Carbonyl ligand

Metal carbonyls are coordination complexes of transition metals with carbon monoxide ligands. Metal carbonyls are useful in organic synthesis and as catalysts or catalyst precursors in homogeneous catalysis, such as hydroformylation and Reppe chemistry. In the Mond process, nickel carbonyl is used to produce pure nickel. In organometallic chemistry, metal carbonyls serve as precursors for the preparation of other organometalic complexes.

Metal carbonyls are toxic by skin contact, inhalation or ingestion, in part because of their ability to carbonylate hemoglobin to give carboxyhemoglobin, which prevents the binding of O₂.

The nomenclature of the metal carbonyls depends on the charge of the complex, the number and type of central atoms, and the number and type of ligands and their binding modes. They occur as neutral complexes, as positively charged metal carbonyl cations or as negatively charged metal carbonylates. The carbon monoxide ligand may be bound terminally to a single metal atom or bridging to two or more metal atoms. These complexes may be homoleptic, that is containing only CO ligands, such as nickel carbonyl (Ni(CO)₄), but more commonly metal carbonyls are heteroleptic and contain a mixture of ligands.

Mononuclear metal carbonyls contain only one metal atom as the central atom. Except vanadium hexacarbonyl only metals with even order number such as chromium, iron, nickel and their homologs build neutral mononuclear complexes. Polynuclear metal carbonyls are formed from metals with odd order numbers and contain a metal-metal bond. Complexes with different metals, but only one type of ligand will be referred to as isoleptic.

The number of carbon monoxide ligands in a metal carbonyl complex is described by a Greek numeral, followed by the word carbonyl. Carbon monoxide has different binding modes in metal carbonyls. They differ in the hapticity and the bridging mode. The hapticity describes the number of carbon monoxide ligands, which are directly bonded to the central atom. The denomination shall be made by the letter ηⁿ, which is prefixed to the name of the complex. The superscript n indicates the number of bounded atoms. In monohapto coordination, such as in terminally bonded carbon monoxide, the hapticity is 1 and it is usually not separately designated. If carbon monoxide is bound via the carbon atom and via the oxygen to the metal, it will be referred to as dihapto coordinated η².

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