Reaction of the alkenyl complex OsCl(E-CH=CHPh)(CO)((PPr3)-Pr-i)(2) (1) with phenyllithiuin gives OsH{C6H4-2-(E-CH=CHPh)}(CO)((PPr3)-Pr-i)(2) (2) The structure of 2 (isomer 2a) has been determined by X-ray diffraction : triclinic, P (1) over bar, a = 9.230(1) Angstrom, b = 10.092(1) Angstrom, c = 18.525(2) Angstrom, alpha = 88.667(7)degrees, beta = 87.172(7)degrees, gamma = 71.110(6)degrees, V = 1630.6(3), Z = 2, R (F, F-o greater than or equal to 4 sigma(F-o)) = 4.01, wR (F-2, all reflections) = 9.94%. The geometry around the osmium can be described as a distorted octahedron with the two triisopropylphosphine ligands occupying two relative trans positions. The remaining perpendicular plane is formed by the carbonyl and the 2-(E-1’-styryl)phenyl ligands mutually trans disposed, the hydride ligand and one olefinic hydrogen of the 2-(E-1’-styryl)phenyl ligand, which shows an agostic interaction with the osmium atom (distance 2.05(7) Angstrom). The solutions of 2 show equilibria between the agostic isomer (2a) and a nonagostic isomer (2b). The thermodynamic magnitudes involved in the equilibrium as well as the activation parameters for the conversion between the two isomers were determined in toluene-d(8) by H-1 NMR spectroscopy. The values obtained were Delta H degrees = -1.6 (+/-0.1) Kcal mol(-1) and Delta S degrees = -9.6 (+/-0.6) cal K-1 mol(-1) for the formation of the agostic isomer, whereas the activation parameters for the breaking of the agostic interaction were Delta H double dagger = 7.6 (+/-0.2) Kcal mol(-1) and Delta S double dagger = -1.0 (+/-0.7) cal K-1 mol(-1). 2 reacts with CO to give the octahedral complex OsH{C6H4-2-(E-CH=CHPh)}(CO)(2)((PPr3)-Pr-i)(2) (3).Reactions of 1 with methyllithium and CD3Li give OsH{C6H4-2-(E-CH=CHCH3)}(CO)((PPr3)-Pr-i)(2) (4) and OsH{C6H4-2-(E-CH=CHCD3)}(CO)((PPr3)-Pr-i)(2) (4-d(3)), respectively. The spectra of these complexes indicate that in solution they also show equilibria between agostic and nonagostic isomers. Reactions of 4 with P(OMe)(3) and CO afford OsH{C6H4-2-(E-CH=CHCH3)}(Co){P(OMe)3}((PPr3)-Pr-i)(2) (5) and OsH{C6H4-2-(E-CH=CHCH3)}(CO)(2)((PPr3)-Pr-i)(2) (6), respectively, in which the incoming ligands coordinate trans to the hydride. 4 and 4-d(3) isomerize in solution to give OsH(eta(3)-CH(2)CHCHPh)(CO)((PPr3)-Pr-i)(2) (11) and OsD(eta(3)-CD(2)CHCHPh)(CO) ((PPr3)-Pr-i)(2) (11-d(3)), respectively. Reaction of 11 with CO leads to OsH(eta(1)-CH2CH=CHPh)(CO)(2)((PPr3)-Pr-i)2 (12). The first order constants k(obs), and k(obs-d3) for the isomerization of 4 and 4-d(3) to 11 and 11-d(3) were obtained in CDCl3, giving activation parameters of Delta H double dagger = 20.8 (+/-1.7) Kcal mol(-1) and Delta S double dagger = -2.8 (+/-2.0) cal K-1 mol(-1) for the isomerization of 4 to 11 and a relation k(obs)/k(obs-d3) = 3.6 at 303 K.