)(2)RuSiPh2OTf. Competition reactions of various para-substituted phenyl isocyanates with 1b show that the rate of cycloaddition increases with the electron-donating ability of the incoming isocyanate. This is consistent with a stepwise cycloaddition mechanism involving initial coordination of the isocyanate nitrogen atom to the Lewis acidic silylene silicon atom.