1a) in 70% yield -1a]/[(OC-6-25)-1a] = 80/20) with concomitant liberation of 2,6-dimethylbenzenethiol. Similar treatment of 2d with PMe 3 in benzene at room temperature rapidly produced cis-Ru[SC 6 H 3 (CH 2 -2)(Me-6)-κ 2 S, C](PMe 3 ) 4 (1d) quantitatively. Treatment of 2b with PMe 3 results in the formation of 1d by the C-H bond cleavage reaction and ligand displacement reaction. The C-H bond cleavage reaction does not occur from 3c under these conditions. Treatment of 2d with PMe 3 in methanol does not give 1d at all but yields cis-Ru(SC 6 H 3 Me 2 -2,6-κ 1 S) 2 (PMe 3 ) 4 (3d), which is not responsible for formation of 1d, suggesting importance of coordinative unsaturation for the C-H bond cleavage reaction. The kinetic study suggests that present C-H bond cleavage reaction proceeds by a concerted mechanism.