in tetrahydrofuran suspn. of Et3P7 and Fe2(CO)9 (molar ratio 1:1) in THF is stirred for 10 min; soln. is concd., THF is distd. off at reduced pressure, residue is dissolved in n-pentane, column chromy. (silicagel, inert atmosphere), yellow fraction is collected, concd., cooled at -30°C, crystn.;