Mechanistic Study of the Reaction of (*)Cr(CO)(3)C(5)Me(5) with H(2)S Yielding HCr(CO)(3)C(5)Me(5), HSCr(CO)(3)C(5)Me(5), and C(5)Me(5)(CO)(2)Cr=S=Cr(CO)(2)C(5)Me(5). Kinetic Evidence for Formation of the Substituted Radical Complex (*)Cr(CO)(2)(H(2)S)C(5)Me(5).

Abstract

Reaction of a large excess of H(2)S with 2 mol of (*)Cr(CO)(3)C(5)Me(5) yields HCr(CO)(3)C(5)Me(5) and HSCr(CO)(3)C(5)Me(5). Kinetic studies of this reaction show two reaction pathways are followed. At pressures of CO above 10-15 atm and temperatures </=10 degrees C, a third-order rate law d[P]/dt = k(3rd) (order)[(*)Cr(CO)(3)C(5)Me(5)](2)[H(2)S] 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 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)(3)C(5)Me(5)][H(2)S]. 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 substitution and formation of the hydrogen sulfide substituted radical complex (*)Cr(H(2)S)(CO)(2)C(5)Me(5). The rate of ligand substitution of (*)Cr(CO)(3)C(5)Me(5) by PMe(2)Ph yielding the phosphine-substituted radical (*)Cr(PMe(2)Ph)(CO)(2)C(5)Me(5) has been reinvestigated and shown to have rate constants and activation parameters similar to those proposed for rate-determining formation of (*)Cr(H(2)S)(CO)(2)C(5)Me(5). A reasonable fit to data at intermediate pressures of CO is obtained at T </= 10 degrees C by combining the 17e(-) second order and 19e(-) third-order mechanisms for oxidative addition. The complex HSCr(CO)(3)C(5)Me(5) can react with an additional 2 mol of (*)Cr(CO)(3)C(5)Me(5) yielding HCr(CO)(3)C(5)Me(5) + C(5)Me(5)(CO)(2)Cr=S=Cr(CO)(2)C(5)Me(5) + 2CO. At a temperature of 50 degrees C under 1 atm of CO the net reaction 4(*)Cr(CO)(3)C(5)Me(5) + H(2)S --> 2HCr(CO)(3)C(5)Me(5) + C(5)Me(5)(CO)(2)Cr=S=Cr(CO)(2)C(5)Me(5) + 2CO occurs within minutes without formation of detectable amounts of HSCr(CO)(3)C(5)Me(5).