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