Reaction of Phenyl and Methyl Disulfide with ·Cr(CO)3C5Me5 and HCr(CO)3C5Me5. Metal Radical and Metal Hydride Reactivity at the Sulfur−Sulfur Bond. Different Mechanisms for Oxidative Addition of Alkyl and Aryl Disulfides

The enthalpies of reaction of the ·Cr(CO) 3 C 5 Me 5 and disulfides (RSSR) forming RS−Cr(CO) 3 C 5 Me 5 have been measured by solution calorimetry, Δ H = −13.3 + 1.5 kcal/mol (R = Ph) and −11.2 + 1.2 kcal/mol (R = Me). These data lead to Cr−SR bond strength estimates of 35 and 43 kcal/mol, respectively. The rates of oxidative addition have been investigated by FTIR spectroscopy. Phenyl disulfide reacts by a second-order mechanism whose rate-determining step is the attack of the chromium radical on the sulfur−sulfur bond, yielding chromium thiolate and a thiyl radical [ k (298 K) = 1.3 M - 1 s - 1, Δ H Φ = +10.2 kcal/mol, Δ S Φ = −24.4 cal/(mol deg)]. Methyl disulfide reacts by a third-order mechanism going through a termolecular transition state in which the stronger sulfur−sulfur bond in the alkyl disulfide is attacked simultaneously by two chromium radicals [ k (298 K = 393 M - 2 s - 1, Δ H Φ = −0.2 kcal/mol, Δ S Φ = −47 cal/(mol deg)]. The rates of reaction of MeSSMe and PhSSPh with the hydride H−Cr(CO) 3 C 5 Me 5 have also been investigated in the presence of varying amounts of added ·Cr(CO) 3 C 5 Me 5 radical. MeSSMe shows no apparent reaction with HCr(CO) 3 C 5 Me 5, even in the presence of added ·Cr(CO) 3 C 5 Me 5 . PhSSPh reacts with HCr(CO) 3 C 5 Me 5 primarily by a radical chain process involving ·Cr(CO) 3 C 5 Me 5 and PhS· radicals. The upper limit to the rate of reaction by nonradical concerted addition is k obs < 4 × 10 - 4 M - 1 s - 1 .