Reaction of a large excess of H2S with 2 mol of ̇Cr(CO)3C5Me5 yields HCr(CO)3C5Me5 and HSCr(CO)3C5Me5. Kinetic studies of this reaction show two reaction pathways are followed. At pressures of CO above 10-15 atm and temperatures ≤ 10°C, a third-order rate law d[P]/dt = k3rd order[̇Cr(CO)3C5Me5] 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°C. As the pressure of CO is reduced, mixed-order kinetics is followed, and under argon atmosphere the reaction obeys the following second-order rate law: d[P]/dt = k2nd order[̇Cr(CO)3C5Me 3][H2S]. The value of k2nd order was found to be 0.20 ± 0.05 M-1 s-1 at 1°C and 0.30 ± 0.05 M-1 s-1 at 10°C. This reaction channel is proposed to proceed by rate-determining ligand substitution and formation of the hydrogen sulfide substituted radical complex ̇Cr(H2S)(CO)2C5Me5. The rate of ligand substitution of ̇Cr(CO)3C5Me5 by PMe2Ph yielding the phosphine-substituted radical ̇Cr(PMe2Ph)(CO)2C5Me5 has been reinvestigated and shown to have rate constants and activation parameters similar to those proposed for rate-determining formation of ̇Cr(H2S)(CO)2C5Me5. A reasonable fit to data at intermediate pressures of CO is obtained at T ≤ 10°C by combining the 17e- second order and 19e- third-order mechanisms for oxidative addition. The complex HSCr(CO)3C5Me5 can react with an additional 2 mol of ̇Cr(CO)3C5Me5 yielding HCr(CO)3C5Me5 + C5Me5(CO)2Cr=S=Cr(CO)2C 5Me5 + 2CO. At a temperature of 50°C under 1 atm of CO the net reaction 4̇Cr(CO)3C5Me5 + H2S → 2HCr(CO)3C5Me5 + C5Me5(CO)2Cr= S=Cr(CO)2C5Me5 + 2CO occurs within minutes without formation of detectable amounts of HSCr(CO)3C5Me5.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry