Reaction of phenyl and methyl disulfide with center dot Cr(CO)(3)C(5)Me(5) and HCr(CO)(3)C(5)Me(5). 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, Delta 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), Delta H-phi = +10.2 kcal/mol, Delta S-phi = -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), Delta H-phi = -0.2 kcal/mol, Delta S-phi = -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 x 10(-4) M(-1) s(-1).