 Anionic metal carbonylates can be obtained for example by reduction of dinuclear complexes with sodium.A familiar example is the sodium salt of iron tetracarbonylate (Na2Fe(CO)4, "Collman's reagent"), which is used in organic synthesis.The cationic hexacarbonyl salts of manganese, technetium and rhenium can be prepared from the carbonyl halides under carbon monoxide pressure by reaction with a Lewis acid.