Irradiation of FeH2(dmpe)2 [dmpe = 1,2-bis(dimethylphosphino)ethane] in a solution of methane in liquefied xenon at -100-degrees-C affords a mixture of cis-FeH(dmpe)2Me and trans-FeH(dmpe)2Me; the cis and trans isomers equilibrate photochemically at -100-degrees-C but decompose thermally with elimination of methane at temperatures above ca. 0-degrees-C following solvent exchange with toluene.