The ruthenium(II) tris-acetonitrile complex [(triphos)Ru(MeCN)(3)]BPh4 (1) is an extremely efficient catalyst precursor for the regioselective hydrogenation of benzo[b]thiophene (BT) to 2,3-dihydrobenzo[b]thiophene (DHBT) in homogeneous phase under mild reaction conditions (THF, 40-100 degrees C, 1-30 bar H-2) [triphos = MeC(CH2PPh2)(3)]. At 30 bar of H-2 and 100 degrees C, BT is converted to DHBT with an average rate of 500 mol of product (mol of cat)(-1) h(-1). During the catalytic reactions with PH2 > 5 bar, the acetonitrile ligands in 1 are transformed into a mixture of NHEt2, NEt3, and NH3, while the termination ruthenium products are the monohydrido complexes [(triphos)Ru(H)(NH3)(2)]BPh4, [(triphos)Ru(H)(NH3)(NH2Et)]BPh4, and [(triphos)Ru(H)(NH3)(eta(1)-SDHBT)]BP4. Below 5 bar of Hz, no hydrogenation of MeCN occurs and all of the ruthenium is recovered as [(triphos)Ru(H)(NCMe)(eta(1)-S-DHBT)]BPh4 All of these Ru(II) hydride complexes catalyze the hydrogenation of BT to DHBT as efficiently as 1. The substitution of D-2 for H2 in a catalytic reaction shows that BT is selectively cis-deuterated to DHBT-d(2) with no deuterium enrichment in either the unreacted BT or the arene ring of DHBT. Water in the reaction mixture decreases the hydrogenation rate of BT due to the formation of the mu-OH and acetate Ru(II) complexes [(triphos)Ru(mu-OH)(3)Ru(triphos)]BPh4 and [(triphos)Ru(O2CCH3)(OH2)]BPh4, which are catalytically inactive. The acetate complex is suggested to form via hydration of a MeCN ligand in the catalyst precursor. Catalytic runs at 30 and 2 bar of H were studied in situ by high-pressure NMR spectroscopy. The kinetics of the hydrogenation of BT in the presence of 1 were studied by gas adsorption techniques at different catalyst, substrate, and dihydrogen concentrations and at different temperatures. The kinetic data together with all of the other evidence accumulated allowed us to deduce a catalytic cycle in which the reversible dissociation of the thioether product from the metal center in the catalyst [(triphos)RuH](+) is a rate-limiting step. A comparison of the hydrogenation reactions of BT catalyzed by either the Ru(II) 14e(-) fragment [(triphos)RuH](+) or the Ru(0) 16e(-) fragment [(triphos)RuH](-) has provided some clues to unravel a number of mechanistic aspects of the HDS of thiophenes over single-component catalysts. In particular, the occurrence of either hydrogenation to thioether or hydrogenolysis to thiol has been related with the metal basicity.