Solvent study of the kinetics of molybdenum radical self-termination

The kinetics of (n-butylCp)Mo(CO)(3) (n-butylCp is n-butyl-eta (5)-cyclopentadienyl) radical self-termination to form a nonequilibrium mixture of trans and gauche-[(n-butylCp)Mo(CO)(3)](2) and the kinetics of the gauche-to-trans isomerization have been determined in the liquid solvents n-heptane, tetrahydrofuran, xenon (350 bar), and CO2 (350 bar) at 283 K by step-scan FTIR. spectroscopy. The overall rate constant for the disappearance, 2k(R), of the (n-butylCp)Mo(CO)(3) radical increases with decreasing solvent viscosity as expected, except in CO2, which is anomalously slower. The slower overall termination rate in liquid CO2 is consistent with the formation of a transient molybdenum radical-CO2 complex. The observed overall rate constants for (n-butylCp)Mo(CO)(3) self-termination, 2k(R), are (7.9 +/- 0.5) x 10(9) M-1 s(-1) in xenon; (3.2 +/- 0.5) x 10(9) M-1 s(-1) in heptane; (2.2 +/- 0.8) x 10(9) M-1 s(-1) in THF; and (1.7 +/- 0.5) x 10(9) M-1 s(-1) in CO2. The first determinations of the radical self-termination-to-gauche rate constants, k(G), are presented. The values of K-G are much slower than the corresponding recombination to trans, k(T), reflecting a steric contribution to the rate. The rate of isomerization (rotation about the molydenum-molybdenum bond) from gauche to trans is unaffected by the solvent and is 3 times faster than the reported isomerization rate for the nonsubstituted [CpMo(CO)(3)](2) molecule.