Several S-alkylthiophenium complexes of the type [Os(NH3)(5)(4,5-eta(2)-L)](OTf)(3) (where L = S-alkylthiophenium, S-methylbenzo[b]thiophenium) are prepared by alkylation of the corresponding thiophene complexes. The S-alkylthiophenium species are proposed to undergo rapid and reversible cleavage of the C5-S bond, forming highly electrophilic metallacyclopropene intermediates. Although not directly observable, these vinyl cation intermediates may be trapped with both anionic and neutral nucleophiles affording eta(2)-4-(alkylthio)-1,3-butadiene complexes. Treatment of selected 4-(alkylthio)-1,3-butadiene complexes with an oxidant (e.g., DDQ) affords the organic ligand in good yield.