Highly Reduced Organometallics. 9. Synthesis and Characterization of the Tetrasodium Tetracarbonylmetalates(4-) of Chromium, Molybdenum, and Tungsten, Na₄M(CO)₄: Their Reactions with Weak Acids To Generate H₂M₂(CO)₈^2- (M = Cr, Mo, and W)

Alkali metal reductions of M(CO)₄(TMED), TMED = N,N,N′,N′-tetramethylethylenediamine, in liquid ammonia provide high yields of the “super-reduced” species Na₄M(CO)₄ which contain chromium, molybdenum, and tungsten in their lowest known formal Oxidation states. These substances have been isolated as somewhat shock-sensitive yellow to orange solids which give acceptable elemental analyses for the proposed formulations. Treatment of liquid ammonia slurries of Na₄M(CO)₄ with 4 equiv of NH₄C1 produces good yields (50-80%) of the corresponding M(CO)₄(NH₃)₂, while 2.5 equiv of NH₄Cl or excess acetonitrile gives 20-50% yields of H₂M₂(CO)₈^2- which have been isolated as Et₄N⁺ or (Ph₃P)₂N⁺ salts. The chromium and molybdenum dianions are new species and are characterized by their elemental compositions and infrared and ¹H NMR spectra. The first well-defined reactions of H₂W₂(CO)₈^2- with nucleophiles are reported. New Compounds formed in these reactions are [Et₄N]₂[W₂(CO)₈(PMe₃)₂], [Et₄N]₂[W₂(CO)₈(PMe₂Ph)₂], [Et₄N]₂[W₂(CO)₈(P(OMe)₃)₂], and K₂H₂W(CO)₄. The former are the first reported bisphosphine-substituted derivatives of M₂(CO)₁₀^2- dianions, while H₂W(CO)₄^2- is formally a diprotonated derivative of W(CO)₄^4-. High yields (70%) of H₂W(CO)₄^2- are also obtained by the reaction of W(CO)₄(TMED) with excess K[sec-Bu₃BH] in THF. Treatment of W₂(CO)₈L^22- with water provides the hydride anions, HW₂(CO)₈L^2-, which are isolated as Et₄N⁺ salts and characterized by elemental analyses and infrared and ¹H NMR spectra.