Activation parameters in flash photolysis studies of Mo(CO) 6

Abstract

Reported is a combined time-resolved optical (TRO) and infrared (TRIR) spectroscopic investigation of the flash photolysis of Mo(CO) 6 in cyclohexane solution. TRIR studies using 308 nm excitation led to transient bleaching of the strong ν CO band at 1987 cm −1 of Mo(CO) 6 and appearance of new bands at 1931 and 1964 cm −1 attributed to Mo(CO) 5(Sol). Using a high pressure/variable temperature flow cell, the kinetics of back reaction with CO ( k CO) to regenerate the hexacarbonyl was studied over the P CO range 1–20 atm and at five temperatures. These data gave k CO=4.6±0.2×10 6 M −1 s −1 (298 K) and the activation parameters ΔH CO ‡ =32.6±3 kJ/mol and ΔS CO ‡ =−7.3±11 J mol −1 K −1 from which an interchange mechanism was proposed. The analogous species seen in the TRO experiment displayed a transient absorbance at 420 nm and analogous kinetics properties although at lower P CO self-trapping with Mo(CO) 6 (to give Mo 2(CO) 11) is a competitive process. The Mo(CO) 5(Sol) transient could also be trapped by n PrBr ( k RBr=5.3±0.7×10 7 M −1 s −1).