Mechanism of displacement of alkanes from photogenerated (alkane) Cr(CO)5 complexes

The kinetics and mechanism of displacement of n-heptane from (n-heptane)Cr(CO)(5) generated via pulsed laser flash photolysis in the presence of trapping nucleophiles, piperidine, tetrahydrofuran, pyridine, acetonitrile, 2-picoline, ethanol, l-hexene, chlorobenzene and 2,6-lutidine) have been studied. Activation parameters suggest that the reactions take place via concurrent interchange and dissociative mechanisms, with the latter prominent where L-s is more sterically demanding. The lack of correlation between rate constants and activation parameters is interpreted to mean that for the dissociative pathway there is significant discrimination among the nucleophiles by Cr(CO)(5) produced via dissociation of n-heptane from (n-heptane)Cr(CO)(5). The results are discussed in terms of time-resolved photocalorimetric experiments reported for closely-related systems.