Alkali metal amalgam reductions of M(CO)6 in tetrahydrofuran (THF) have provided salts of the pentacarbonylmetallate dianions. Cesium amalgam reacts with M(CO)6 at room temperature to yield Cs2M(CO)5, while sodium or potassium amalgams provide only (Na or K)2Cr(CO)5 in good yield in addition to a substantial amount of (Na or K)2Cr2(CO)10. Molybdenum and tungsten carbonyl give primarily (Na or K)2M2(CO)10 in THF even after 24 h of reflux; however, infrared evidence indicates the presence of monomeric dianions in these solutions. Reaction of the monomeric dianions with Ph3MCl (M = Ge, Sn, Pb) provides air- and moisture-stable substituted carbonyl anions of the type Ph3MM′(CO)5- (M′ = Cr, Mo, W) which have been isolated as tetraethylammonium salts. Pure Na2Cr(CO)5, prepared from the interaction of Cr(CO)6 with sodium metal in liquid ammonia or HMPA, has also been used to provide similar substituted carbonyl anions of the type ECr(CO)5- where E = Me3Sn and C5H5Fe(CO)2. The triphenyltin derivatives have also been prepared independently by the interaction of Ph3SnLi with M′(CO)6 in THF at room temperature as well as from the reaction of Ph3SnCl with M′2(CO)102- in THF.