 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.Photolysis or thermolysis of mononuclear carbonyls generates bi- and multimetallic carbonyls such as diiron nonacarbonyl (Fe2(CO)9).