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

The first systematic TRIR study of the photolysis of M(CO) 6 in supercritical Ar, Kr, Xe, and CO 2 permits the observation of M(CO) 5 L (M = Cr, Mo, and W; L = Ar (W only), Kr, Xe, and CO 2 ). The second-order rate constants for the reaction of M(CO) 5 L with CO have been evaluated and the reactivity for each metal is Kr > Xe ≈ CO 2 . For M(CO) 5 Kr, M(CO) 5 Xe, or M(CO) 5 (CO 2 ) the reactivity is Cr ≈ Mo > W. In supercritical Kr doped with either Xe or CO 2, the M(CO) 5 moiety interacts with Xe or CO 2 in preference to Kr. The effect of solvent density on the rate of the reaction of W(CO) 5 (CO 2 ) 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 (CO 2 ) with CO in scCO 2 is predominantly a dissociative process. The activation energies for the reaction of W(CO) 5 Xe and W(CO) 5 (CO 2 ) with CO and the relative wavelength of the visible absorption maxima for Cr(CO) 5 Xe and Cr(CO) 5 (CO 2 ) all indicate a similar strength of interaction for Xe and CO 2 with the M(CO) 5 moiety.