The chemistry of metal carbonyl anions. II. Synthesis and reactions of the pentacarbonylmetallate dianions of chromium, molybdenum, and tungsten with metallic monohalides

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)₁₀^2- in THF.