Can Organometallic Noble Gas Compounds Be Observed in Solution at Room Temperature? A Time-Resolved Infrared (TRIR) and UV Spectroscopic Study of the Photochemistry of M(CO)₆ (M = Cr, Mo, and W) in Supercritical Noble Gas and CO₂ Solution

Abstract

The first systematic TRIR study of the photolysis of M(CO)6 in supercritical Ar, Kr, Xe, and CO2 permits the observation of M(CO)5L (M = Cr, Mo, and W; L = Ar (W only), Kr, Xe, and CO2). The second-order rate constants for the reaction of M(CO)5L with CO have been evaluated and the reactivity for each metal is Kr > Xe ≈ CO2. For M(CO)5Kr, M(CO)5Xe, or M(CO)5(CO2) the reactivity is Cr ≈ Mo > W. In supercritical Kr doped with either Xe or CO2, the M(CO)5 moiety interacts with Xe or CO2 in preference to Kr. The effect of solvent density on the rate of the reaction of W(CO)5(CO2) with CO has been investigated. This is the first time that the density dependence of any dissociative reaction has been followed in this way in supercritical solution. Our observations demonstrate that the reaction of W(CO)5(CO2) with CO in scCO2 is predominantly a dissociative process. The activation energies for the reaction of W(CO)5Xe and W(CO)5(CO2) with CO and the relative wavelength of the visible absorption maxima for Cr(CO)5Xe and Cr(CO)5(CO2) all indicate a similar strength of interaction for Xe and CO2 with the M(CO)5 moiety.