Treatment of [Et_4N] [Mo(CO)_5(PPh_2CS_2) ] , 1 with iodoacetonitrile afforded the neutral complex Mo(CO)_5(PPh_2CS_2CH_2CN), 2 in which the alkylation reaction took place at the sulfur atom. Protonation of complex 2 with HBF_4 produced the intramolecular cyclization product [Mo(CO)_5(PPh_2CSC(H)C(NH_2)S][BF_4] , 3. In complex 3, a five-membered cationic 1,3-dithiolium ring formed. Deprotonation of 3 by n-BuLi or PPh_3 is reversible and forming complex 2 quantitatively.