INTERMEDIATES RELEVANT TO THE CARBONYLATION OF MANGANESE ALKYL COMPLEXES INTERROGATED BY TIME-RESOLVED INFRARED AND OPTICAL SPECTROSCOPY

Intermediates relevant to the carbonylation of metal alkyl complexes h ave been generated by laser flash photolysis of the manganese acyl com plexes RC(O)Mn(CO)(5) (R = CH3, CD3, CH2CH3, CH2F, CF3). This results in immediate CO photodissociation to give intermediate acyl complexes which were observed by time resolved infrared (TRIR) and time resolved optical (TRO) spectroscopy. In the presence of added ligands, such in termediates are trapped to form stable cis-substituted octahedral comp lexes in competition with alkyl migration from the acyl group to give the alkyl pentacarbonyl complexes RMn(CO)(5). The spectra and reactivi ty of the intermediate (I) derived from CH3C(O)Mn(CO)(5) (A) indicate that this exists as the chelated acyl complex (eta(2)-CH3CO)Mn(CO)(4) in weakly coordinating solvents such as cyclohexane but as the solvent o species cis-CH3C(O)Mn(CO)(4)(THF) in tetrahydrofuran. Comparisons wi th thermal reaction kinetics support the assertion that the intermedia tes generated photochemically are indeed relevant to understanding the mechanism for CH3Mn(CO)(5) carbonylation. The CF3 and CH2F analogs of I are much more reactive than I in cyclohexane solution, and this has been interpreted in terms of the eta(2)-acyl configuration being dest abilized by these electron-withdrawing groups. Solvent effects on the rates of methyl migration and ligand trapping reactions of the interme diate species are described and analyzed in terms of their relevance t o the migratory insertion mechanism.