Kinetic and Thermodynamic Studies of Reaction of *Cr(CO)(3)C(5)Me(5), HCr(CO)(3)C(5)Me(5), and PhSCr(CO)(3)C(5)Me(5) with *NO. Reductive Elimination of Thermodynamically Unstable Molecules HNO and RSNO Driven by Formation of the Strong Cr-NO Bond.

Abstract

Reaction of H-Cr(CO)(3)C(5)Me(5) with *NO at 1-2 atm pressure in toluene solution yields Cr(NO)(CO)(2)C(5)Me(5) as the sole metal-containing product in addition to N(2)O and HNO(2) as the principle nitrogen-containing products. N(2)O and HNO(2) are attributed to decomposition of the initial product HNO. Kinetic studies yield the rate law d[P]/dt = -k(2nd)( )(order)[HCr(CO)(3)C(5)Me(5)][*NO]; k(2nd)( )(order) = 0.14 M(-)(1) s(-)(1) at 10 degrees C, with DeltaH() = 11.7 +/- 1.5 kcal/mol and DeltaS() = -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)C(5)Me(5) 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 approximately 150-times slower reaction of H-Mo(CO)(3)C(5)Me(5) and failure to observe reaction for H-W(CO)(3)C(5)Me(5) in keeping with metal-hydrogen bond strengths Cr < Mo < W. Reaction of PhS-Cr(CO)(3)C(5)Me(5) with NO at 1-2 atm pressure in toluene solution also forms Cr(NO)(CO)(2)C(5)Me(5) 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)C(5)Me(5)]; k(1st)( )(order) = 3.1 +/- 0.3 x 10(-)(3) s(-)(1) at 10 degrees C, with DeltaH() = 21.6 +/- 1.2 kcal/mol, DeltaS() = + 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)C(5)Me(5). The enthalpy of reaction of *Cr(CO)(3)C(5)Me(5) and NO yielding Cr(NO)(CO)(2)C(5)Me(5) and CO has been measured by solution calorimetry: DeltaH degrees = -33.2 +/- 1.8 kcal/mol. The Cr-NO bond strength is estimated as approximately 70 kcal/mol and provides the net thermodynamic driving force for the proposed elimination of the unstable molecules HNO and PhSNO.