The bis(triphenylphosphine)iminium (PPN+) salts of HFeM(CO)9-(M = Cr, Mo. W) were shown to be olefin isomerization catalysts under mild conditions (25 C, fluorescent lighting) toward the conversion of allylbenzene to cis and trans-propenylbenzenes and 1-hexene to internal olefins. When DFeM(CO)9- was used as catalyst, deuterium label was incorporated into the isomerized products. The olefin isomerization capability of HFe(CO)4- was reviewed. The HFe(CO)4- anion is inactive in THF solution with PPN+ as counterion, but activity may be generated in the presence of cocatalysts such as BF3 and Ph3C+ (via a 1, 3-shift mechanism) or alkali cations, Na+ and Li+ (via a reversible Fe-H addition mechanism). Evidence is presented which supports the action of the group 6 metal carbonyl fragment, M(CO)5, to be similar to that of the alkali cations in promoting catalysis on the Fe-H- center. 1985, American Chemical Society. All rights reserved.