Kinetic and thermodynamic studies of reaction of •Cr(CO)3C5Me5, HCr(CO)3C5Me5, and PhSCr(CO)3C5Me5 with •NO.: Reductive elimination of thermodynamically unstable molecules HNO and RSNO driven by formation of the strong Cr-NO bond

Reaction of H-Cr(CO)(3)C5Me5 with . NO at 1-2 atm pressure in toluene solution yields Cr(NO)(CO)(2)C5Me5 as the sole metal-containing product in addition to N2O and HNO2 as the principle nitrogen-containing products. N2O and HNO2 are attributed to decomposition of the initial product TWO. Kinetic studies yield the rate law d[P]/dr = -k(2nd) (order)[HCr(CO)(3)C5Me5][. NO]; k(2nd order) = 0.14 M-1 s(-1) at 10 degrees C, with Delta H double dagger = 11.7 +/- 1.5 kcal/mol and Delta S double dagger = -16.3 +/- 3.5 cal/(mol deg). The rate of reaction is not inhibited by CO. The kinetic isotope effect for reaction of D-Cr(CO)(3)C5Me5 is k(H)/k(D) = 1.7. These observations are consistent with a first step involving direct H (D) atom transfer from the metal hydride to . NO, forming HNO. Also supporting this mechanism is the similar to 150-times slower reaction of H-Mo(CO)(3)C5Me5 and Failure to observe reaction for H-W(CO)(3)C5Me5 in keeping with metal-hydrogen bond strengths Cr < Mo < W. Reaction of PhS-Cr(CO)(3)C5Me5 with NO at 1-2 atm pressure in toluene solution also forms Cr(NO)(CO)(2)C5Me5 as the sole metal-containing product. The initial product is the unstable nitrosothiol PhS-NO. Kinetic studies yield the rate law d[P]/dt = -k(1st order)[PhS-Cr(CO)(3)C5Me5]; k(1st order) = 3.1 +/- 0.3 x 10(-3) s(-1) at 10 degrees C, with Delta H double dagger = 21.6 +/- 1.2 kcal/mol, Delta S double dagger = +3.9 +/- 1.5 cal/(mol deg). The rate of reaction is independent of both NO and CO pressure. The transition state in the first-order process is proposed to involve migration of bound thiolate to coordinated CO, forming Cr(CO)(2) (eta(2)-C(=O)SPh)C5Me5. The enthalpy of reaction of . Cr(CO)(3)C5Me5 and NO yielding Cr(NO)(CO)(2)C5Me5 and CO has been measured by solution calorimetry: Delta H degrees = -33.2 +/- 1.8 kcal/mol. The Cr-NO bond strength is estimated as similar to 70 kcal/mol and provides the net thermodynamic driving force for the proposed elimination of the unstable molecules HNO and PhSNO.