Organolithium reagents add with metal carbonyls to acylmetal carbonyl anions. Like many metal carbonyls, dicobalt octacarbonyl abstracts halides from alkyl halides. Soon thereafter cationic Rh and Ir were found catalyze the hydrogenation of alkenes and carbonyls. Heteroatom-containing dipolarophiles such as carbonyls and imines can also undergo 1,3-dipolar cycloaddition. Some salts of cationic and anionic metal carbonyls are soluble in water or lower alcohols. Nickel carbonyl (Ni(CO)) and other metal carbonyls present an interesting case. The most important active alkenes are the aforementioned conjugated carbonyls and acrylonitriles. The synthesis of metal carbonyls is subject of intense organometallic research. The Riley oxidation is a selenium dioxide-mediated oxidation of methylene groups adjacent to carbonyls. Similar species are known for the metal carbonyls and the early metal halides. CAN is traditionally used to release organic ligands from metal carbonyls. Carbon monoxide has distinct binding modes in metal carbonyls. The 18-electron rule is helpful in predicting the stabilities of metal carbonyls and related compounds. Most are prepared by condensation of the active methylene group with carbonyls. Another important reaction catalyzed by metal carbonyls is the hydrocarboxylation.