Editing Carbon monoxide (section)

==Physical and chemical properties==
Carbon monoxide is the simplest [[oxocarbon]] and is [[isoelectronic]] with other triply bonded [[diatomic]] species possessing 10 valence electrons, including the [[cyanide]] anion, the [[nitrosonium]] cation, [[boron monofluoride]] and molecular [[nitrogen]]. It has a [[molar mass]] of 28.0, which, according to the [[ideal gas law]], makes it slightly less dense than air, whose average molar mass is 28.8.

The carbon and oxygen are connected by a [[triple bond]] that consists of a net two [[pi bond]]s and one [[sigma bond]]. The [[bond length]] between the carbon atom and the oxygen atom is 112.8&nbsp;[[picometre|pm]].<ref name="gilliam">{{Cite journal|author1=Gilliam, O. R. |author2=Johnson, C. M. |author3=Gordy, W. |title=Microwave Spectroscopy in the Region from Two to Three Millimeters|year=1950|journal=[[Physical Review]]|volume=78|issue=2|pages=140–144|doi=10.1103/PhysRev.78.140|bibcode = 1950PhRv...78..140G }}</ref><ref>{{CRC91|page=9–33}}</ref> This bond length is consistent with a triple bond, as in molecular nitrogen ({{chem2|N2}}), which has a similar bond length (109.76&nbsp;pm) and nearly the same [[molecular mass]]. Carbon–oxygen double bonds are significantly longer, 120.8&nbsp;pm in [[formaldehyde]], for example.<ref>{{CRC91|page=9–39}}</ref> The boiling point (82&nbsp;K) and melting point (68&nbsp;K) are very similar to those of {{chem2|N2}} (77&nbsp;K and 63&nbsp;K, respectively). The [[bond-dissociation energy]] of 1072&nbsp;kJ/mol is stronger than that of {{chem2|N2}} (942&nbsp;kJ/mol) and represents the strongest chemical bond known.<ref>[http://www.wiredchemist.com/chemistry/data/bond_energies_lengths.html Common Bond Energies (D) and Bond Lengths (r)]. wiredchemist.com</ref>

The [[ground state|ground]] [[electronic state]] of carbon monoxide is a [[singlet state]]<ref>{{cite web|title=Highly Excited Triplet States of Carbon Monoxide| first=C. R. |last=Vidal|date=28 June 1997 |url=http://www.mpe.mpg.de/lab/CO/co.html |archive-url=https://web.archive.org/web/20060828201131/http://www.mpe.mpg.de/lab/CO/co.html|archive-date=2006-08-28|access-date= August 16, 2012}}</ref> since there are no unpaired electrons.

{| class="wikitable mw-collapsible"
|+
Thermal and physical properties of carbon monoxide (CO) at atmospheric pressure<ref>{{Cite book |last=Holman |first=Jack P. |url=https://www.worldcat.org/oclc/46959719 |title=Heat transfer |publisher=McGraw-Hill Companies, Inc. |year=2002 |isbn=9780072406559 |edition=9th |location=New York, NY |pages=600–606 |language=English |oclc=46959719}}</ref><ref>{{Cite book |last=Incropera 1 Dewitt 2 Bergman 3 Lavigne 4 |first=Frank P. 1 David P. 2 Theodore L. 3 Adrienne S. 4 |url=https://www.worldcat.org/oclc/62532755 |title=Fundamentals of heat and mass transfer. |publisher=John Wiley and Sons, Inc. |year=2007 |isbn=9780471457282 |edition=6th |location=Hoboken, NJ |pages=941–950 |language=English |oclc=62532755}}</ref>
!Temperature (°C) || Temperature (K) || Density (kg/m<sup>3</sup>) || Specific heat (J/g&nbsp;°C) || Dynamic viscosity (cg/m&nbsp;s) || Kinematic viscosity (cm<sup>2</sup>/s) || Thermal conductivity (cW/m&nbsp;°C) || Thermal diffusivity (cm<sup>2</sup>/s) || [[Prandtl number]]
|-
| −73.15
|200||1.6888 ||1.045||1.27||0.0752||1.7 ||0.0963||0.781
|-
| −53.15
|220||1.5341 ||1.044||1.37||0.0893||1.9 ||0.119 ||0.753
|-
| −33.15
|240||1.4055 ||1.043||1.47||0.105 ||2.06||0.141 ||0.744
|-
| −13.15
|260||1.2967 ||1.043||1.57||0.121 ||2.21||0.163 ||0.741
|-
|6.85
|280||1.2038 ||1.042||1.66||0.138 ||2.36||0.188 ||0.733
|-
|26.85
|300||1.1233 ||1.043||1.75||0.156 ||2.5 ||0.213 ||0.73
|-
|46.85
|320||1.0529 ||1.043||1.84||0.175 ||2.63||0.239 ||0.73
|-
|66.85
|340||0.9909 ||1.044||1.93||0.195 ||2.78||0.269 ||0.725
|-
|86.85
|360||0.9357 ||1.045||2.02||0.216 ||2.91||0.298 ||0.725
|-
|106.85
|380||0.8864 ||1.047||2.1 ||0.237 ||3.05||0.329 ||0.729
|-
|126.85
|400||0.8421 ||1.049||2.18||0.259 ||3.18||0.36  ||0.719
|-
|176.85
|450||0.7483 ||1.055||2.37||0.317 ||3.5 ||0.443 ||0.714
|-
|226.85
|500||0.67352||1.065||2.54||0.377 ||3.81||0.531 ||0.71
|-
|276.85
|550||0.61226||1.076||2.71||0.443 ||4.11||0.624 ||0.71
|-
|326.85
|600||0.56126||1.088||2.86||0.51  ||4.4 ||0.721 ||0.707
|-
|376.85
|650||0.51806||1.101||3.01||0.581 ||4.7 ||0.824 ||0.705
|-
|426.85
|700||0.48102||1.114||3.15||0.655 ||5  ||0.933 ||0.702
|-
|476.85
|750||0.44899||1.127||3.29||0.733 ||5.28||1.04  ||0.702
|-
|526.85
|800||0.42095||1.14 ||3.43||0.815 ||5.55||1.16  ||0.705
|}

===Bonding and dipole moment===
The strength of the {{chem2|C\tO}} bond in carbon monoxide is indicated by the high frequency of its vibration, 2143&nbsp;cm<sup>−1</sup>.<ref name=nuCO>{{cite journal |doi=10.1139/v62-306 |title=Infrared Spectra of Carbon Monoxide and Carbon Dioxide Adsorbed on Chromia–Alumina and on Alumina |date=1962 |last1=Little |first1=L. H. |last2=Amberg |first2=C. H. |journal=Canadian Journal of Chemistry |volume=40 |issue=10 |pages=1997–2006 }}</ref> For comparison, organic carbonyls such as ketones and esters absorb at around 1700&nbsp;cm<sup>−1</sup>.

Carbon and oxygen together have a total of 10 [[electrons]] in the [[valence shell]]. Following the [[octet rule]] for both carbon and oxygen, the two atoms form a [[triple bond]], with six shared electrons in three bonding molecular orbitals, rather than the usual double bond found in organic carbonyl compounds. Since four of the shared electrons come from the oxygen atom and only two from carbon, one bonding orbital is occupied by two electrons from oxygen, forming a dative or [[dipolar bond]]. This causes a C←O [[Chemical polarity|polarization]] of the molecule, with a small negative charge on carbon and a small positive charge on oxygen. The other two bonding orbitals are each occupied by one electron from carbon and one from oxygen, forming (polar) covalent bonds with a reverse C→O polarization since oxygen is more [[electronegativity|electronegative]] than carbon. In the free carbon monoxide molecule, a net negative charge δ<sup>−</sup> remains at the carbon end and the molecule has a small [[bond dipole moment|dipole moment]] of 0.122&nbsp;[[Debye|D]].<ref>{{cite journal | last1=Scuseria | first1=Gustavo E. | last2=Miller | first2=Michael D. | last3=Jensen | first3=Frank | last4=Geertsen | first4=Jan |title=The dipole moment of carbon monoxide|journal=J. Chem. Phys. |volume=94 |issue= 10 | pages=6660 |year=1991 |doi=10.1063/1.460293|bibcode = 1991JChPh..94.6660S }}</ref>

The molecule is therefore asymmetric: oxygen is more electron dense than carbon and is also slightly positively charged compared to carbon being negative. 
[[File:Carbon-monoxide-resonance-2D.png|thumb|The most important resonance form of carbon monoxide is {{chem2|-C\tO+}}. An important minor resonance contributor is the non-octet carbenic structure :C=O.]]
Carbon monoxide has a computed fractional bond order of 2.6, indicating that the "third" bond is important but constitutes somewhat less than a full bond.<ref>{{Cite journal|last1=Martinie|first1=Ryan J.|last2=Bultema|first2=Jarred J.|last3=Vander Wal|first3=Mark N.|last4=Burkhart|first4=Brandon J.|last5=Vander Griend|first5=Douglas A.|last6=DeKock|first6=Roger L.|s2cid=11905354|date=2011-08-01|title=Bond Order and Chemical Properties of BF, CO, and N2|journal=Journal of Chemical Education|volume=88|issue=8|pages=1094–1097|doi=10.1021/ed100758t|issn=0021-9584|bibcode=2011JChEd..88.1094M}}</ref>  Thus, in valence bond terms, {{chem2|-C\tO+}} is the most important structure, while :C=O is non-octet, but has a neutral formal charge on each atom and represents the second most important resonance contributor.  Because of the lone pair and divalence of carbon in this resonance structure, carbon monoxide is often considered to be an extraordinarily stabilized [[carbene]].<ref>{{Cite book|title=Cumulenes in click reactions|last=Ulrich|first=Henri|date=2009|publisher=Wiley|others=Wiley InterScience (Online service)|isbn=9780470747957|location=Chichester, U.K.|pages=45|oclc=476311784}}</ref>  [[Isocyanide]]s are compounds in which the O is replaced by an NR (R = alkyl or aryl) group and have a similar bonding scheme.

If carbon monoxide acts as a [[Metal carbonyl|ligand]], the polarity of the dipole may reverse with a net negative charge on the oxygen end, depending on the structure of the [[coordination complex]].<ref>{{cite journal | last1 = Lupinetti | first1 = Anthony J. | last2 = Fau | first2 = Stefan | last3 = Frenking | first3 = Gernot | last4 = Strauss | first4 = Steven H. | title = Theoretical Analysis of the Bonding between CO and Positively Charged Atoms| journal = J. Phys. Chem. A | year= 1997 |volume= 101 |issue=49 |pages= 9551–9559 |doi=10.1021/jp972657l|bibcode= 1997JPCA..101.9551L }}</ref>
See also the section ''[[#Coordination_chemistry|"Coordination chemistry"]]'' below.

===Bond polarity and oxidation state===
Theoretical and experimental studies show that, despite the greater electronegativity of oxygen, the dipole moment points from the more-negative carbon end to the more-positive oxygen end.<ref>{{cite journal | last1 = Blanco | first1 = Fernando | last2 = Alkorta | first2 = Ibon | last3 = Solimannejad | first3 = Mohammad | last4 = Elguero | first4 = Jose | title = Theoretical Study of the 1:1 Complexes between Carbon Monoxide and Hypohalous Acids | journal = J. Phys. Chem. A |year= 2009 |volume= 113 |issue=13 | pmid = 19275137 |pages= 3237–3244 |doi = 10.1021/jp810462h |bibcode = 2009JPCA..113.3237B | hdl = 10261/66300 | hdl-access = free }}</ref><ref>{{cite journal | last1 = Meerts | first1 = W | title = Electric and magnetic properties of carbon monoxide by molecular-beam electric-resonance spectroscopy | journal = Chemical Physics | volume =22 | issue =2 |date= 1 June 1977 | pages =319–324 |doi=10.1016/0301-0104(77)87016-X|bibcode = 1977CP.....22..319M | last2 = De Leeuw | first2 = F.H. | last3 = Dymanus | first3 = A. }}</ref> The three bonds are in fact [[polar covalent bond]]s that are strongly polarized. The calculated polarization toward the oxygen atom is 71% for the [[sigma bond|σ-bond]] and 77% for both [[pi bond|π-bonds]].<ref name="Stefan">{{cite journal | last1 = Stefan | first1 = Thorsten | last2 = Janoschek | first2 = Rudolf | title = How relevant are S=O and P=O Double Bonds for the Description of the Acid Molecules H<sub>2</sub>SO<sub>3</sub>, H<sub>2</sub>SO<sub>4</sub>, and H<sub>3</sub>PO<sub>3</sub>, respectively? | journal= Journal of Molecular Modeling | volume =6 | issue=2 |year=2000 |pages=282–288 |doi=10.1007/PL00010730| s2cid = 96291857 }}</ref>

The [[oxidation state]] of carbon in carbon monoxide is +2 in each of these structures. It is calculated by counting all the bonding electrons as belonging to the more electronegative oxygen. Only the two non-bonding electrons on carbon are assigned to carbon. In this count, carbon then has only two valence electrons in the molecule compared to four in the free atom.