Reductions of metal carbonyls by quaternary ammonium borohydrides

Quaternary ammonium borohydrides, used directly or generated in phase transfer reactions, are highly effective reagents for preparing metal carbonyl anions from metal carbonyls Mo(CO)₆, Mn₂(CO)₁₀, Re₂(CO)₁₀, CO₂(CO)₈, Fe₃(CO)₁₂, Ru₃(CO)₁₂ and (η⁵-C₅H₅)₂Mo₂(CO)₆] and from some metal carbonyl halides BrMn(CO)₅ and η⁵-C₅H₅Mo(CO)₃Cl]. Where strongly basic anions would be formed from a halide BrMn(CO)₄PPh₃ and η⁵-C₅H₅Ru(CO)₂Br], the reactions provide efficient syntheses of the corresponding hydrides instead. The anion η⁵-C₅H₅Fe(CO)₂^? is not accessible by these techniques; reaction of η⁵-C₅H₅Fe(CO)₂Br yields the iron dimer (via the highly nucleophilic anion) and the dimer is unreactive toward Q⁺BH₄^?. Reductions of Re₂(CO)₁₀ conducted in CH₂Cl₂ provide Re₂(CO)₉Cl^? in high yield.