Mechanistic study of the reaction of Cr-center dot(CO)(3)C5Me5 with H2S yielding HCr(CO)(3)C5Me5, HSCr(CO)(3)C5Me5 and C5Me5(CO)(2)Cr=S=Cr(CO)(2)C5Me5. Kinetic evidence for formation of the substituted radical complex Cr-center dot(CO)(2)(H2S)C5Me5

Reaction of a large excess of H2S with 2 mol of Cr-.(CO)(3)C5Me5 yields HCr (CO)(3)C5Me5 and HSCr(CO)(3)C5Me5. Kinetic studies of this reaction show tw o reaction pathways are followed. At; pressures of CO above 10-15 atm and t emperatures less than or equal to 10 degrees C, a third-order rate law d[P] /dt = k(3rd order)[Cr-.(CO)(3)C5Me5](2)[H2S] is followed. The value of the third-order rate constant 70 +/- 5 M-2 s(-1) is essentially independent of temperature in the range -30 to +10 degrees C. As the pressure of CO is red uced, mixed-order kinetics is followed, and under argon atmosphere the reac tion obeys the following second-order rate law: d[P]/dt = k(2nd order)[Cr-. (CO)(3)C5Me5][H2S]. The value of k(2nd order) was found to be 0.20 +/- 0.05 M-1 s(-1) at 1 degrees C and 0.30 +/- 0.05 M-1 s(-1) at 10 degrees C. This reaction channel is proposed to proceed by rate-determining ligand substit ution and formation of the hydrogen sulfide substituted radical complex Cr- .(H2S)(CO)(2)C5Me5. The rate of ligand substitution of Cr-.(CO)(3)C5Me5 by PMe2Ph yielding the phosphine-substituted radical Cr-.(PMe2Ph)(CO)(2)C5Me5 has been reinvestigated and shown to have rate constants and activation par ameters similar to those proposed for rate-determining formation of Cr-.(H2 S)(CO)(2)C5Me5. A reasonable fit to data at intermediate pressures of CO is obtained at T less than or equal to 10 degrees C by combining the 17e(-) s econd order and 19e(-) third-order mechanisms for oxidative addition. The c omplex HSCr(CO)(3)C5Me5 can react with an additional 2 mol of Cr-.(CO)(3)C5 Me5 yielding HCr(CO)(3)C5Me5 + C5Me5(CO)(2)Cr = S = Cr(CO)(2)C5Me5 + 2CO. A t a temperature of 50 degrees C under 1 atm of CO the net reaction 4(.)Cr(C O)(3)C5Me5 + H2S --> 2HCr(CO)(3)C5Me5 + C5Me5(CO)(2)Cr = S = Cr(CO)(2)C5Me5 + 2CO occurs within minutes without formation of detectable amounts of HSC r(CO)(3)C5Me5.