Metallabenzene
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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 The parent metallabenzenes can be viewed as derivatives of benzene wherein a CH center has been replaced by a transition metal complex. [2]
Classification
All known metallabenzenes are 18-electron complexes,[3] and have been classified into three varieties.[which?] In modeling metallabenzenes, the parent acyclic hydrocarbon ligand is viewed as the anion C5H5−.[4]
Early computations suggested that the six π-electrons in the metallacycle conform to the Hückel (4n+2) theory;[4] however, interactions with an additional d orbital suggest that metallabenzenes may instead be an 8-π Möbius aromat.[2] 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 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 dxz orbital, whose four lobes are all circumferential and effect a Möbius twist (see Fig. 4 in the cited paper).[3] Still other authors instead argue that metallabenzenes are Hückel aromats but with 10 π-electrons.[5]
Also, a large number of multinuclear metal complexes can be notionally decomposed into a metallabenzene ligand facially coordinated to another metal center.[2]
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.[6][2] 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.[2] Osmabenzene and its derivatives can be regarded as an Os(II), d6 octahedral complex.
Metallabenzenes have also been characterized with metals ruthenium,[7][8][9][10] iridium,[11][12] platinum,[13][14][15] and rhenium.[16] The iridabenzenes can be produced from ligand substitution, with a 3-vinylcyclopropene-derived terminal carbanion or a linear (CH)−
5-skeletal carbanion displacing an X-type ligand.[2] As of 2020, there remained no general method for the synthesis of metallabenzenes, with most techniques applicable to only two or three metals.[17]
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.[3]