page_title: Carbon
section_title: Organometallic compounds
hierarchical_section_title: Carbon / Compounds / Organometallic compounds
caption_attribution_description: English: depiction of a carbon with 5 bonds around it.
context_page_description: Carbon is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon makes up only about 0.025 percent of Earth’s crust. Three isotopes occur naturally, ¹²C and ¹³C being stable, while ¹⁴C is a radionuclide, decaying with a half-life of about 5,730 years. Carbon is one of the few elements known since antiquity.
  Carbon is the 15th most abundant element in the Earth's crust, and the fourth most abundant element in the universe by mass after hydrogen, helium, and oxygen. Carbon's abundance, its unique diversity of organic compounds, and its unusual ability to form polymers at the temperatures commonly encountered on Earth enables this element to serve as a common element of all known life. It is the second most abundant element in the human body by mass after oxygen.
  The atoms of carbon can bond together in diverse ways, resulting in various allotropes of carbon. The best known allotropes are graphite, diamond, and buckminsterfullerene. The physical properties of carbon vary widely with the allotropic form.
context_section_description: Organometallic compounds by definition contain at least one carbon-metal covalent bond. A wide range of such compounds exist; major classes include simple alkyl-metal compounds (for example, tetraethyllead), η²-alkene compounds (for example, Zeise's salt), and η³-allyl compounds (for example, allylpalladium chloride dimer); metallocenes containing cyclopentadienyl ligands (for example, ferrocene); and transition metal carbene complexes. Many metal carbonyls and metal cyanides exist (for example, tetracarbonylnickel and potassium ferricyanide); some workers consider metal carbonyl and cyanide complexes without other carbon ligands to be purely inorganic, and not organometallic. However, most organometallic chemists consider metal complexes with any carbon ligand, even 'inorganic carbon' (e.g., carbonyls, cyanides, and certain types of carbides and acetylides) to be organometallic in nature. Metal complexes containing organic ligands without a carbon-metal covalent bond (e.g., metal carboxylates) are termed metalorganic compounds.
  While carbon is understood to strongly prefer formation of four covalent bonds, other exotic bonding schemes are also known. Carboranes are highly stable dodecahedral derivatives of the [B₁₂H₁₂]²⁻ unit, with one BH replaced with a CH⁺.  Thus, the carbon is bonded to five boron atoms and one hydrogen atom. The cation [(Ph₃PAu)₆C]²⁺ contains an octahedral carbon bound to six phosphine-gold fragments. This phenomenon has been attributed to the aurophilicity of the gold ligands, which provide additional stabilization of an otherwise labile species. In nature, the iron-molybdenum cofactor (FeMoco) responsible for microbial nitrogen fixation likewise has an octahedral carbon center (formally a carbide, C(-IV)) bonded to six iron atoms. In 2016, it was confirmed that, in line with earlier theoretical predictions, the hexamethylbenzene dication contains a carbon atom with six bonds.  More specifically, the dication could be described structurally by the formulation [MeC(η⁵-C₅Me₅)]²⁺, making it an "organic metallocene" in which a MeC³⁺ fragment is bonded to a η⁵-C₅Me₅⁻ fragment through all five of the carbons of the ring.
  It is important to note that in the cases above, each of the bonds to carbon contain less than two formal electron pairs.  Thus, the formal electron count of these species does not exceed an octet.  This makes them hypercoordinate but not hypervalent. Even in cases of alleged 10-C-5 species (that is, a carbon with five ligands and a formal electron count of ten), as reported by Akiba and co-workers, electronic structure calculations conclude that the electron population around carbon is still less than eight, as is true for other compounds featuring four-electron three-center bonding.
caption: This anthracene derivative contains a carbon atom with 5 formal electron pairs around it.
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