Theoretical and synthetic studies on dihaptoacyl and beta-agostic acyl complexes of molylbdenum

Molybdenum acyl complexes of formula Mo(C(O)CH2SiMe2R)(S2CX)(CO)(PMe3)(2) ( R = Me, Ph; X = NMe2 (2-Me, 2-Ph), N-i-Pr-2 (3-Me), NC4H4 (4-Me), O-i-Pr (5 -Me, 5-Ph), O-t-Bu (6-Me)) containing several S-donor ligands have been pre pared and characterized. Compounds 2-Me and 2-Ph exist in solution as equil ibrium 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)(S2CNMe 2)CO(PMe3)(2) and by ab initio calculations performed with the representati ve model species Mo(C(O)CH2R)(S2CNH2)(CO)(PH3)(2) (R = H (8), SiH3 (9)), wh ich show good agreement with the structural data of the parent compounds. F or 8 a value of 12.7 kcal mol(-1) has been obtained for the agostic stabili zation. Calculation of the energy profile for the CO deinsertion in 8 gives an energy barrier of 4.0 kcal mol(-1) for the C-alpha-C-beta breaking proc ess, in good accordance with the available experimental data.