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)-cyclopen tadienyl) radical self-termination to form a nonequilibrium mixture of tran s and gauche-[(n-butylCp)Mo(CO)(3)](2) and the kinetics of the gauche-to-tr ans isomerization have been determined in the liquid solvents n-heptane, te trahydrofuran, xenon (350 bar), and CO2 (350 bar) at 283 K by step-scan FTI R. spectroscopy. The overall rate constant for the disappearance, 2k(R), of the (n-butylCp)Mo(CO)(3) radical increases with decreasing solvent viscosi ty as expected, except in CO2, which is anomalously slower. The slower over all termination rate in liquid CO2 is consistent with the formation of a tr ansient 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 con stants, k(G), are presented. The values of K-G are much slower than the cor responding recombination to trans, k(T), reflecting a steric contribution t o the rate. The rate of isomerization (rotation about the molydenum-molybde num 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.