The reaction of Pt(tfd)(2) [tfd=S2C2(CF3)(2)] with excess 2,3-dimethyl-1,3-butadiene initially yields the expected 1:1 adduct, in which the diene has added across two sulfur atoms on separate tfd ligands. However, within 1 day at 50 degrees C, this kinetic product quantitatively converts into a thermodynamic product where two dienes have added to one tfd ligand via unprecedented addition across the dithiolene CS bonds. The new reaction is highly selective for the C-2-Symmetric diastereomer. A new chiral bisthioether chelate ligand has formed in the product, which has been characterized crystallographically.