The enthalpies of reaction of the ·Cr(CO)3C5Me5 and disulfides (RSSR) forming RS-Cr(CO)3C5Me5 have been measured by solution calorimetry, ΔH = -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, ΔHΦ = +10.2 kcal/mol, ΔSΦ = -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, ΔHΦ = -0.2 kcal/mol, ΔSΦ = -47 cal/(mol deg)]. The rates of reaction of MeSSMe and PhSSPh with the hydride H-Cr(CO)3C5Me5 have also been investigated in the presence of varying amounts of added ·Cr(CO)3C5Me5 radical. MeSSMe shows no apparent reaction with HCr(CO)3C5Me5, even in the presence of added ·Cr(CO)3C5Me5. PhSSPh reacts with HCr(CO)3C5Me5 primarily by a radical chain process involving ·Cr(CO)3C5Me5 and PhS· radicals. The upper limit to the rate of reaction by nonradical concerted addition is kobs < 4 × 10-4 M-1 s-1.
|Original language||English (US)|
|Number of pages||8|
|State||Published - Feb 12 1997|
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry