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Carbynes

Carbyne is often a general term for any compound whose molecular structure includes an electrically neutral carbon atom with three non-bonded electrons, connected to another atom by a single bond. A carbyne has the general formula R-C3•, where R is any monovalent group and the superscript 3• indicates the three unbounded valences. Carbynes are named after the simplest such compound, HC3•, the methylidyne radical or (unsubstituted) carbyne.

Electronic configuration

Carbyne molecules are generally found to be in electronic doublet states: the nonbonding electrons on carbon are arranged as one radical (unpaired electron) and one electron pair, leaving a vacant atomic orbital, rather than being a tri-radical (the quartet state). The simplest case is the CH radical, which has an electron configuration222 1π.[2] Here the 1σ molecular orbital is essentially the carbon 1s atomic orbital, and the 2σ is the C-H bonding orbital formed by overlap of a carbon s-p hybrid orbital with the hydrogen 1s orbital. The 3σ is a carbon non-bonding orbital pointing along the C-H axis away from the hydrogen, while there are two non-bonding 1π orbitals perpendicular to the C-H axis. However the 3σ is an s-p hybrid which has lower energy than the 1π orbital which is pure p, so the 3σ is filled before the 1π.

 

         

Figure: doublet (1 radical, 1 pair, 1 vacant orbital) (lefT) quartet (3 radicals)
 

The CH radical is in fact isoelectronic with the nitrogen atom which does have three unpaired electrons in accordance with Hund's rule of maximum multiplicity. However the N atom has three degenerate p orbitals, in contrast to the CH radical where hybridization of one orbital (the 3σ) leads to an energy difference.

References

  1. IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006–) "carbynes".
  2. Ruzsicska, B. P.; Jodhan, A.; Choi; H. K. J., Strausz, O. P.; Bell, T. N. (1983). "Chemistry of carbynes: reaction of CF, CCl, and CBr with alkenes". J. Am. Chem. Soc. 105: 2489–2490. doi:10.1021/ja00346a072.
  3. Furno, F.; Fox, T.; Berke, H. "trans-W(CMes)(dmpe)2H: a W+ H ion pair". EURO-HYDRIDES 2000.
  4. Details of their reactivity and that of the related carbenes is shown at http://www.thieme-chemistry.com/thieme-chemistry/sos/info/include/pdf/sc02.pdf.
  5. Ernst Otto Fischer, Gerhard Kreis, Cornelius G. Kreiter, Jorn Miiller, Gottfried Huttner, and Hans Lorenz (1973), trans-Halogeno[alkyl(aryl)carbyne]tetracarbonyl complexes of chromium, molybdenum, and tungsten - a new class of compounds having a transition metal-carbon triple bond. Angewandte Chemie International Edition in English, volume 12, issue 7, 564--565. doi:10.1002/anie.197305641
  6. Jaeger, M.; Stumpf, R.; Troll, C.; Fischer, H. (2000). "Novel hepta-coordinated molybdenum(II) and tungsten(II) carbene complexes by oxidative decarbonylation of Mo(0) and W(0) carbene complexes". Chem. Commun.: 931–932. doi:10.1039/B002228O.

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