TIME-RESOLVED INFRARED SPECTRAL STUDIES OF INTERMEDIATES FORMED IN THE LASER FLASH-PHOTOLYSIS OF MN(CO)(5)CH3

Laser flash photolysis (308 nm) of Mn(CO)(5)CH3 (I) in different solve nts leads predominantly to CO photodissociation and the formation of r eactive intermediates formulated as the solvento complexes cis-Mn(CO)( 4)(solv)CH3. This conclusion is based upon the solvent sensitivity of the time-resolved infrared spectra and the dynamics of reactions with various ligands. For example, the reaction of cis-Mn(CO)(4)(solv)CH3 w ith CO displays second-order kinetics with a rate constant k(co) nearl y 8 orders of magnitude larger for solv = perfluoro-(methylcyclohexane ) than for solv = tetrahydrofuran. The k(L) values for the second-orde r reaction of cis-Mn(CO)(4)(solv)CH3 in cyclohexane with various ligan ds L follow the approximate order 4-phenylpyridine approximate to H2O approximate to THF > P(OMe)(3) approximate to PPh(3) > CO approximate to N-2. The quantitative behaviors of intermediates seen in the flash photolysis of I are compared with those seen in similar studies of the acetyl complex Mn(CO)(5)(COCH3) (II) (Organometallics, 1993, 12, 4739 -4741), and this comparison is used to substantiate the earlier conclu sion that CO dissociation from II in weakly coordinating solvents lead s to an eta(2)-acyl-coordinated intermediate rather than to a solvento complex.