Molybdenum acyl complexes of formula Mo(C(O)CH2SiMe2R)(S2CX)(CO)(PMe 3)2 (R = Me, Ph; X = NMe2 (2-Me, 2-Ph), N-i-Pr2 (3-Me), NC4H4 (4-Me), O-i-Pr (5-Me, 5-Ph), O-t-Bu (6-Me)) containing several S-donor ligands have been prepared and characterized. Compounds 2-Me and 2-Ph exist in solution as equilibrium mixtures of the agostic and the dihaptoacyl species and crystallize, respectively, as the isomeric mixture (2-Me) and as the agostic compound (2-Ph). Complexes 3-6 show dihapto coordination both in solution and in the solid state. Confirmation of the agostic coordination of the Mo-acyl moiety has been provided by an X-ray diffraction study of Mo(C(O)CH2SiMe3)(S2CNMe2)CO(PMe 3)2 and by ab initio calculations performed with the representative model species Mo(C(O)CH2R)(S2-CNH2)(CO)(PH3) 2 (R = H (8), SiH3 (9)), which show good agreement with the structural data of the parent compounds. For 8 a value of 12.7 kcal mol-1 has been obtained for the agostic stabilization. Calculation of the energy profile for the CO deinsertion in 8 gives an energy barrier of 4.0 kcal mol-1 for the Cα-Cβ breaking process, in good accordance with the available experimental data.