acetic anhydride in acetic acid acetic acid and acetic anhydride added to Mo2Br4(CO)8 at -40°C under Ar, mixt. stirred, left to warm to room temp. overnight, refluxed 4h; slowly cooled to room temp., pptn.; concn. of mixt. to 1/4 of initial volume, filtered, washed (benzene), dried (vac.);