Metallabenzene
The metallabenzenes are class of chemical compound of the form LnM(CH)5, or derivatives thereof. Most metallabenzenes do not feature the M(CH)5 ring itself, but, instead, some of the H atoms are replaced by other substituents. The parent metallabenzenes can be viewed as derivatives of benzene wherein a CH center has been replaced by a transition metal complex. {{cite journal|title=Metallabenzenes|journal=Chem. Rev.|year=2001|volume=101|issue=5|pages=1205–27|doi=10.1021/cr990337n|author1=Bleeke, J.R.|pmid=11710218}}
Classification
File:Interactions between orbitals.PNG
All known metallabenzenes are 18-electron complexes, and have been classified into three varieties.{{which|date=March 2025}} In modeling metallabenzenes, the parent acyclic hydrocarbon ligand is viewed as the anion C5H5−.
Early computations suggested that the six π-electrons in the metallacycle conform to the Hückel (4n+2) theory;{{cite journal|title=Delocalization in Metallocycles|journal=Nouv. J. Chim.|year=1979|volume=3|issue=1|page=39|author1=Thorn, D.L.|author2=Hoffmann, R.|url=https://roaldhoffmann.com/sites/default/files/fromd6/149s_0.pdf}} however, interactions with an additional d orbital suggest that metallabenzenes may instead be an 8-π Möbius aromat. Specifically, if the ring is located in the xy plane with y measuring radial distance at the metal center, then the Hückel orbitals treat the two lobes of the {{math|dyz}} orbital inside the ring as though a main-group π orbital. One resulting orbital has vanishing metal-orbital component. That Hückel orbital is split by interaction with the {{math|dxz}} orbital, whose four lobes are all circumferential and effect a Möbius twist (see Fig. 4 in the cited paper). Still other authors instead argue that metallabenzenes are Hückel aromats but with 10 π-electrons.{{cite journal|doi=10.1039/C5CS00004A|journal=Chem Soc Rev|publisher=Royal Society of Chemistry|title=Aromaticity of metallabenzenes and related compounds|first1=Israel|last1=Fernández|first2=Gernot|last2=Frenking|first3=Gabriel|last3=Merino|orig-date=2nd January 2015|year=2015|volume=44 |issue=18 |pages=6452–6463 |pmid=25692666 }}
Also, a large number of multinuclear metal complexes can be notionally decomposed into a metallabenzene ligand facially coordinated to another metal center.
Preparation and structure
The first reported stable metallabenzene was the osmabenzene Os(C5H4S)CO(PPh3)2, produced from double addition of acetylene to the corresponding thiocarbonyl complex.{{cite journal|title= Metallacyclohexatrienes or 'metallabenzenes.' Synthesis of osmabenzene derivatives and X-ray crystal structure of [Os(CSCHCHCHCH)(CO)(PPh3)2]|journal=J. Chem. Soc., Chem. Commun.|year=1982| pages=811–813|author1=Elliott, G.P.|author2=Roper, W.R.|author3=Waters, J.M.| doi = 10.1039/C39820000811|issue= 14 }} Characteristic of other metallaarenes, the Os-C bonds are about 0.6 Å longer than the C-C bonds (in benzene these are 1.39 Å), resulting in a distortion of the hexagonal ring. 1H NMR signals for the ring protons are downfield, consistent with aromatic "ring current", and the ring readily undergoes electrophilic aromatic substitution. Osmabenzene and its derivatives can be regarded as an Os(II), d6 octahedral complex.
Metallabenzenes have also been characterized with metals ruthenium,{{Cite journal| doi = 10.1002/anie.200600055| pmid = 16566052| year = 2006| last1 = Zhang | first1 = H.| last2 = Xia | first2 = H.| last3 = He | first3 = G.| last4 = Wen | first4 = T.| last5 = Gong | first5 = L.| last6 = Jia | first6 = G.| title = Synthesis and characterization of stable ruthenabenzenes| volume = 45| issue = 18| pages = 2920–2923| journal = Angewandte Chemie International Edition in English }}{{Cite journal| doi = 10.1021/om070195k| title = Synthesis and Characterization of Stable Ruthenabenzenes Starting from HC⋮CCH(OH)C⋮CH| year = 2007| last1 = Zhang | first1 = H.| last2 = Feng | first2 = L.| last3 = Gong | first3 = L.| last4 = Wu | first4 = L.| last5 = He | first5 = G.| last6 = Wen | first6 = T.| last7 = Yang | first7 = F.| last8 = Xia | first8 = H.| journal = Organometallics| volume = 26| issue = 10| page = 2705 }}{{Cite journal| doi = 10.1021/jo800052u| pmid = 18336045| year = 2008| last1 = Wu | first1 = L.| last2 = Feng | first2 = L.| last3 = Zhang | first3 = H.| last4 = Liu | first4 = Q.| last5 = He | first5 = X.| last6 = Yang | first6 = F.| last7 = Xia | first7 = H.| title = Synthesis and characterization of a novel dialdehyde and cyclic anhydride| volume = 73| issue = 7| pages = 2883–2885| journal = The Journal of Organic Chemistry }}{{Cite journal| doi = 10.1021/om800857k| title = Stable Cationic and Neutral Ruthenabenzenes| year = 2009| last1 = Clark | first1 = G. R.| last2 = O'neale | first2 = T. R.| last3 = Roper | first3 = W. R.| last4 = Tonei | first4 = D. M.| last5 = Wright | first5 = L. J.| journal = Organometallics| volume = 28| issue = 2| page = 567 }} iridium,{{Cite journal| doi = 10.1021/ja00193a064| title = Metallabenzene: synthesis, structure, and spectroscopy of a 1-irida-3,5-dimethylbenzene complex| year = 1989| last1 = Bleeke | first1 = J. R.| last2 = Xie | first2 = Y. F.| last3 = Peng | first3 = W. J.| last4 = Chiang | first4 = M.| journal = Journal of the American Chemical Society| volume = 111| issue = 11| page = 4118 | bibcode = 1989JAChS.111.4118B}}{{Cite journal| doi = 10.1021/ja00012a061| title = Metallacyclohexadiene and metallabenzene chemistry. 5. Chemical reactivity of metallabenzene| year = 1991| last1 = Bleeke | first1 = J. R.| last2 = Xie | first2 = Y. F.| last3 = Bass | first3 = L.| last4 = Chiang | first4 = M. Y.| journal = Journal of the American Chemical Society| volume = 113| issue = 12| page = 4703 | bibcode = 1991JAChS.113.4703B}} platinum,{{Cite journal| doi = 10.1002/1521-3773(20020916)41:18<3470::AID-ANIE3470>3.0.CO;2-4| title = Metallabenzenes and Valence Isomers. Synthesis and Characterization of a Platinabenzene| year = 2002| last1 = Jacob | first1 = V.| last2 = Weakley | first2 = T. J. R.| last3 = Haley | first3 = M. M.| journal = Angewandte Chemie International Edition| volume = 41| issue = 18| pages = 3470–3473 | pmid=12298068}}{{Cite journal| doi = 10.1021/om034371a| title = Rational Synthesis of Platinabenzenes†| year = 2004| last1 = Landorf | first1 = C. W.| last2 = Jacob | first2 = V.| last3 = Weakley | first3 = T. J. R.| last4 = Haley | first4 = M. M.| journal = Organometallics| volume = 23| issue = 6| page = 1174 }}{{Cite journal| doi = 10.1021/om900439z| title = Platinabenzenes: Synthesis, Properties, and Reactivity Studies of a Rare Class of Metalla-aromatics†| year = 2009| last1 = Jacob | first1 = V.| last2 = Landorf | first2 = C. W.| last3 = Zakharov | first3 = L. N.| last4 = Weakley | first4 = T. J. R.| last5 = Haley | first5 = M. M.| journal = Organometallics| volume = 28| issue = 17| page = 5183 }} and rhenium.{{Cite journal| doi = 10.1002/anie.200907014| title = Synthesis and Characterization of Rhenabenzenes| pmid = 20229549| year = 2010| last1 = Poon | first1 = K. C.| last2 = Liu | first2 = L.| last3 = Guo | first3 = T.| last4 = Li | first4 = J.| last5 = Sung | first5 = H. H. Y.| last6 = Williams | first6 = I. D.| last7 = Lin | first7 = Z.| last8 = Jia | first8 = G.| journal = Angewandte Chemie International Edition| volume = 49| issue = 15| pages = 2759–2762 | s2cid = 45468728| doi-access = free}} The iridabenzenes can be produced from ligand substitution, with a vinylcyclopropenide or a linear (CH){{su|b=5|p=−}}-skeletal carbanion displacing an X-type ligand. As of 2020, there remained no general method for the synthesis of metallabenzenes, with most techniques applicable to only two or three metals.{{cite journal|doi=10.1021/acs.chemrev.0c00392|journal=Chemical Reviews|title=Metallaaromatic chemistry|author1=Chen Dafa|author2=Hua Yuhui|author3=Xia Haiping|date=2020 |volume=120 |issue=23 |pmid=33073994 |orig-date=April 30, 2020|at=§ 3.1}}
Metallabenzenes typically exhibit a slight nonplanarity, with the metal nucleus shifted perpendicular to the ring plane. However, ligands that strongly accept π electrons reduce the nonplanarity. These geometric effects are one of the pieces of evidence suggesting that metallabenzenes are Möbius aromats, not Hückel ones.{{cite journal|doi=10.1021/om0701367|journal=Organometallics|year=2007|volume=26|pages=1986–1995|title=Understanding nonplanarity in metallabenzene complexes|author1=Zhu Jun|author2=Jia Guochen|author3=Lin Zhenyang|issue=8 |orig-date=Feb 11, 2007|publisher=American Chemical Society}}