Mechanistic study of the reaction of^̇Cr(CO)₃C₅Me₅ with H₂S yielding HCr(CO)₃C₅Me₅, HSCr(CO)₃C₅Me₅, and C₅Me₅(CO)₂Cr=S=Cr(CO)₂C ₅Me₅.

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 = k^{3rd 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 = k^{2nd order}[^̇Cr(CO)₃C₅Me ₃][H₂S]. The value of k^{2nd 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 4^̇Cr(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.