Editing Carbon (section)

===Organometallic compounds===
{{Main|Organometallic chemistry}}
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]]), η{{sup|2}}-alkene compounds (for example, [[Zeise's salt]]), and η{{sup|3}}-allyl compounds (for example, [[allylpalladium chloride dimer]]); [[metallocene]]s containing cyclopentadienyl ligands (for example, [[ferrocene]]); and [[transition metal carbene complex]]es. Many [[metal carbonyl]]s and [[cyanometalate|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. [[Carborane]]s are highly stable dodecahedral derivatives of the [B<sub>12</sub>H<sub>12</sub>]<sup>2-</sup> unit, with one BH replaced with a CH<sup>+</sup>. Thus, the carbon is bonded to five boron atoms and one hydrogen atom. The cation [(Ph{{sub|3}}PAu){{sub|6}}C]{{sup|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.<ref>{{cite journal |author=Scherbaum, Franz |journal=[[Angew. Chem. Int. Ed. Engl.]] |volume=27 |issue=11 |pages=1544–1546 |date=1988 |doi=10.1002/anie.198815441 |title="Aurophilicity" as a consequence of Relativistic Effects: The Hexakis(triphenylphosphaneaurio)methane Dication [(Ph{{sub |3}}PAu){{sub|6}}C]{{sup|2+}} |display-authors=1|last2=Grohmann|first2=Andreas|last3=Huber|first3=Brigitte|last4=Krüger|first4=Carl|last5=Schmidbaur|first5=Hubert}}</ref> 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|hexamethylbenzene dication]] contains a carbon atom with six bonds. More specifically, the dication could be described structurally by the formulation [MeC(η<sup>5</sup>-C<sub>5</sub>Me<sub>5</sub>)]<sup>2+</sup>, making it an "organic [[metallocene]]" in which a MeC<sup>3+</sup> fragment is bonded to a η<sup>5</sup>-C<sub>5</sub>Me<sub>5</sub><sup>−</sup> fragment through all five of the carbons of the ring.<ref>{{cite magazine |url=http://cen.acs.org/articles/94/i49/Six-bonds-carbon-Confirmed.html?type=paidArticleContent |url-status=live |title=Six bonds to carbon: Confirmed |first=Stephen K. |last=Ritter |magazine=Chemical & Engineering News |archive-url=https://web.archive.org/web/20170109183800/http://cen.acs.org/articles/94/i49/Six-bonds-carbon-Confirmed.html?type=paidArticleContent |archive-date=2017-01-09}}</ref>

[[File:Akiba's "hypervalent carbon" compound.png|thumb|This anthracene derivative contains a carbon atom with 5 formal electron pairs around it.]]
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,<ref>{{cite journal |last1=Yamashita |first1=Makoto |last2=Yamamoto |first2=Yohsuke |last3=Akiba |first3=Kin-ya |last4=Hashizume |first4=Daisuke |last5=Iwasaki |first5=Fujiko |last6=Takagi |first6=Nozomi |last7=Nagase |first7=Shigeru |date=2005-03-01 |title=Syntheses and Structures of Hypervalent Pentacoordinate Carbon and Boron Compounds Bearing an Anthracene Skeleton − Elucidation of Hypervalent Interaction Based on X-ray Analysis and DFT Calculation |journal=Journal of the American Chemical Society |volume=127 |issue=12 |pages=4354–4371 |doi=10.1021/ja0438011 |pmid=15783218 |bibcode=2005JAChS.127.4354Y |issn=0002-7863}}</ref> 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 bond]]ing.