metal carbonyl

Metal carbonyls are coordination complexes of transition metals with carbon monoxide. These complexes may be homoleptic, i.e. contain only CO ligands, such as nickel carbonyl (Ni(CO)4), but more commonly metal carbonyls contain a mix of ligands, such as Re(CO)3(2,2'-bipyridine)Cl.  Carbon monoxide is an important building block for the synthesis for many compounds, e.g. hydroformylation, and metal carbonyl catalysts are central to its utilization. Metal carbonyls are toxic, in part because of their ability to carbonylate hemoglobin to give carboxyhemoglobin, which will not bind O2.
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