Intramolecular and intermolecular activation of carbon-hydrogen bonds has been investigated by the thermolysis of cis-L40s(H)(neopenty1) (1, L = PMe,). Thermolysis of 1 at 80 "C in cycloalkane solvents affords quantitative yields of fac-L30s(H)(v2-CH2PMe2) (8). The reaction is inhibited by added L up to a point after which koM is independent of L. Thermolysis of 1 in neat SiMe, at 80 "C results in formation of cis-L,Os(H)(CH,SiMe,) (2) and 8. The total rates of reaction (/cow) are the same in either alkane or SiMe, solvent at a given concentration of L. Reaction of 1 with a mixture of Si(CH3), and Si(CD,), reveals an isotope effect of 3.6 for the C-H activation step, but no effect on koM is found when neat SI(CD,), is used as solvent. Thermolysis of 1 containing a mixture of L and P(CD3), (L') ligands reveals an isotope effect for the C-H activating step in cyclometalation of 2.2, but only a secondary isotope effect is observed on ka upon thermolysis of (L'),Os(H)Np (l-d3). The secondary kH/kD appears to arise essentially from dissociation of L from 1 since the isotope effect for this dissociation for I-do vs l-d36 is 1.3. Thermolysis of 1 in Si(CD3), solvent results in no crossover into the neopentane product. These data are most consistent with a mechanism wherein the L-dependent thermolyses proceed via initial reversible L dissociation followed by reductive elimination of neopentane to form L30s0 and, finally, oxidative addition of C-H bonds intramolecularly in L or intermolecularly in SiMe4. The ratio of 2 to 8 formed from L30s is ca. 2.4 and is independent of the concentration of L up to 0.01 M, suggesting that these reactions are much more rapid than uptake of L by L30s to form L40s. Thermolysis of 1 in mesitylene solvent at 80 "C forms cis-L40s(H)[CH2(3,5-MezC6H3)] (12) and 8 in a ratio of 5/1. Thermolysis of l-d36 in the presence of excess L' proceeds via the L-independent path to form 8 with an isotope effect on koM of 1.4. The isotope effect on the cyclometalation step itself under these conditions was shown to be 4.7 by thermolysis of 1 in a large excess of 3/1 L'/L. No crossover of deuterium into neopentane product was detected in these experiments. The L-independent path is, therefore, believed to proceed via initial NpH reductive elimination to form L40s0 and then intramolecular L activation to form 8. Thermolysis of 8 at 178 OC in cycloalkane solvent with 1 M C6H6 forms a mixture of 8 and L,Os(H)(phenyl) (7) with an equilibrium constant very near to 1. After 7 days in neat SiMe, at 178 OC, 8 undergoes no conversion to 2. Thermolysis of 8 at 155 "C in cycloalkane solvent with excess L' results in incorporation of L' into all positions at the same rate, including the cyclometalated position. We have recently described the mild intermolecular activation of the carbon-hydrogen bond in benzene by intermediates generated by the thermolysis of cis-L40s(H)R (L = P(CH3),: R = CH2C(CH3),, 1; R = CH2Si(CH3)3, 2; R = CH,, 3).'*2 This (1) Desrosiers, P. J.; Shinomoto, R. S.; Flood, T. C. J . Am. Chem. Sac. 1986...