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 ca lorimetry, Delta H = -13.3 + 1.5 kcal/mol (R = Ph) and -11.2 + 1.2 kca l/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 b een investigated by FTIR spectroscopy. Phenyl disulfide reacts by a se cond-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 kc al/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 at tacked 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)]. Th e 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 amount s of added . Cr(CO)(3)C(5)Me(5) radical. MeSSMe shows no apparent reac tion 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 rad ical chain process involving . Cr(CO)(3)C(5)Me(5) and PhS . radicals. The upper limit to the rate of reaction by nonradical concerted additi on is k(obs) < 4 x 10(-4) M(-1) s(-1).