Kinetics and Mechanism of the Hydrogenation of the CpCr(CO)3 •/[CpCr(CO)3]2 Equilibrium to CpCr(CO)3H

The kinetics of the hydrogenation of 2 CpCr(CO)3 •/[CpCr(CO)3]2 to CpCr(CO)3H has been investigated. The reaction is second-order in Cr and first-order in H2, with a rate constant (if the rate law is written with [CpCr(CO)3 •]2) of 12(2) M–2 s–1 at 25 °C in benzene. DFT calculations rule out a side-on H2 complex as an intermediate and suggest either (1) homolytic cleavage via a collinear Cr–H–H–Cr transition state or (2) end-on approach of H2 to one Cr as charge is transferred to the other, followed by heterolytic cleavage of the coordinated H2 between the first Cr and the O of a carbonyl ligand on the second Cr, and eventual isomerization of the resulting O-protonated intermediate to CpCr(CO)3H.