Persistent carbene
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[[Image:Stable_carbene3.jpg|thumb|A stable carbene: isolated 1,3-dimesitylimidazol-2-ylidene in a [[Schlenk flask]]]]
A '''persistent carbene''' (also known as a stable carbene) is a type of [[carbene]] demonstrating particular stability despite also being a [[reactive intermediate]]. The ''instability'' in these carbenes involves reactivity with [[Substrate (chemistry)|substrate]]s, or [[dimerisation]] (see [[Persistent carbene#Wanzlick equilibrium|Wanzlick equilibrium]]). Persistent carbenes can exist in the [[Diradical|singlet state]] or the [[triplet state]], with the singlet state carbenes being more stable.
The field of stable carbene research was awakened in 1991 with a landmark discovery by the research group of [[Anthony J. Arduengo, III]].<ref name=Arduengo1>{{cite journal
| author = A. J. Arduengo, R. L. Harlow and M. Kline
| title = A stable crystalline carbene
| year = 1991
| journal = [[J. Am. Chem. Soc.]]
| volume = 113
| issue = 1
| pages = 361–363
| doi = 10.1021/ja00001a054}}</ref>, which managed to isolate and obtain an X-ray structure of the stable carbene N,N'-diadamantyl-imidazol-2-ylidene:
[[Image:Arduengo2.png|center|thumb|600px|Preparation of N,N'-diadamantyl-imidazol-2-ylidene]]
However, prior to this isolation, persistent carbenes had been proposed to exist by [[Ronald Breslow]] in 1957.<ref name=Breslow2>{{cite journal
| title = Mechanism of Thiamine Action: Participation of a Thiazolium Zwitterion
| author = [[Ronald Breslow]]
| journal = Chem. and Ind.
| volume =
| issue =
| pages = 893
| year = 1957
| url =
| doi = }}</ref><ref name=Breslow3>{{cite journal
| title = Rapid Deuterium Exchange inThiazolium Salts
| author = R. Breslow
| journal = [[Journal of the American Chemical Society]]
| volume = 79
| issue = 7
| pages = 1762–1763
| year = 1957
| url =
| doi = 10.1021/ja01564a064 }}</ref> The [[Hans-Werner Wanzlick]] group <ref name=Wanzlick1>{{cite journal
| title = Ein neuer Zugang zur Carben-Chemie
| author = [[Hans-Werner Wanzlick]] and E. Schikora
| journal = [[Angewandte Chemie]]
| volume = 72
| issue = 14
| pages = 494
| year = 1960
| url =
| doi = 10.1002/ange.19600721409 }}</ref><ref name=Wanzlick2>{{cite journal
| title = Ein nucleophiles Carben
| author = H. W. Wanzlick and E. Schikora
| journal = [[Chemische Berichte]]
| volume = 94
| issue = 9
| pages = 2389–2393
| year = 1960
| url =
| doi = 10.1002/cber.19610940905 }}</ref> were the first group to make (but not isolate) a stable carbene. In 1989 [[Guy Bertrand (chemist)|Guy Bertrand]]'s group <ref name=Bertrand4>{{cite journal
| title = Analogous α,α'-bis-carbenoid, triply bonded species: synthesis of a stable λ<sup>3</sup>-phosphino carbene-λ<sup>3</sup>-phosphaacetylene
| author = A. Igau, H. Grutzmacher, A. Baceiredo, G. Bertrand
| journal = [[J. Am. Chem. Soc.]]
| volume = 110
| issue =
| pages = 6463–6466
| year = 1988
| url =
| doi = 10.1021/ja00227a028 }}</ref><ref name=Bertrand5>{{cite journal
| title = λ<sup>3</sup>-Phosphinocarbenes λ<sup>5</sup>-phosphaacetylenes
| author = G. Bertrand, R. Reed
| journal = [[Coordination Chemistry Reviews]]
| volume = 137
| issue =
| pages = 323–355
| year = 1994
| url =
| doi = 10.1016/0010-8545(94)03005-B }}</ref> were the first to make and isolate a stable carbene.
Typically, ''normal'' [[carbenes]] are very [[reactive]] short lived [[molecules]] that cannot be isolated, and are usually studied by observing the reactions they undergo. However, persistent carbenes are much more stable and considerably longer lived. This means that in many cases these carbenes are thermodynamically stable in the absence of moisture and (in most cases) [[oxygen]], and can be isolated and indefinitely stored. Some persistent carbenes are not thermodynamically stable and [[dimerise]] slowly over days. The less stable [[triplet state]] carbenes have [[half-lives]] measured in seconds, and cannot be stored but merely observed.
==Classes of stable carbenes==
The following are examples of the classes of stable carbenes isolated to date:
===Imidazol-2-ylidenes===
Imidazol-2-ylidenes were the first (and the most stable) family of stable carbenes isolated, and hence are the most well studied and understood. A considerable range of imidazol-2-ylidenes have been synthesised, including those in which the 1,3-positions have been functionalised with [[alkyl]], [[aryl]],<ref name=Arduengo2>A. J. Arduengo, H. V. R. Dias, R. L. Harlow, and M. Kline, J. Am. Chem. Soc., 1992, 114, 5530</ref> alkyloxy, alkylamino, alkylphosphino<ref name=Herman1>(W. A. Herrmann, C. Kocher, L. J. Goossen, and G. R. J. Artus, ''Chemistry-a European Journal'' '''1996''', ''2'', 1627.</ref> and even [[Chirality (chemistry)|chiral]] substituents:<ref name=Herman1/>
[[Image:Imidazol2ylidenes1.png|center|thumb|600px|Stable imidazol-2-ylidenes. a) Low solubility b) No value given.]]
Arguably one of the more interesting functionalisations occurred with the 4,5-dichlorination of the [[imidazole]] moiety, resulting in an air-stable carbene.<ref name=Arduengo5>A. J. Arduengo, F. Davidson, H. V. R. Dias, J. R. Goerlich, D. Khasnis, W. J. Marshall, T. K. Prakasha, ''[[J. Am. Chem. Soc.]]'' '''1997''', ''119'', 12742.</ref> Molecules containing two and even three imidazol-2-ylidenes have also been synthesised.<ref name=Herman2>W. A. Herrmann, M. Elison, J. Fischer, C. Kocher, and G. R. J. Artus, ''Chemistry-a European Journal'' '''1996''', ''2'', 772.</ref><ref name=Dias1>H. V. R. Dias and W. C. Jin, ''[[Tetrahedron Lett.]]'' '''1994''', ''35'', 1365.</ref>
Imidazol-2-ylidenes have been prepared by the deprotonation of [[imidazolium]] salts, and by the [[desulfurisation]] of [[thioureas]] with molten [[potassium]]. Imidazole-based carbenes are thermodynamically stable and generally have diagnostic <sup>13</sup>C [[NMR]] chemical shift values between 210-230 ppm for the carbenic carbon. Typically, X-ray structures of these molecules show N-C-N bond angles of ca. 101-102°.
===Triazol-5-ylidenes===
The triazol-5-ylidenes pictured below were first prepared by Enders and co-workers<ref name=Ender1>D. Enders, K. Breuer, G. Raabe, J. Runsink, J. H. Teles, J. P. Melder, K. Ebel, and S. Brode, Angew. Chem., Int. Ed. Engl., 1995, 34, 1021.</ref> by [[vacuum pyrolysis]] through loss of methanol from 2-methoxytriazoles. Only a limited range of these molecules have been reported, with the triphenyl substituted molecule being commercially available.
[[Image:Triazol5ylidenes.png|thumb|600px|center|Triazol-5-ylidenes]]
[[Triazole]]-based carbenes are thermodynamically stable and have diagnostic <sup>13</sup>C NMR chemical shift values between 210-220 ppm for the carbenic carbon. The X-ray structure of the triphenyl substituted carbene above shows an N-C-N bond angle of ca. 101°. The 5-methoxytriazole precursor to this carbene was made by the treatment of a triazolium salt with sodium methoxide, which attacks as a [[nucleophile]].<ref name=Ender1/> This may indicate that these carbenes are less aromatic than imidazol-2-ylidenes, as the imidazolium precursors do not react with nucleophiles due to the resultant loss of [[aromaticity]].
===Cyclic and acyclic diaminocarbenes===
A range of cyclic diaminocarbenes have been prepared principally by the Alder group in which the N-C-N unit is a member of a 5 or 6 membered ring,<ref name=Arduengo3>J. Arduengo, J. R. Goerlich, and W. J. Marshall, J. Am. Chem. Soc., 1995, 117, 11027</ref><ref name=Denk1>M. K. Denk, A. Thadani, K. Hatano, and A. J. Lough, Angew. Chem., Int. Ed. Engl., 1997, 36, 2607</ref><ref name=Alder5>R. W. Alder, M. E. Blake, C. Bortolotti, S. Buffali, C. P. Butts, E. Lineham, J. M. Oliva, A. G. Orpen, and M. J. Quayle, Chem. Commun., 1999, 241.)</ref> including a bicyclic example. The Alder group have prepared a range of acyclic diaminocarbenes.<ref name=Alder1>R. W. Alder, P. R. Allen, M. Murray, and A. G. Orpen, Angew. Chem., Int. Ed. Engl., 1996, 35, 1121</ref><ref name=Alder2>R. W. Alder and M. E. Blake, Chem. Commun., 1997, 1513</ref><ref name=Alder3>R. W. Alder, M. E. Blake, and J. M. Oliva, J. Phys. Chem. A, 1999, 103, 11200.</ref>
[[Image:diaminocarbenes1.png|center|thumb|600px|Synthesised cyclic and acyclic diaminocarbenes]]
Unlike the aromatic imidazol-2-ylidenes or triazol-5-ylidenes these carbenes appear not to be thermodynamically stable, as shown by the dimerisation of some un[[hindered]] cyclic and acyclic examples.<ref name=Denk1/><ref name=Alder2/> However more recent work by Alder<ref name=AlderEtDAC/> suggests that these carbenes dimerise via acid catalysed dimerisation (see [[Persistent carbene#Wanzlick equilibrium|Wanzlick equilibrium]]). Dihydroimidazole carbenes were prepared via the desulfurisation of thioureas with molten potassium<ref name=Denk1/> [[deprotonation]] of the respective dihydroimidazolium salts. The acyclic carbenes<ref name=Alder1/><ref name=Alder2/> and the tetrahydropyrimidinyl<ref name=Alder5/> based carbenes were prepared by deprotonation using strong homogeneous bases. Diaminocarbenes have diagnostic <sup>13</sup>C NMR chemical shift values between 230-270 ppm for the carbenic carbon. The X-ray structure of dihydroimidazole carbene shows a N-C-N bond angle of ca. 106°, whilst the angle of the acyclic carbene is 121°, both greater than those seen for imidazol-2-ylidenes.
===Heteroamino carbenes===
Stable [[nucleophilic]] carbenes in which one of the nitrogen atoms adjacent to the carbene center has been replaced by an alternative heteroatom (e.g. O, S or P)<ref name=Arduengo4>{{cite journal | author = A. J. Arduengo, J. R. Goerlich and W. J. Marshall | title = A Stable Thiazol-2-ylidene and Its Dimer | year = 1997 | journal = [[Liebigs Annalen]] | volume = 1997 | issue = 2 | pages = 365–374 | doi = 10.1002/jlac.199719970213}}</ref><ref name=Alder4>R. W. Alder, C. P. Butts, and A. G. Orpen, J. Am. Chem. Soc., 1998, 120, 11526</ref><ref name=Bertrand4/><ref name=Bertrand5/> have been prepared, as well as a species in which both [[nitrogens]] have been replaced by [[phosphorus]] atoms.<ref name=Bertrand6>G. Bertrand, A. Igau, A. Baceiredo, and G. Trinquier, Angew. Chem. Int. Ed. Engl., 1989, 28, 621.)</ref> However, these phosphorus substituted “carbenes” seem to exhibit some alkynic properties, and when published the exact carbenic nature of these red oils was in debate.<ref name=Bertrand5/>
[[Image:Heteroaminocarbenes1.png|center|thumb|600px|Synthesised heteroamino carbenes]]
An aromatic heteroamino [[thiazole]] based carbene (analogous to the carbene postulated by Breslow) (R. Breslow, J. Am. Chem. Soc., 1957, 79, 1762.) 5 has been prepared and characterised by X-ray crystallography.<ref name=Arduengo4/> Other formally aromatic α-heteroatom substituted carbenes have perhaps not been synthesised as they have the potential to dissociate into [[alkynes]] (i.e. R1CCR2 as well as X=C=X,). The reaction of carbon disulfide with electron deficient acetylenes gives transient 1,3-dithiolium carbenes (i.e. where X = S) which then dimerise. Thus it is possible that the reverse of this process might be occurring in similar carbenes.<ref name=Haztzler1>H. D. Haztzler, J. Am. Chem. Soc., 1970, 92, 1412., H. D. Hartzler, J. Am. Chem. Soc., 1972, 95, 4379.</ref>
[[Image:DecompositionHeteroaminocarbenes.png|center|thumb|600px|A possible decomposition pathway for aromatic N-C-X (X = O, S) substituted carbenes]]
Acyclic non-aromatic carbenes with O, S and P atoms adjacent (i.e. alpha) to the carbene centre have been prepared, e.g. thio- and oxy-iminium based carbenes have been characterised by X-ray crystallography.<ref name=Alder4/>
Since [[oxygen]] and sulfur are [[divalent]], [[steric]] protection of the carbenic centre is limited especially when the N-C-X unit is part of a ring. These acyclic carbenes have diagnostic <sup>13</sup>C NMR chemical shift values between 250-300 ppm for the carbenic carbon, further downfield than any other types of stable carbene. X-ray structures have show N-C-X bond angles of ca. 104 ° and 109 ° respectively.
===Other nucleophilic carbenes===
One stable N-heterocyclic carbene<ref name=Präsang1>''Stable Planar Six--Electron Six-Membered N-Heterocyclic Carbenes with Tunable Electronic Properties'' Carsten Präsang, Bruno Donnadieu, and Guy Bertrand [[J. Am. Chem. Soc.]]; '''2005'''; 127(29) pp 10182 - 10183 [http://pubs.acs.org/cgi-bin/abstract.cgi/jacsat/2005/127/i29/abs/ja052987g.html Abstract]</ref> has a structure analogous to [[borazine]] with one [[boron]] atom replaced by methylene. This results in a planar 6 electron compound.
[[Image:Borazine carbene.gif|center|thumb|600px|In the second step of this reaction sequence the proton is abstracted by [[Lithium tetramethylpiperidide|LiTMP]], two [[cyclohexane|cyclohexyl]] groups shield the carbene.]]
===Triplet state carbenes===
[[Image:Persistent triplet carbene.png|right|400px|Persistent triplet carbene Itoh 2006]]In 2001, [[Hideo Tomioka]] and his associates were able to produce a comparatively stable triplet carbene, taking advantage of [[Resonance (chemistry)|resonance]]. Triplet bis(9-anthryl)carbene has a half-life of 19 minutes.<ref name=Tomioka1>'''Nature''', 412, 626 (2001)</ref><ref name=Tomioka2>{{cite journal | journal = [[Chemical & Engineering News]] | date = August 13, 2001 | volume = 79 | issue = 33 | pages = 11 | url = http://pubs.acs.org/cen/topstory/7933/7933notw5.html | title = Triplet Carbene has Long Life}}</ref>
In 2006 the same group reported a triplet carbene with a [[half-life]] of 40 minutes.<ref name=Tomioka3>{{cite journal | title = Triplet Diphenylcarbenes
Protected by [[Trifluoromethyl]] and [[Bromine]] Groups. A Triplet Carbene Surviving a Day in Solution at Room Temperature | author = Tetsuji Itoh, Yoshimaru Nakata, Katsuyuki Hirai, Hideo Tomioka | journal = [[J. Am. Chem. Soc.]] | year = 2006 | volume = 128 | issue = 3 | pages = 957–967 | doi = 10.1021/ja056575j}}</ref> This carbene is prepared by a [[photochemistry|photochemical]] [[chemical decomposition|decomposition]] of a [[diazomethane]] with expulsion of [[nitrogen]] gas at a [[wavelength]] of 300 [[nanometer]]s in benzene. As with the other carbenes this species contains large bulky substituents, in this molecule [[bromine]] and the trifluoromethyl groups, that shield the carbene and prevent or slow down the process of dimerisation to a 1,1,2,2-tetra(phenyl)alkene. [[In silico]] experiments show that the [[bond length|distance]] of the divalent carbon atom to its neighbours is 138 [[picometer]]s with a [[bond angle]] of 158.8°. The [[dihedral angle]] is 85.7° which makes the phenyl groups almost at right angles to each other. Exposure to oxygen (diradical) converts the carbene to the corresponding [[benzophenone]] and the diphenylmethane compound is formed when it is trapped by [[1,4-cyclohexadiene]].
==History of stable carbenes==
'''1957''': Breslow proposed that a thiazol-2-ylidene was involved in the [[catalytic cycle]] of [[vitamin B1]].<ref name=Breslow2/> This was the first example of a stable [[nucleophilic]] carbene being implicated in a [[reaction mechanism]]. In the [[catalytic cycle]] shown below two [[molecules]] of [[furfural]] react to give furoin, via a thiazol-2-ylidene [[catalyst]], generated in situ by C2-[[deprotonation]] of a [[thiazolium salt]] moiety:
[[Image:breslow1.png|center|thumb|600px|Furoin formation from furfural, catalysed by thiamine]]
Evidence that the thiazol-2-ylidene was a stable intermediate in the above catalytic cycle was suggested by a [[deuterium]] exchange experiment. Breslow demonstrated that under standard reaction conditions (in [[heavy water|deuterated water]]) the thiazolium C2-proton was rapidly exchanged for a deuteron in a statistical [[Chemical equilibrium|equilibrium]].<ref name=Breslow3/>
[[Image:breslow2.png|center|thumb|600px|Deuterium exchange of the C2-proton of thiazolium salt]]
This confirmed that the C2-proton of the salt was labile, and was proposed to be exchanged as a result of the generation of a stable thiazol-2-ylidene intermediate.
===Wanzlick equilibrium===
'''1960''': Wanzlick ''et al.'' proposed that dihydroimidazol-2-ylidenes were generated from 2-trichloromethyl [[dihydroimidazole]]s, with the loss of [[chloroform]] by [[vacuum pyrolysis]].<ref name=Wanzlick1/><ref name=Wanzlick2/>
[[Image:Wanzlick1.png|center|thumb|600px|Wanzlick's mechanism for the reaction of dihydroimidazol-2-ylidene with electrophiles]]
Wanzlick ''et al.'' believed that once prepared these carbenes existed in an unfavourable equilibrium with its corresponding [[dimer]]. This assertion was based on reactivity studies which they believed showed that the free carbene reacted with [[electrophile]]s (E-X). The dimer ([[tetraaminoethylene]]) was believed to be inactive to the electrophiles (E-X), and thought to merely act as a stable carbene reservoir.<ref name=Wanzlick3>H. W. Wanzlick, Angew. Chem., Int. Ed. Engl., 1962, 1, 75.</ref>
Lemal<ref name=Lemal1>D. M. Lemal, R. A. Lovald, and K. I. Kawano, J. Am. Chem. Soc., 1964, 86, 2518.</ref> and separately Winberg<ref name=Winberg1>H. E. Winberg, J. E. Carnahan, D. D. Coffman, and M. Brown, J. Am. Chem. Soc., 1965, 87, 2055</ref> tested Wanzlick’s hypothesis of a carbene-dimer equilibrium by heating two differently N-aryl substituted [[tetraaminoethylene]]s together:
[[Image:lemal1.png|center|thumb|600px|Dimer cross-over experiment]]
This reaction did not produce a mixed dimeric product, and accordingly indicated that a 'carbene-dimer equilibrium' did not exist for these dihydroimidazol-2-ylidenes.
Lemal<ref name=Lemal1/> proposed an alternative mechanism to account for the reactions observed by Wanzlick’s<ref name=Wanzlick3/> by considering the reactivity of the electron rich [[tetraaminoethylene]]s and not the then hypothetical stable carbenes.<ref name=Wanzlick2/>
[[Image:lemal2.png|center|thumb|600px|Lemal's mechanism for the reaction of [[tetraaminoethylene]] with electrophiles. In conditions of excess E-X, the salt (blue) is formed. In conditions of catalytic E-X, the dimer (purple) will be formed. However, this is based on the assumption that the dimer is more stable than the carbene, however, see [[Persistent_carbene#Dimerisation|Chen's work]] below. E-X may be an acid or even a metal salt e.g. Li-Cl]]
Lemal believed that the [[tetraaminoethylene]], not the carbene, reacted with the electrophile (E-X) to generate a transient [[cationic]] species. He proposed that this cation then dissociated into the free carbene plus the resultant salt. The free carbene could then either re-dimerise (regenerating the [[tetraaminoethylene]] starting material) or react with E-X (as Wanzlick originally predicted), with either route eventually giving the same reaction product, the dihydroimidazolium salt. More recent work by Alder<ref name=AlderEtDAC/> has shown that un[[hindered]] diaminocarbenes form dimers by acid catalysed dimerisation as shown in the Lemal reaction above. In excess acid conditions the dimer forms the salt.
'''1970''': Wanzlick ''et al.'' prepared but did not isolate the first imidazol-2-ylidene by the deprotonation of imidazolium salt.<ref name=Wanzlick4>H. W. Wanzlick and H. J. Schonherr, ''Liebigs Ann. Chem.'' '''1970''', ''731'', 176.</ref> Wanzlick<ref name=Wanzlick3/> as well as Hoffmann<ref name=Hoffmann1>R. Gleiter and R. Hoffmann, ''[[J. Am. Chem. Soc.]]'' '''1968''', ''90'', 5457</ref> believed that these imidazole-based carbenes, with a [[Hückel's rule|4n+2]] π-electron ring system, should be more stable than the 4,5-dihydro analogues, due to Hückel-type [[aromaticity]]. Unfortunately, perhaps believing that these carbenes were still too reactive to be isolated, they resorted to trapping these carbenes with reagents such as [[Mercury (element)|mercury]] and [[isothiocyanate]]:
[[Image:Wanzlick2.png|center|thumb|600px|Preparation and trapping of an imidazol-2-ylidene]]
'''1991''': After nearly 30 years Arduengo ''et al.'' revisited this area, and remarkably managed not only to isolate a stable carbene but also to acquire an [[X-ray structure]] of it.<ref name=Arduengo1/> Given the prevailing belief at that time that all carbenes existed only as highly reactive, transient species, it is understandable that few attempts had been made prior to this to isolate these species. Arduengo ''et al.'' found that simple [[deprotonation]] of an imidazolium chloride with a strong base gave the carbene:
[[Image:Arduengo2.png|center|thumb|600px|Preparation of N,N'-diadamantyl-imidazol-2-ylidene]]
This carbene was found to be indefinitely stable at room temperature (in the absence of oxygen and moisture), and melted at 240-241 °C without decomposition. Another interesting chemical property of this molecule was a characteristic resonance in the <sup>13</sup>C [[NMR]] spectrum at 211 ppm for the deshielded carbenic carbon. The [[X-ray diffraction|X-ray]] structure revealed longer N–C [[bond length]]s in the ring of the carbene than in the parent imidazolium compound, indicating that there was very little [[double bond]] character to these bonds.
<Table align="center" cellpadding="10" border="1">
<tr >
<td>
[[Image:Ardueng1.svg|center|150px]]
</td>
<td>
[[Image:Arduengo3.svg|center|100px]]
</td>
<td>
[[Image:Arduengo4.svg|center|150px]]
</td>
<td>
[[Image:Alder1.svg|center|150px]]
</td>
<td>
[[Image:Arduengo5.svg|center|150px]]
</td>
</tr>
<tr>
<td>'''1991''': Hindered N,N'-diadamantyl-imidazol-2-ylidene. ([http://home.att.net/~ajarduengo/RotateStructures/0153Viewer.html external viewer])</td>
<td>'''1992''': Less hindered tetramethyl-imidazol-2-ylidene</td>
<td>'''1995''': Cyclic diaminocarbene 1,3-dimesityl-imidazol-4,5-dihydro-2-ylidene ([http://home.att.net/~ajarduengo/RotateStructures/0159Viewer.html external viewer]) </td>
<td>'''1996''': Acyclic diaminocarbene bis(diisopropylamino)carbene</td>
<td>'''1997''': Air-stable 1,3-dimesityl-4,5-dichloroimidazol-2-ylidene. ([http://home.att.net/~ajarduengo/RotateStructures/0173Viewer.html external viewer])</td>
</tr>
</table>
<center>'''Isolation and characterisation by X-ray crystallography of some important stable carbenes (1991-97)'''</center>
'''1992''': Initially many researchers believed that this carbene's unique stability was due to the bulky N-[[adamantyl]] substituents, which prevented the carbene from dimerising due to [[steric hindrance]]. However, the Arduengo laboratory later also isolated and acquired an X-ray structure of an imidazol-2-ylidene in which the bulky N-adamantyl groups were replaced with smaller [[methyl]] groups.<ref name=Arduengo2/>
This showed that [[steric hindrance|steric]] factors were not the predominant stabilising factors, and that imidazole-2-ylidenes were [[thermodynamic reaction control|thermodynamically stable]].
'''1995''': Arduengo and co-workers also went on to obtain an X-ray structure of the first dihydroimidazol-2-ylidene, a cyclic diaminocarbene.<ref name=Arduengo3/> This hindered molecule demonstrated that the aromatic imidazolium ring system, with the 4-5 carbon double bond, was also not critical to the stability of these carbenes. Later work performed by Denk ''et al.'' suggested that these dihydroimidazole carbenes were in part reliant on steric protection to prevent dimerisation, and thus not thermodynamically stable, unlike their aromatic imidazol-2-ylidene analogues<ref name=Denk1/>. However, in light of the work of Alder<ref name=AlderEtDAC/> it would seem the dimerisation was acid (or metal) catalysed (see [[Persistent carbene#Wanzlick equilibrium|Wanzlick equilibrium]]).
'''1996''': Alder ''et al.'' isolated and acquired an X-ray structure of the first acyclic diaminocarbene. <ref name=Alder1/> This carbene showed that diaminocarbenes without a cyclic backbone could be prepared. However, the real virtue of this carbene was that it has the ability to rotate around the N-C carbene bonds. By measuring the [[bond rotation barrier|barrier to rotation]] of these bonds, the extent of double bond character in these bonds could be measured. This allowed the [[ylide|ylidic]] nature of this carbene to be determined. Like the cyclic diaminocarbenes, unhindered examples tend to dimerise.<ref name=Alder2/><ref name=Alder3/><ref name=AlderEtDAC/>
'''1997''': Arduengo and co-workers reported the synthesis of the first air-stable carbene, 1,3-dimesityl-4,5-dichloroimidazol-2-ylidene:<ref name=Arduengo5/>
'''1997-1998''': The preparation of a thiazol-2-ylidene by Arduengo ''et al.''<ref name=Arduengo4/> and an aminothiocarbene and aminooxycarbene by Alder ''et al.'',<ref name=Alder4/> demonstrated that at least one [[nitrogen]] adjacent to the carbene centre could be replaced by another heteroatom without destroying the stability of these molecules:
[[Image:Alder2.png|center|thumb|600px|Heteroatomic (O, S) stabilised carbenes. ([http://home.att.net/~ajarduengo/RotateStructures/0165Viewer.html External viewer])]]
However, these carbenes are not thermodynamically stable as decomposition and dimerisation have been observed for unhindered examples.
Some time before Arduengo’s initial discovery in 1988, Bertrand ''et al.'' had isolated a red oil, the molecular structure of which can be represented as either a λ³-[[phosphinocarbene]] or λ<sup>5</sup>-phosphaacetylene:<ref name=Bertrand4/><ref name=Bertrand5/>
[[Image:Bertrand1.png|center|thumb|600px|Alkyne and carbene resonances structures of Bertrand’s carbene.]]
These molecules exhibit both carbenic and [[alkyne|alkynic]] reactivity. An X-ray structure of this molecule has not been obtained and at the time of publication some doubt remained as to their exact carbenic nature. The carbene was made by the reaction of an imidazol-2-ylidene with [[carbon tetrachloride]]. This extra stability probably results from the [[electron-withdrawing group|electron-withdrawing]] effect of the [[chlorine]] atoms, which must reduce the [[electron density]] on the carbon atom bearing the [[lone pair]], via [[inductive effect|induction]] through the sigma-backbone.
Arduengo’s initial publication has excited considerable interest in the field of stable carbenes.<ref name=Arduengo1/> Since publication, this paper has been cited many hundreds of times. Work in this field has included a diverse range of topics from theoretical calculations, to the practical application of these carbenes as metal [[ligands]] in catalysis, e.g. the [[Grubbs' catalyst#Second generation catalyst|second generation Grubbs' catalyst]]:
[[Image:Grubbs'-2G-3D-balls.png|center|thumb|300px|Grubbs Catalyst 2nd Generation]]
With the establishment of some of the fundamental principles of this chemistry, it is clear that this subject is no longer a laboratory curiosity, but has established itself as a chemical research field in its own right, and is set to grow still further in the future.
==General methods of preparing stable carbenes==
Stable carbenes are very reactive [[molecules]] and so it is important to consider the reaction conditions carefully when attempting to prepare these molecules. Stable carbenes are strongly [[Base (chemistry)|basic]] (the [[pKa]] value of the [[conjugate acid]] of an imidazol-2-ylidene was measured at ca. 24)<ref name=Alder6>R. W. Alder, P. R. Allen, and S. J. Williams, ''J. Chem. Soc., Chem. Commun.'' '''1995''', 1267.</ref> and react with [[oxygen]]. Clearly these reactions must be performed under a dry, inert atmosphere, avoiding protic solvents or compounds of even moderate [[acidity]]. Furthermore, one must also consider the relative stability of the starting materials. Whilst imidazolium salts are stable to [[nucleophilic]] addition, other non-aromatic salts are not (i.e. [[formamidinium]] salts)<ref name=Alderformamidinium>Preparation of tetraalkylformamidinium salts and related species as precursors to stable carbenes, J. Chem. Soc., Roger W. Alder, Michael E. Blake, Simone Bufali, Craig P. Butts, A. Guy Orpen,
Jan Schütz and Stuart J. Williams, Perkin Trans. 1, 2001, 1586–1593</ref>. Consequently in these cases, strong unhindered nucleophiles must be avoided whether they are generated in ''situ'' or are present as an impurity in other reagents (e.g. LiOH in BuLi).
Several approaches have been developed in order to prepare stable carbenes, these are outlined below.
===Deprotonation===
[[Deprotonation]] of carbene precursor salts with strong bases has proved a reliable route to almost all stable carbenes:
[[Image:deprotonation1.png|center|thumb|600px|Deprotonation of precursor salts to give stable carbenes.]]
Several bases and reaction conditions have been employed with varying success. The degree of success has been principally dependent on the nature of the [[wiktionary:Precursor|precursor]] being deprotonated. The major drawback with this method of preparation is the problem of isolation of the free carbene from the metals ions used in their preparation.
====Metal hydride bases====
One might believe that sodium or [[potassium hydride]]<ref name=Arduengo3/><ref name=Arduengo4/> would be the ideal base for deprotonating these precursor salts. The hydride should react irreversibly with the loss of [[hydrogen]] to give the desired carbene, with the [[inorganic]] by-products and excess hydride being removed by filtration. In practice this reaction is often too slow in suitable solvents (e.g. THF) due to the relative insolubility of the metal hydride and the salt.
The addition of soluble “[[catalysts]]” ([[DMSO]], [[tBuOH]])<ref name=Arduengo1/><ref name=Arduengo2/> considerably improves the rate of reaction of this heterogeneous system, via the generation of tert-butoxide or dimsyl anion. However, these catalysts have proved ineffective for the preparation of non-imidazolium adducts as they tend to act as nucleophiles towards the precursor salts and in so doing are destroyed. The presence of [[hydroxide]] ions as an impurity in the metal hydride could also destroy non-aromatic salts.
Deprotonation with [[sodium]] or [[potassium]] hydride in a mixture of liquid [[ammonia]]/THF at -40 °C has been reported to work well by Hermann ''et al.''<ref name=Herman1/> for imidazole based carbenes. Arduengo and co-workers<ref name=Arduengo4/> managed to prepare a dihydroimidazol-2-ylidene using NaH. However, this method has not been applied to the preparation of diaminocarbenes.
====Potassium tert-butoxide====
Arduengo and co-workers<ref name=Arduengo2/> have used [[potassium tert-butoxide]] without the addition of a metal hydride to deprotonate precursor salts.
====Alkyllithiums====
The use of [[alkyllithiums]] as strong bases<ref name=Arduengo1/> has not been extensively studied, and have been unreliable for deprotonation of precursor salts. With non-aromatic salts, n-BuLi and PhLi can act as nucleophiles whilst t-BuLi can on occasion act as a source of hydride, reducing the salt with the generation of [[isobutene]]:
[[Image:Basebutyllithium.png|center|thumb|600px|Reduction of [[formamidinium]] salts with tert-butyllithium]]
====Lithium amides====
Lithium amides like [[Lithium diisopropylamide|LDA]] and lithium tetramethylpiperidide ([[LiTMP]])<ref name=Alder1/><ref name=Alder2/> generally work well for the deprotonation of all types of salts, providing that not too much [[LiOH]] is present in the [[n-BuLi]] used to make the lithium amide. Titration of lithium amide can be used to determine the amount of hydroxide in solution.
====Metal hexamethyldisilazides====
The deprotonation of precursor salts with metal [[hexamethyldisilazides]]<ref name=Alder5/> works very cleanly for the deprotonation of all types of salts, except for unhindered formamidinium salts, where this base can act as a nucleophile to give a triaminomethane adduct.
===Metal free carbene preparation===
[[Image:Kcarbene.png|right|300px|Stable carbenes readily coordinate to metals; in this case a diaminocarbene co-ordinates to [[KHMDS]] to form a complex.]] The preparation of stable carbenes free from metal cations has been keenly sought to allow further study of the carbene species in isolation from these metals. Separating a carbene from a carbene-metal complex can be problematic due to the stability of the complex. Accordingly, it is preferable to make the carbene free from these metals in the first place. Indeed, some metal ions, rather than stabilising the carbene, have been implicated in the catalytic dimerisation of unhindered examples.
Shown right is an x-ray structure showing a complex between a diaminocarbene and potassium [[HMDS]]. This complex was formed when excess [[potassium bis(trimethylsilyl)amide| KHMDS]] was used as a strong base to deprotonate the [[formamidinium]] salt. Removing lithium ions resulting from deprotonation with reagents such as [[LDA]] can be especially problematic. Potassium and sodium salt by-products tend to precipitate from solution and can be removed. Lithium ions may be chemically removed by binding to species such as [[kryptane]]s or [[crown ether]]s.
Metal free carbenes have been prepared in several ways as outlined below:
====Dechalcogenation====
Another approach of preparing carbenes has relied on the [[desulfurisation]] of [[thiourea]]s with molten [[potassium]] in boiling [[THF]].<ref name=Denk1/><ref name=Kuhn1>>N. Kuhn and T. Kratz, ''Synthesis'' '''1993''', 561.</ref> A contributing factor to the success of this reaction is that the byproduct, [[potassium sulfide]], is insoluble in the solvent. The elevated temperatures suggest that this method is not suitable for the preparation of unstable dimerising carbenes. A single example of the [[deoxygenation]] of a [[urea]] with a [[fluorene]] derived carbene to give the tetramethyldiaminocarbene and fluorenone has also been reported:<ref name=Kovacs1>D. Kovacs, M. S. Lee, D. Olson, and J. E. Jackson, ''[[J. Am. Chem. Soc.]]'' '''1996''', ''118'', 8144.</ref>
[[Image:dechalcogenation.png|center|thumb|600px|Preparation of carbenes by dechalcogenation.]]
The [[desulfurisation]] of [[thiourea]]s with molten [[potassium]] to give imidazol-2-ylidenes or diaminocarbenes has not been widely used.
====Vacuum pyrolysis====
Vacuum pyrolysis, with the removal of neutral volatile by-products (CH<sub>3</sub>OH, CHCl<sub>3</sub>), has been used to prepare dihydroimidazole and triazole based carbenes:
[[Image:vaccumpyrolysis.png|center|thumb|600px|Preparation of carbenes via vacuum pyrolysis.]]
Historically the removal of chloroform by [[vacuum pyrolysis]] of '''d''' adducts was used by Wanzlick<ref name=Wanzlick2/> in his early attempts to prepare dihydroimidazol-2-ylidenes but this method is not widely used. The Enders laboratory<ref name=Ender1/> has used vacuum pyrolysis of a '''c''' adduct to generate a triazolium-5-ylidene '''c'''.
====Bis(trimethylsilyl)mercury====
[[Bis(trimethylsilyl)mercury]] (CH<sub>3</sub>)<sub>3</sub>Si-Hg-Si(CH<sub>3</sub>)<sub>3</sub> reacts with chloro-[[iminium]] and chloro-[[amidinium]] salts to give a metal-free carbene and elemental [[Mercury (element)|mercury]].<ref name=Bertrand7>{{cite journal
| title = Mono- and Diaminocarbenes from Chloroiminium and -amidinium Salts: Synthesis of Metal-Free Bis(dimethylamino)carbene
| author = Michael Otto, Salvador Conejero, Yves Canac, Vadim D. Romanenko, Valentyn Rudzevitch, and Guy Bertrand
| journal = [[J. Am. Chem. Soc.]]
| volume = 126
| issue = 4
| pages = 1016–1017
| year = 2004
| url =
| doi = 10.1021/ja0393325 }}</ref> ''e.g.'': (CH<sub>3</sub>)<sub>3</sub>Si-Hg-Si(CH<sub>3</sub>)<sub>3</sub> + R<sub>2</sub>N=C(Cl)-NR<sub>2</sub><sup>+ </sup>Cl<sup>-</sup> → R<sub>2</sub>N-C:-NR<sub>2</sub> + Hg<sub>(l)</sub> + (CH<sub>3</sub>)<sub>3</sub>Si-Cl
====Photochemical decomposition====
Persistent triplet state carbenes have been prepared by [[photochemical]] decomposition of a [[diazomethane]] product via the expulsion of [[nitrogen]] gas, at a wavelength of 300 nm in benzene.
===Purification===
[[Image:Air-free sublimation.png|right|300px|Sublimation of a carbene]] Stable carbenes are very reactive, and so the minimum amount of handling is desirable using [[air-free technique]]s. However, provided rigorously dry, relatively non-acidic and air-free materials are used, stable carbenes are reasonably robust to handling ''per se''. By way of example, a stable carbene prepared from potassium hydride can be filtered through a dry celite pad to remove excess KH (and resulting salts) from the reaction. On a relatively small scale, a suspension containing a stable carbene in solution can be allowed to settle and the supernatant solution pushed through a dried membrane syringe-filter. Stable carbenes are readily soluble in non-polar solvents such as hexane, and so typically [[recrystallisation]] of stable carbenes can be difficult, due to the unavailability of suitable non-acidic polar solvents. Air-free [[sublimation]] as shown right can be an effective method of purification, although temperatures below 60 <sup>o</sup>C under high vacuum are preferable as these carbenes are relatively volatile and also could begin to decompose at these higher temperatures. Indeed, sublimation in some cases can give single crystals suitable for x-ray analysis. However, strong complexation to metal ions like [[lithium]] will in most cases prevent sublimation.
==Chemistry of stable carbenes==
===Basicity and nucleophilicity===
The nucleophilicity and basicity of imidazol-2-ylidenes have been studied by Alder ''et al.''<ref name=Alder6/> who revealed that these molecules are strong bases, having a [[pKa]] of ca. 24 for the conjugate acid in DMSO:
[[Image:imidazol2ylidene pka.png|center|thumb|600px|Measurement of the pKa value for the conjugate acid of an imidazol-2-ylidene.]]
However, further work by Alder has shown that diaminocarbenes will deprotonate the DMSO solvent, with the resulting anion reacting with the resulting amidinium salt.
[[Image:AlderTIPcarbeneDMSO.png|center|thumb|600px|Using D6-[[DMSO]] as an NMR solvent can have unexpected results.]]
Reaction of imidazol-2-ylidenes with [[1-bromohexane]] gave 90% of the 2-substituted adduct, with only 10% of the corresponding [[alkene]], indicating that these molecules are also reasonably [[nucleophilic]].
===Dimerisation===
Imidazol-2-ylidenes and triazol-5-ylidenes are thermodynamically stable and do not [[dimerise]], and have been stored in [[solution]] in the absence of water and air for years. This is presumably due to the [[aromatic]] nature of these carbenes, which is lost upon dimerisation. In fact imidazol-2-ylidenes are so thermodynamically stable that only in highly constrained conditions are these carbenes forced to dimerise.
Chen and Taton<ref name=Chen>T. A. Taton and P. Chen, Angew. Chem., Int. Ed. Engl., 1996, 35, 1011</ref> made a doubly-tethered diimidazol-2-ylidene by [[deprotonating]] the respective diimidazolium salt. Only the deprotonation of the doubly-tethered diimidazolium salt with the shorter [[methylene]] (-CH<sub>2</sub>-) linkage resulted in the dicarbene dimer:
[[Image:Chen dimer.png|center|thumb|300px|Dimerisation of tethered diimidazol-2-ylidenes.]]
If this dimer existed as a dicarbene, the electron [[lone pair]]s on the carbenic carbon would be forced into close proximity. Presumably the resulting repulsive [[electrostatic]] interactions would have a significant destabilising effect. To avoid this electronic interaction, the [[carbene]] units dimerise.
On the other hand, heteroamino carbenes (''e.g.'' R<sub>2</sub>N-C:-OR or R<sub>2</sub>N-C:-SR) and non-aromatic carbenes such as diaminocarbenes (''e.g.'' R<sub>2</sub>N-C:-NR<sub>2</sub>) have been shown to dimerise,<ref name=Alderreview>Alder, R. W., Blake, M. E.; Chaker, L.; Harvey, J. N.; Paolini, F. P. V.; Schütz, J. (2004). "When and How Do Diaminocarbenes Dimerize?". Angew. Chem. Int. Ed. Engl. 43 (44): 5896–5911.</ref> albeit quite slowly. This has been presumed to be due to the high barrier to [[singlet state]] dimerisation:
[[Image:diaminocarbene dimer.png|center|thumb|600px|"Least motion" (path A - not allowed) and "non-least motion" (path B) routes of carbene dimerisation.]]
However, more recent work by Alder <ref name=AlderEtDAC>Bis(diethylamino)carbene and the mechanism of dimerisation for simple diaminocarbenes, Roger W. Alder, Leila Chaker and François P. V. Paolini, Chem. Commun., 2004, 2172 - 2173, DOI: 10.1039/b409112d</ref> suggests that diaminocarbenes do not truly dimerise, but rather form the dimer by reaction via [[formamidinium]] salts, a protonated precursor species (see [[Persistent carbene#Wanzlick equilibrium|Wanzlick equilibrium]]). Accordingly, this reaction can be acid catalysed. This reaction occurs because unlike imidazolium based carbenes, there is no loss of aromaticity in protonation of the carbene.
Unlike the dimerisation of [[triplet state]] carbenes, these [[singlet state]] carbenes do not approach head to head (“least motion”), but rather the carbene [[lone pair]] attacks the empty carbon [[p-orbital]] (“non-least motion”). Carbene dimerisation can also be acid or metal catalysed, and so care must be taken when determining if the carbene is undergoing true dimerisation.
===Reactivity of stable carbenes===
The chemistry of stable carbenes has not been fully explored. However, Enders ''et al''.<ref name=Ender1/><ref name=Ender4>D. Enders, K. Breuer, J. Runsink, and J. H. Teles, Liebigs Ann. Chem., 1996, 2019.</ref><ref name=Enders3>D. Enders, K. Breuer, J. H. Teles, and K. Ebel, ''Journal Fur Praktische Chemie-Chemiker-Zeitung'' '''1997''', ''339'', 397.</ref> have performed a range of organic reactions involving a triazol-5-ylidene. These reactions are outlined below and may be considered as a model for other carbenes.
[[Image:triazol5ylidene reactions.png|center|thumb|600px|Reactions of triazol-5-ylidene.<ref name=Enders3/>]]
[[Image:triazol5ylidene reactions text.png|center|600px|]]
These carbenes tend to behave in a [[nucleophilic]] fashion ('''e''' and '''f'''), performing [[insertion reaction]]s ('''b'''), [[addition reaction]]s ('''c'''), [2+1] [[cycloaddition]]s ('''d''', '''g''' and '''h'''), [4+1] [[cycloaddition]]s ('''a''') as well as simple [[deprotonation]]s. The [[insertion reaction]]s ('''b''') probably proceed via [[deprotonation]], resulting in the generation of a nucleophile (<sup>-</sup>XR) which can attack the generated salt giving the impression of a H-X insertion.
Care must be taken to check that a stable carbene is truly stable. The discovery of a stable [[isothiazole]] carbene ('''2''') from an isothiazolium perchlorate ('''1''') by one research group <ref>''Synthesis of Stable Isothiazole Carbenes'' Janine Wolf, Winfried Bohlmann, Matthias Findeisen, Thomas Gelbrich, Hans-Jorg Hofmann, and Borbel Schulze [[Angew. Chem. Int. Ed.]] 2007, 46, 3118 –3121 {{DOI|10.1002/anie.200604305}}</ref> was questioned by another group <ref>''Recently Reported Crystalline Isothiazole Carbenes: Myth or Reality'' Alan DeHope, Vincent Lavallo, Bruno Donnadieu, Wolfgang W. Schoeller, and Guy Bertrand [[Angew. Chem. Int. Ed.]] 2007, 46, 6922 –6925 {{DOI|10.1002/anie.200702272}}</ref> who were only able to isolate ''2-imino-2H-thiete'' ('''4'''). The intermediate '''3''' was proposed through a [[rearrangement reaction]]. This carbene is no longer considered stable <ref>''Reply to “Recently Reported Crystalline Isothiazole
Carbenes: Myth or Reality”'' Janine Wolf, Winfried Bhlmann, Matthias Findeisen, Thomas Gelbrich, Hans-Jorg Hofmann, and Borbel Schulze [[Angew. Chem. Int. Ed.]] 2007, 46, 6926 {{DOI|10.1002/anie.200702746}}</ref>.
[[Image:IsothiazoleCarbene.png|400px|center|Isothiazole carbene DeHope 2007]]
===Carbene complexation===
Imidazol-2-ylidenes, triazol-5-ylidenes (and less so, diaminocarbenes) have been shown to co-ordinate to a plethora of elements, from [[alkali metals]], [[main group element]]s, [[transition metal]]s and even [[lanthanides]] and [[actinides]]. A [[periodic table]] of elements gives some idea of the complexes which have been prepared, and in many cases these have been identified by single crystal [[X-ray crystallography]].<ref name=Herrmann3>''N-Heterocyclic Carbenes'' [[Angewandte Chemie International Edition]] in English Volume 36, Issue 20, Date: November 3, '''1997''', Pages: 2162-2187 Wolfgang A. Herrmann, Christian Köcher {{DOI|10.1002/anie.199721621}}.</ref> <ref name=Alder5/> <ref name=Boche1>''Crystal Structure of the Dimeric (4-tert-Butylthiazolato)(glyme)lithium: Carbene Character of a Formyl Anion Equivalent'' [[Angewandte Chemie International Edition]] in English Volume 34, Issue 4, Date: March 7, '''1995''', Pages: 487-489 Gernot Boche, Christof Hilf, Klaus Harms, Michael Marsch, John C. W. Lohrenz. {{DOI|10.1002/anie.199504871}}</ref>
<!-- Note to editors; to make an elemental box green (i.e to show that carbenes co-ordinate to that element) from a gray box (i.e. no co-ordination), replace "Poor metals" with "Nonmetals" field, and the opposite to turn a gray box green -->
<center>
{| border="0" cellpadding="0" cellspacing="1" style="width:80%; {{{1|}}}"
|-
! width="1.0%" | [[Periodic table group|Group]] →
! width="5.5%" | [[Alkali metal|1]]
! width="5.5%" | [[Alkaline earth metal|2]]
! width="5.5%" | [[Group 3 element|3]]
! width="5.5%" | [[Group 4 element|4]]
! width="5.5%" | [[Group 5 element|5]]
! width="5.5%" | [[Group 6 element|6]]
! width="5.5%" | [[Group 7 element|7]]
! width="5.5%" | [[Group 8 element|8]]
! width="5.5%" | [[Group 9 element|9]]
! width="5.5%" | [[Group 10 element|10]]
! width="5.5%" | [[Group 11 element|11]] <!-- also [[Coinage metal|11]] -->
! width="5.5%" | [[Group 12 element|12]]
! width="5.5%" | [[Boron group|13]]
! width="5.5%" | [[Carbon group|14]]
! width="5.5%" | [[Nitrogen group|15]] <!-- also [[Pnictogen|15]] -->
! width="5.5%" | [[Chalcogen|16]]
! width="5.5%" | [[Halogen|17]]
! width="5.5%" | [[Noble gas|18]]
|-
! ↓ [[Periodic table period|Period]]
| colspan="19"|<br />
|-
! [[Period 1 element|1]]
| {{element cell| 1|hydrogen |H |1.00794(7) |Gas |Nonmetals|Primordial}}
| colspan="16"|<br />
| {{element cell| 2|helium |He|4.002602(2) |Gas |Poor metals|Primordial}}
|-
! [[Period 2 element|2]]
| {{element cell| 3|lithium |Li|6.941(2) |Solid|Nonmetals|Primordial}}
| {{element cell| 4|beryllium |Be|9.012182(3) |Solid|Nonmetals|Primordial}}
| colspan="10"|<br />
| {{element cell| 5|boron |B |10.811(7) |Solid|Nonmetals|Primordial}}
| {{element cell| 6|carbon |C |12.0107(8) |Solid|Nonmetals|Primordial}}
| {{element cell| 7|nitrogen |N |14.00674(7) |Gas |Nonmetals|Primordial}}
| {{element cell| 8|oxygen |O |15.9994(3) |Gas |Poor metals|Primordial}}
| {{element cell| 9|fluorine |F |18.9984032(5) |Gas |Poor metals|Primordial}}
| {{element cell|10|neon |Ne|20.1797(6) |Gas |Poor metals|Primordial}}
|-
! [[Period 3 element|3]]
| {{element cell|11|sodium |Na|22.98976928(2)|Solid|Nonmetals|Primordial}}
| {{element cell|12|magnesium |Mg|24.3050(6) |Solid|Nonmetals|Primordial}}
| colspan="10"|<br />
| {{element cell|13|aluminium |Al|26.9815386(8) |Solid|Nonmetals|Primordial}}
| {{element cell|14|silicon |Si|28.0855(3) |Solid|Nonmetals|Primordial}}
| {{element cell|15|phosphorus|P |30.973762(2) |Solid|Nonmetals|Primordial}}
| {{element cell|16|sulfur |S |32.066(6) |Solid|Nonmetals|Primordial}}
| {{element cell|17|chlorine |Cl|35.4527(9) |Gas |Poor metals|Primordial}}
| {{element cell|18|argon |Ar|39.948(1) |Gas |Poor metals|Primordial}}
|-
! [[Period 4 element|4]]
| {{element cell|19|potassium |K |39.0983(1) |Solid|Nonmetals|Primordial}}
| {{element cell|20|calcium |Ca|40.078(4) |Solid|Nonmetals|Primordial}}
| {{element cell|21|scandium |Sc|44.955912(6) |Solid|Poor metals|Primordial}}
| {{element cell|22|titanium |Ti|47.867(1) |Solid|Nonmetals|Primordial}}
| {{element cell|23|vanadium |V |50.9415(1) |Solid|Nonmetals|Primordial}}
| {{element cell|24|chromium |Cr|51.9961(6) |Solid|Nonmetals|Primordial}}
| {{element cell|25|manganese |Mn|54.938045(5) |Solid|Nonmetals|Primordial}}
| {{element cell|26|iron |Fe|55.845(2) |Solid|Nonmetals|Primordial}}
| {{element cell|27|cobalt |Co|58.933195(5) |Solid|Nonmetals|Primordial}}
| {{element cell|28|nickel |Ni|58.6934(2) |Solid|Nonmetals|Primordial}}
| {{element cell|29|copper |Cu|63.546(3) |Solid|Nonmetals|Primordial}}
| {{element cell|30|zinc |Zn|65.39(2) |Solid|Nonmetals|Primordial}}
| {{element cell|31|gallium |Ga|69.723(1) |Solid|Nonmetals|Primordial}}
| {{element cell|32|germanium |Ge|72.61(2) |Solid|Nonmetals|Primordial}}
| {{element cell|33|arsenic |As|74.92160(2) |Solid|Poor metals|Primordial}}
| {{element cell|34|selenium |Se|78.96(3) |Solid|Nonmetals|Primordial}}
| {{element cell|35|bromine |Br|79.904(1) |Liquid|Poor metals|Primordial}}
| {{element cell|36|krypton |Kr|83.80(1) |Gas |Poor metals|Primordial}}
|-
! [[Period 5 element|5]]
| {{element cell|37|rubidium |Rb|85.4678(3) |Solid|Poor metals|Primordial}}
| {{element cell|38|strontium |Sr|87.62(1) |Solid|Nonmetals|Primordial}}
| {{element cell|39|yttrium |Y |88.90585(2) |Solid|Nonmetals|Primordial}}
| {{element cell|40|zirconium |Zr|91.224(2) |Solid|Nonmetals|Primordial}}
| {{element cell|41|niobium |Nb|92.90638(2) |Solid|Nonmetals|Primordial}}
| {{element cell|42|molybdenum|Mo|95.94(1) |Solid|Nonmetals|Primordial}}
| {{element cell|43|technetium|Tc|[97.9072] |Solid|Poor metals|Synthetic}}
| {{element cell|44|ruthenium |Ru|101.07(2) |Solid|Nonmetals|Primordial}}
| {{element cell|45|rhodium |Rh|102.90550(2) |Solid|Nonmetals|Primordial}}
| {{element cell|46|palladium |Pd|106.42(1) |Solid|Nonmetals|Primordial}}
| {{element cell|47|silver |Ag|107.8682(2) |Solid|Nonmetals|Primordial}}
| {{element cell|48|cadmium |Cd|112.411(8) |Solid|Poor metals|Primordial}}
| {{element cell|49|indium |In|114.818(3) |Solid|Poor metals|Primordial}}
| {{element cell|50|tin |Sn|118.710(7) |Solid|Nonmetals|Primordial}}
| {{element cell|51|antimony |Sb|121.760(1) |Solid|Poor metals|Primordial}}
| {{element cell|52|tellurium |Te|127.60(3) |Solid|Nonmetals|Primordial}}
| {{element cell|53|iodine |I |126.90447(3) |Solid|Nonmetals|Primordial}}
| {{element cell|54|xenon |Xe|131.29(2) |Gas |Poor metals|Primordial}}
|-
! [[Period 6 element|6]]
| {{element cell|55|caesium |Cs|132.9054519(2)|Solid|Poor metals|Primordial}}
| {{element cell|56|barium |Ba|137.327(7) |Solid|Nonmetals|Primordial}}
| {{element cell | * |Lanthanides| | |Solid|Poor metals|Undiscovered}}
| {{element cell|72|hafnium |Hf|178.49(2) |Solid|Nonmetals|Primordial}}
| {{element cell|73|tantalum |Ta|180.94788(2) |Solid|Nonmetals|Primordial}}
| {{element cell|74|tungsten |W |183.84(1) |Solid|Nonmetals|Primordial}}
| {{element cell|75|rhenium |Re|186.207(1) |Solid|Nonmetals|Primordial}}
| {{element cell|76|osmium |Os|190.23(3) |Solid|Nonmetals|Primordial}}
| {{element cell|77|iridium |Ir|192.217(3) |Solid|Nonmetals|Primordial}}
| {{element cell|78|platinum |Pt|195.084(9) |Solid|Nonmetals|Primordial}}
| {{element cell|79|gold |Au|196.966569(4) |Solid|Nonmetals|Primordial}}
| {{element cell|80|mercury |Hg|200.59(2) |Liquid|Nonmetals|Primordial|mercury (element)}}
| {{element cell|81|thallium |Tl|204.3833(2) |Solid|Poor metals|Primordial}}
| {{element cell|82|lead |Pb|207.2(1) |Solid|Poor metals|Primordial}}
| {{element cell|83|bismuth |Bi|208.98040(1) |Solid|Poor metals|Primordial}}
| {{element cell|84|polonium |Po|[208.9824] |Solid|Poor metals|Natural radio}}
| {{element cell|85|astatine |At|[209.9871] |Solid|Poor metals|Natural radio}}
| {{element cell|86|radon |Rn|[222.0176] |Gas |Poor metals|Natural radio}}
|-
! [[Period 7 element|7]]
| {{element cell|87|francium |Fr|[223.0197] |Solid|Poor metals|Natural radio}}
| {{element cell|88|radium |Ra|[226.0254] |Solid|Poor metals|Natural radio}}
| {{element cell | ** |Actinides| | |Solid|Poor metals|Undiscovered}}
| {{element cell|104|rutherfordium|Rf|[263.1125] |Solid|Poor metals|Synthetic}}
| {{element cell|105|dubnium |Db|[262.1144] |Solid|Poor metals|Synthetic}}
| {{element cell|106|seaborgium |Sg|[266.1219] |Solid|Poor metals|Synthetic}}
| {{element cell|107|bohrium |Bh|[264.1247] |Solid|Poor metals|Synthetic}}
| {{element cell|108|hassium |Hs|[269.1341] |Solid|Poor metals|Synthetic}}
| {{element cell|109|meitnerium |Mt|[268.1388] |Solid|Poor metals|Synthetic}}
| {{element cell|110|darmstadtium |Ds|[272.1463] |Solid|Poor metals|Synthetic}}
| {{element cell|111|roentgenium |Rg|[272.1535] |Solid|Poor metals|Synthetic}}
| {{element cell|112|ununbium |Uub|[277] |Liquid|Poor metals|Synthetic}}
| {{element cell|113|ununtrium |Uut|[284] |Solid|Poor metals|Synthetic}}
| {{element cell|114|ununquadium |Uuq|[289] |Solid|Poor metals|Synthetic}}
| {{element cell|115|ununpentium |Uup|[288] |Solid|Poor metals|Synthetic}}
| {{element cell|116|ununhexium |Uuh|[292] |Solid|Poor metals|Synthetic}}
| {{element cell|117|ununseptium |Uus|[291]‡ |Solid|Poor metals|Synthetic}}
| {{element cell|118|ununoctium |Uuo|[293]‡ |Solid|Poor metals|Synthetic}}
|-
| colspan="21"|<br />
|-
| colspan="4" style="text-align:right"|* '''[[Lanthanide|Lanthanoids]]'''
| {{element cell|57|lanthanum |La|138.90547(7)|Solid|Poor metals|Primordial}}
| {{element cell|58|cerium |Ce|140.116(1) |Solid|Poor metals|Primordial}}
| {{element cell|59|praseodymium|Pr|140.90765(2)|Solid|Poor metals|Primordial}}
| {{element cell|60|neodymium |Nd|144.242(3) |Solid|Poor metals|Primordial}}
| {{element cell|61|promethium |Pm|[144.9127] |Solid|Poor metals|Synthetic}}
| {{element cell|62|samarium |Sm|150.36(2) |Solid|Nonmetals|Primordial}}
| {{element cell|63|europium |Eu|151.964(1) |Solid|Nonmetals|Primordial}}
| {{element cell|64|gadolinium |Gd|157.25(3) |Solid|Poor metals|Primordial}}
| {{element cell|65|terbium |Tb|158.92535(2)|Solid|Poor metals|Primordial}}
| {{element cell|66|dysprosium |Dy|162.500(1) |Solid|Poor metals|Primordial}}
| {{element cell|67|holmium |Ho|164.93032(2)|Solid|Poor metals|Primordial}}
| {{element cell|68|erbium |Er|167.259(3) |Solid|Poor metals|Primordial}}
| {{element cell|69|thulium |Tm|168.93421(2)|Solid|Poor metals|Primordial}}
| {{element cell|70|ytterbium |Yb|173.04(3) |Solid|Nonmetals|Primordial}}
| {{element cell|71|lutetium |Lu|174.967(1) |Solid|Poor metals|Primordial}}
|-
| colspan="4" style="text-align:right"|** '''[[Actinide|Actinoids]]'''
| {{element cell|89|actinium |Ac|[227.0277] |Solid|Poor metals|Natural radio}}
| {{element cell|90|thorium |Th|232.03806(2)|Solid|Poor metals|Primordial}}
| {{element cell|91|protactinium|Pa|231.03588(2)|Solid|Poor metals|Natural radio}}
| {{element cell|92|uranium |U |238.02891(3)|Solid|Nonmetals|Primordial}}
| {{element cell|93|neptunium |Np|[237.0482] |Solid|Poor metals|Natural radio}}
| {{element cell|94|plutonium |Pu|[244.0642] |Solid|Poor metals|Natural radio}}
| {{element cell|95|americium |Am|[243.0614] |Solid|Poor metals|Synthetic}}
| {{element cell|96|curium |Cm|[247.0703] |Solid|Poor metals|Synthetic}}
| {{element cell|97|berkelium |Bk|[247.0703] |Solid|Poor metals|Synthetic}}
| {{element cell|98|californium |Cf|[251.0796] |Solid|Poor metals|Synthetic}}
| {{element cell|99|einsteinium |Es|[252.0830] |Solid|Poor metals|Synthetic}}
| {{element cell|100|fermium |Fm|[257.0951] |Solid|Poor metals|Synthetic}}
| {{element cell|101|mendelevium|Md|[258.0984] |Solid|Poor metals|Synthetic}}
| {{element cell|102|nobelium |No|[259.1011] |Solid|Poor metals|Synthetic}}
| {{element cell|103|lawrencium |Lr|[262.110] |Solid|Poor metals|Synthetic}}
|}
* '''Green box''' = Carbene complex with element known.
* '''Grey box''' = No carbene complex with element known.
Figure: Periodic Table featuring elements that have formed stable carbenes complexes.
</center>
Stable carbenes are believed to behave in a similar fashion to [[organophosphine]]s in their co-ordination properties to metals. These [[ligand]]s are said to be good σ-donors through the carbenic [[lone pair]], but poor π-acceptors due to internal [[ligand]] [[back-donation]] from the [[nitrogen]] atoms adjacent to the carbene centre, and so are able to co-ordinate to even relatively electron deficient metals. Enders <ref name=Enders10>D. Enders, H. Gielen, G. Raabe, J. Runsink, and J. H. Teles, ''[[Chem. Ber.]]'' '''1996''', ''129'', 1483</ref> and Hermann <ref name=Herrman4>''Metal Complexes of N-Heterocyclic Carbenes - A New Structural Principle for Catalysts in Homogeneous Catalysis'' [[Angewandte Chemie International Edition]] in English Volume 34, Issue 21, Date: November 17, '''1995''', Pages: 2371-2374 Wolfgang A. Herrmann, Martina Elison, Jakob Fischer, Christian Köcher, Georg R. J. Artus {{DOI|10.1002/anie.199523711}}.</ref><ref name=Herrman5>''Chiral Heterocylic Carbenes in Asymmetric Homogeneous Catalysis'' [[Angewandte Chemie International Edition]] in English Volume 35, Issue 23-24, Date: December '''1996''', Pages: 2805-2807 Wolfgang A. Herrmann, Lukas J. Goossen, Christian Köcher, Georg R. J. Artus {{DOI|10.1002/anie.199628051}}</ref> have shown that these carbenes are suitable replacements for [[phosphine]] [[ligands]] in several [[catalytic cycles]]. Whilst they have found that these ligands do not activate the metal catalyst as much as phosphine ligands they often result in more robust catalysts. Several catalytic systems have been looked into by Hermann and Enders, using catalysts containing imidazole and triazole carbene ligands, with moderate success.<ref name=Herrmann3/><ref name=Enders10/><ref name=Herrman4/><ref name=Herrman5/>Grubbs <ref name=Grubbs1> (M. Scholl, T. M. Trnka, J. P. Morgan, and R. H. Grubbs, ''[[Tetrahedron Lett.]]'' '''1999''', ''40'', 2247.</ref> has reported replacing a phosphine ligand (PCy<sub>3</sub>) with an imidazol-2-ylidene in the [[olefin metathesis]] catalyst RuCl<sub>2</sub>(PCy<sub>3</sub>)<sub>2</sub>CHPh, and noted increased ring closing metathesis as well as exhibiting “a remarkable air and water stability”. Molecules containing two and three carbene moieties have been prepared as potential [[bidentate]] and [[tridentate]] carbene ligands.<ref name=Herman2/>.<ref name=Dias1/>
===Carbenes in organometallic chemistry===
Carbenes can be stabilised as [[organometallic chemistry|organometallic]] species. These [[transition metal carbene complex]]es fall into two categories:
*[[Ernst Otto Fischer|Fischer]] carbenes in which carbenes are tethered to a metal and an [[electron-withdrawing group]] (usually a carbonyl),
*[[Richard R. Schrock|Schrock]] carbenes; in which carbenes are tethered to a metal and an [[electron-donating group]]. The reactions that such carbenes participate in are very different from those in which organic carbenes participate.
Some persistent carbenes are used as [[Ligand|ancillary ligand]] in [[organometallic]] chemistry. One of the most notable examples is in the second generation [[Grubbs' catalyst]]s.
===Triplet state carbene chemistry===
Persistent triplet state carbenes are likely to have very similar reactivity as other non-persistent triplet state [[carbenes]].
==Physical properties==
[[Image:Carbene 13C NMR.png|left|150px|Carbene peak in 13NMR]] Those carbenes that have been isolated to date tend to be white solids with low melting points.
These carbenes tend to sublime at low temperatures under high vacuum.
One of the more useful physical properties is the diagnostic chemical shift of the carbenic carbon atom in the 13C-[[NMR]] spectrum. Typically this peak is in the range between 200 and 300 ppm, where few other peaks appear in the 13C-[[NMR]] spectrum. An example is shown left for a cyclic diaminocarbene which has a carbenic peak at 238 ppm.
==References==
{{reflist}}
==Further reading==
Reviews on persistent carbenes:
* [http://books.google.com/books?id=ElV85eHbubQC&pg=PA153&dq=Alder+Bertrand&lr=&sig=wNCxFAU9c83Sa-o5Ck7KCkN6LnA Carbene Chemistry: From Fleeting Intermediates to Powerful Reagents, (Chapter 4, Hideo Tomioka (triplet state); Chapter 5 (singlet state), Roger W. Alder) - ed. Guy Bertrand]
* [http://books.google.com/books?id=wp0C10qO0A8C&pg=PA329&dq=moss+jones+platz+Bertrand&lr=&source=gbs_toc_s&cad=1&sig=PcsZRp-ha0h09pG06s4Ip11Q4K8 Reactive Intermediate Chemistry By Robert A. Moss, Matthew Platz, Maitland Jones (Chapter 8, Stable Singlet Carbenes, Guy Bertrand)]
*{{cite journal | last=Alder | first=R. W. | coauthors=Blake, M. E.; Chaker, L.; Harvey, J. N.; Paolini, F. P. V.; Schütz, J. | title=When and How Do Diaminocarbenes Dimerize? | journal=Angew. Chem. Int. Ed. Engl. | year=2004 | volume=43 | issue=44 | pages=5896–5911 | doi=10.1002/anie.200400654 }}
*R. W. Alder, in 'Diaminocarbenes: exploring structure and reactivity', ed. G. Bertrand, New York, 2002
*{{cite journal | last=Herrmann | first=W. A. | coauthors=Köcher, C. | title=N-Heterocyclic Carbenes | journal=Angew. Chem., Int. Ed. Engl. | year=1997 | volume=36 | issue=20 | pages=2162–2187
| doi = 10.1002/anie.199523711 <!--Retrieved from Yahoo! by DOI bot-->}}
*{{cite journal | last=Regitz | first=M. | title=Stable Carbenes—Illusion or Reality? | journal=Angew. Chem., Int. Ed. Engl. | year=1996 | volume=30 | issue=6 | pages=674–676 | doi=10.1002/anie.199106741 }}
[[Category:Functional groups]]
[[Category:Reactive intermediates]]
[[Category:organometallic chemistry]]