![]() Because of the multiple bond character of the M–CO linkage, the distance between the metal and carbon atom is relatively short, often less than 1.8 Å, about 0.2 Å shorter than a metal– alkyl bond. As electrons from the metal fill the π-antibonding orbital of CO, they weaken the carbon–oxygen bond compared with free carbon monoxide, while the metal–carbon bond is strengthened. The latter kind of binding requires that the metal have d-electrons, and that the metal is in a relatively low oxidation state (0 or +1) which makes the back-donation of electron density favorable. A pair of pi (π) bonds arises from overlap of filled d-orbitals on the metal with a pair of π*- antibonding orbitals projecting from the carbon atom of the CO. A sigma (σ) bond arises from overlap of the nonbonding (or weakly anti-bonding) sp-hybridized electron pair on carbon with a blend of d-, s-, and p-orbitals on the metal. The M-C bonding has three components, giving rise to a partial triple bond. Structure and bonding ĭiagram showing synergic π backbonding in transition metal carbonylsĬarbon monoxide bonds to transition metals using "synergistic pi* back-bonding". ![]() Less common are bonding modes in which both C and O bond to the metal, such as μ 3 η 2. These ligands are denoted μ 3-CO and μ 4-CO. ![]() In certain higher nuclearity clusters, CO bridges between three or even four metals. This bonding mode is observed in the commonly available metal carbonyls: Co 2(CO) 8, Fe 2(CO) 9, Fe 3(CO) 12, and Co 4(CO) 12. In the most common bridging mode, denoted μ 2 or simply μ, the CO ligand bridges a pair of metals. The carbonyl ligand engages in a wide range of bonding modes in metal carbonyl dimers and clusters. More commonly only carbon is bonded, in which case the hapticity is not mentioned. In η 2-CO complexes, both the carbon and oxygen are bonded to the metal. They differ in terms of their hapticity, denoted η, and their bridging mode. Ĭarbon monoxide has distinct binding modes in metal carbonyls. Complexes with different metals but only one type of ligand are called isoleptic. Polynuclear metal carbonyls are formed from metals with odd atomic numbers and contain a metal–metal bond. Except vanadium hexacarbonyl, only metals with even atomic number, such as chromium, iron, nickel, and their homologs, build neutral mononuclear complexes. Mononuclear metal carbonyls contain only one metal atom as the central atom. These complexes may be homoleptic, containing only CO ligands, such as nickel tetracarbonyl (Ni(CO) 4), but more commonly metal carbonyls are heteroleptic and contain a mixture of ligands. The carbon monoxide ligand may be bound terminally to a single metal atom or bridging to two or more metal atoms. They occur as neutral complexes, as positively-charged metal carbonyl cations or as negatively charged metal carbonylates. 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. 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 oxygen. In organometallic chemistry, metal carbonyls serve as precursors for the preparation of other organometallic complexes. In the Mond process, nickel tetracarbonyl is used to produce pure nickel. Metal carbonyls are useful in organic synthesis and as catalysts or catalyst precursors in homogeneous catalysis, such as hydroformylation and Reppe chemistry. Metal carbonyls are coordination complexes of transition metals with carbon monoxide ligands. Sample of iron pentacarbonyl, an air-stable liquid.
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