ORGANOMETALLIC CO SUBSTITUTION KINETICS IN LIQUID XE BY FAST TIME-RESOLVED IR SPECTROSCOPY

The reaction of Cp*Rh(CO)Xe (Cp* = C5Me5) with CO was studied using time-resolved IR spectroscopy of liquid rare gas solutions. IR spectra for Cp*Rh(CO)Xe were obtained using pulsed UV laser photolysis of Cp*Rh-(CO)2 in liquid Xe and a rapid-scan FTIR spectrometer with 0.09-s time resolution. Assignment to the Xe complex was confirmed from the similarity of the spectra and lifetime of the complex when a mixture of Xe in liquid Kr was used. The reaction of Cp*Rh(CO)Xe with added CO is very fast, and the rate constant was measured by fast time-resolved IR spectroscopy to be (5.7 +/- 0.6) X 10(5) to (1.9 +/- 0.2) X 10(6) M-1 s-1 over the temperature range 202-242 K. The kinetics are consistent with an associative substitution mechanism with activation parameters for the bimolecular rate constant of log(A) = 8.8 +/- 0.3 (DELTAS double dagger = -20 +/- 1 cal/(mol K)) and E(a) = 2.8 +/- 0.3 kcal/mol (DELTAH double dagger = 2.4 +/- 0.3 kcal/mol).