User:Linksfuss/Metal carbonyl

Nomenclature and terminology
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.

The number of carbon monoxide ligands in a metal carbonyl complex is described by a Greek numeral, followed by the word carbonyl. A complex containing only carbon monoxide as ligands will be referred to as a pure or homoleptic metal carbonyls. If the complexes contain other ligands besides carbon monoxide, the complexes are called mixed or heteroleptic metal carbonyls. Carbon monoxide has different binding modes in metal carbonyls on. They differ in the hapticity and the bridging mode. The hapticity describes the number of carbon monoxide atoms, 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 coordination dihapto coordination η2.

The carbonyl ligand engages in a range of bonding modes in metal carbonyl dimers and clusters. In the most common bridging mode, the CO ligand bridges a pair of metals. This bonding mode is observed in the commonly available metal carbonyls: Co2(CO)8, Fe2(CO)9, Fe3(CO)12, and Co4(CO)12. In certain higher nuclearity clusters, CO bridges between three or even four metals. These ligands are denoted μ3-CO and μ4-CO. Less common are bonding modes in which both C and O bond to the metal, e.g. μ3-η2.


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