text: Some metal carbonyls are prepared by the reduction of metal halides in the presence of high pressure of carbon monoxide. A variety of reducing agents are employed, including copper, aluminum, hydrogen, as well as metal alkyls such as triethylaluminium. Illustrative is the formation of chromium hexacarbonyl from anhydrous chromium(III) chloride in benzene with aluminum as a reducing agent, and aluminum chloride as the catalyst:
  :CrCl + Al + 6 CO → Cr(CO) + AlCl
  The use of metal alkyls, such as triethylaluminium and diethylzinc, as the reducing agent leads to the oxidative coupling of the alkyl radical to form the dimer alkane:
  :WCl + 6 CO + 2 Al(CH) → W(CO) + 2 AlCl + 3 CH
  Tungsten, molybdenum, manganese, and rhodium salts may be reduced with lithium aluminium hydride. Vanadium hexacarbonyl is prepared with sodium as a reducing agent in chelating solvents such as diglyme.
  :VCl + 4 Na + 6 CO + 2 diglyme → Na(diglyme)[V(CO)] + 3 NaCl
  :[V(CO)] + H → H[V(CO)] →  H + V(CO)
  In the aqueous phase, nickel or cobalt salts can be reduced, for example by sodium dithionite. In the presence of carbon monoxide, cobalt salts are quantitatively converted to the tetracarbonylcobalt(−1) anion:
  :Co +   + 6 OH + 4 CO →  + 3  + 3 HO
  Some metal carbonyls are prepared using CO directly as the reducing agent. In this way, Hieber and Fuchs first prepared dirhenium decacarbonyl from the oxide:
  :ReO + 17 CO → Re(CO) + 7 CO
  If metal oxides are used carbon dioxide is formed as a reaction product. In the reduction of metal chlorides with carbon monoxide phosgene is formed, as in the preparation of osmium carbonyl chloride from the chloride salts. Carbon monoxide is also suitable for the reduction of sulfides, where carbonyl sulfide is the byproduct.