W(CO)(5)(NCMe) (1) has been found to transform vinylthiirane and a series of methyl-substituted vinylthiiranes into a Series of 3,6-dihydro-1,2-dithiin compounds. Two equivalents of the vinylthiirane are required,and 1 equiv of a butadiene is formed by the transfer of its sulfur atom to the second vinythiirane, which is then transformed into the dihydrodithiin. The formation of 3,6-dihydro-1,2-dithiin (9) proceeds at 15 turnovers/h at 25 degrees C using vinylthiirane (4) and 1 as the catalyst. The Catalyst is long-lived (up to 2000 turnovers have been obtained without loss of activity) and relatively insensitive to air. Methyl substitutents on the vinyl group increase the rate of reaction while methyl substituents on the thiirane ring slow it considerably. The introduction of phosphine ligands to the catalyst also leads to significant increases in the rate of reaction. The dithiin complex W(CO)(5)(SSCH2CH=CHCH2) (13) was isolated from the catalytic reactions and was structurally characterized. The dihydrodithiin is coordinated to the tungsten atom through one of its two sulfur atoms. Compound 13 was shown to be a species in the catalytic cycle. A mechanism involving a vinylthiirane intermediate that undergoes spontaneous ring opening, followed by addition of a second vinylthiirane to the terminal carbon of the chain, elimination of 1 equiv of butadiene, and formation of a sulfur-sulfur bond leading to 13 is proposed. The vinylthiirane intermediate is regenerated by ligand substitution which releases the dihydrodithiin product. Compound 9 readily polymerizes when its pure form is exposed to visible light, if the polymerization is interrupted at an early stage, 1,2,7,8-tetrathiacyclododeca-4, IO-diene (14), a dimer of 9, can be isolated. Compound 14 was obtained in 5.6% yield and was structurally characterized crystallographically.