Kenneth B. Capps

THE RATE AND MECHANISM OF OXIDATIVE ADDITION OF H2 TO THE CR(CO)3C5ME5 RADICAL-GENERATION OF A MODEL FOR REACTION OF H2 WITH THE CO(CO)4 RADICAL

Abstract The rate of reaction of hydrogen with the 17 e − metal centered radical Cr(CO) 3 C 5 Me 5 obeys the third-order rate law d[P]/d t = k obs [ Cr] 2 [H 2 ] in toluene solution. In the temperature range 20–60°C, k obs =330±30 M −2 s −1 , Δ H ≠ =0±1 kcal mol −1 , Δ S ≠ =−47±3 cal mol −1 deg −1 . The rate of oxidative addition is not inhibited by added pressure of CO. The rate of binding of D 2 is slower than that of H 2 : k (H 2 )/ k (D 2 )=1.18. These results are combined with earlier work to generate a complete reaction profile for hydrogenation of the metal–metal bonded dimer [Cr(CO) 3 C 5 Me 5 ] 2 +H 2 →2H–Cr(CO) 3 C 5 Me 5 . A similar reaction profile for Co 2 (CO) 8 +H 2 →2H–Co(CO) 4 under high pressures of CO is constructed based on literature data and estimated activation parameters for reaction of the Co(CO) 4 radical with hydrogen.

First- and second-order mechanisms for oxidative addition for bound methyl disulfide in the complex W(CO)3(phen)(MeSSMe)

Abstract The rate of oxidative addition of methyl disulfide in the complex W(CO)3(1,10-phenanthroline) (MeSSMe) in methylene chloride has been studied. The dominant reaction pathway is second order in metal complex and inhibited by excess methyl disulfide. Formation of a dinuclear complex [W(CO)3(phen)]2(MeSSMe) is proposed to lead to the transition state for cleavage of the sulfur-sulfur bond in the second-order mechanism. In neat methyl disulfide, or in concentratred solutions of methyl disulfide at low metal complex concentrations, the reaction occurs at reduced rate and follows a first-order mechanism. Addition of Mo(CO)3(1,10-phenanthroline) (MeSSMe) to the corresponding tungsten complex results in a ten-fold increase in the rate of oxidative addition of the tungsten complex and production of Mo(CO)4(1,10-phenanthroline) as the sole molybdenum-containing product. The faster rate of reaction in the presence of the molybdenum complex is attributed to the faster formation of the heteronuclear dinuclear intermediate by initial loss of MeSSMe from the molybdenum versus tungsten center. Additional kinetic/mechanistic studies are described using a new flow-through FT-IR/microscope reaction system designed to allow convenient monitoring of small quantities of sensitive/hazardous reactants.