Half sandwich compound

Half sandwich compounds, also known as piano stool complexes, are organometallic complexes that feature a cyclic polyhapto ligand bound to an ML_n center, where L is a unidentate ligand. Thousands of such complexes are known.^{[1][page needed]} Well-known examples include cyclobutadieneiron tricarbonyl and (C₅H₅)TiCl₃. Commercially useful examples include (C₅H₅)Co(CO)₂, which is used in the synthesis of substituted pyridines, and methylcyclopentadienyl manganese tricarbonyl, an antiknock agent in petrol.

• 
  MMT is a commercially useful antiknock compound.
• 
  CpCo(CO)₂ is a catalyst for the synthesis of pyridines.
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  (C₄H₄)Fe(CO)₃.
• 
  CpFe(CO)₂I is an example of a piano stool complex with two different monodentate ligands.
• 
  The diruthenium of cymene is readily cleaved by ligands to give monoRu half-sandwich derivatives.
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  Cycloheptatriene molybdenum tricarbonyl
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  Cp₂V₂(CO)₅ featuring a pair of semi-bridging CO ligands.^[2]

Half sandwich complexes containing cyclopentadienyl ligands are common. Well studied examples include (η⁵-C₅H₅)V(CO)₄, (η⁵-C₅H₅)Cr(CO)₃H, (η⁵-CH₃C₅H₄)Mn(CO)₃, (η⁵-C₅H₅)Cr(CO)₃H, [(η⁵-C₅H₅)Fe(CO)₃]⁺, (η⁵-C₅H₅)V(CO)₄I, and (η⁵-C₅H₅)Ru(NCMe)⁺
₃. (η⁵-C₅H₅)Co(CO)₂ is a two-legged piano stool complex. Bulky cyclopentadienyl ligands such as 1,2,4-C₅H₂(tert-Bu)₃⁻ form unusual half-sandwich complexes.^[3]

In organometallic chemistry, (η⁶-C₆H₆) piano stool compounds are half-sandwich compounds with (η⁶-C₆H₆)ML₃ structure (M = Cr, Mo, W, Mn(I), Re(I) and L = typically CO). (η⁶-C₆H₆) piano stool complexes are stable 18-electron coordination compounds with a variety of chemical and material applications. Early studies on (η⁶-C₆H₆)Cr(CO)₃ were carried out by Natta, Ercoli and Calderazzo,^[4] and Fischer and Ofele,^[5][6] and the crystal structure was determined by Corradini and Allegra in 1959.^[7] The X-ray data indicate that the plane of the benzene ring is nearly parallel to the plane defined by the oxygen atoms of the carbonyl ligands, and so the structure resembles a benzene seat mounted on three carbonyl legs tethered by the metal atom.

Piano stool complexes of the type (η⁶-C₆H₆)M(CO)₃ are typically synthesized by heating the appropriate metal carbonyl compound with benzene. Alternately, the same compounds can be obtained by carbonylation of the bis(arene) sandwich compounds, such as (η⁶-C₆H₆)₂M compound with the metal carbonyl compound. This second approach may be more appropriate for arene ligands containing thermally fragile substituents.^[8]

The benzene ligand in (η⁶-C₆H₆)Cr(CO)₃Mi is prone to deprotonation.^[9] For example, Organolithium compounds form adducts featuring cyclohexadienyl ligands. Subsequent oxidation of the complex results in the release of a substituted benzene.^[10][11] Oxidation of the chromium atom by I₂ and other iodine reagents has been shown to promote exchange of arene ligands, but the intermediate chromium iodide species has not been characterized.^[12]

(η⁶-C₆H₆)Cr(CO)₃ complexes exhibit "cine" and "tele" nucleophilic aromatic addition.^[13] Processes of this type involve reaction of (η⁶-C₆H₆)Cr(CO)₃ with an alkyl lithium reagent. Subsequent treatment with an acid results in the addition of a nucleophile to the benzene ring at a site ortho ("cine"), meta or para ("tele") to the ipso carbon (see Arene substitution patterns).

Reflecting its increased acidity, the benzene ligand can be lithiated with n-butyllithium. The resulting organolithium compound serves as a nucleophile in various reactions, for example, with trimethylsilyl chloride:^{[citation needed]}

(η⁶-C₆H₆)Cr(CO)₃ is a useful catalyst for the hydrogenation of 1,3-dienes. The product alkene results from 1,4-addition of hydrogen. The complex does not hydrogenate isolated double bonds.^{[citation needed]}

A variety of arenes ligands have been installed aside from benzene.^[14] Weakly coordinating ligands may be employed to improve ligand exchange and thus the turnover rates for (η⁶-C₆H₆)M(CO)₃ complexes.^[8]: 248(η⁶-C₆H₆)M(CO)₃ complexes have been incorporated into high surface area porous materials.^[15]

(η⁶-C₆H₆)M(CO)₃ complexes serve as models for the interaction of metal carbonyls with graphene and carbon nanotubes.^[16] The presence of M(CO)₃ on extended π-network materials has been shown to improve electrical conductivity across the material.^[17]

Typical arene tricarbonyl piano stool complexes of Mn(I) and Re(I) are cationic and thus exhibit enhanced reactivity toward nucleophiles. Subsequent to nucleophilic addition, the modified arene can be recovered from the metal.^[18][19]

Half-sandwich compounds employing Ru(II), such as (cymene)ruthenium dichloride dimer, have been mainly investigated as catalysts for transfer hydrogenation.^[20] These complexes feature three coordination sites that are susceptible to substitution, while the arene ligand is tightly bonded and protects the metal against oxidation to Ru(III). They are prepared by reaction of RuCl₃·x(H₂O) with 1,3-cyclohexadienes.^[21] Work is also conducted on their potential as anticancer drugs.^[22]

(η⁶-C₆H₆)RuCl₂ readily undergoes ligand exchange via cleavage of the chloride bridges, making this complex a versatile precursor to Ru(II) piano stool derivatives.^[23]