Abstract
Metallocenes
are known with most metals (discovery of ferrocene in 1951 and report of its sandwich structure in 1952), and have applications in organic, polymer and medicinal chemistry.
With the MCp2 structure, they are sandwich complexes in which the two Cp rings are parallel (ferrocene). The sandwich MCp2 structure is very robust with NVE = 18 (Fe, Ru, Os). The neutral metallocenes of the 1st-row with 15 to 20 e− (V, Cr, Mn, Fe, Co, Ni) correspond to the filling of the 5d orbitals split in three MO levels (e1g: bonding, double; a1g: non-bonding, simple, e*1g; antibonding, double) under the influence of the pseudo-octahedral field of the two Cp ligands. The metallocenes are all the more fragile as their NVE is further from 18, but they are stabilized by the Cp* (η5-C5Me5) ligand.
Ferrocene
is a diamagnetic (low spin) orange, crystalline solid stable up to 400°C, covalent (nil dipole moment), soluble in hydrocarbons, with free rotation about the Fe-Cp ring axis. It readily oxidizes to blue ferrocenium [FeCp2]+, (most often synthesized as the PF6 − salt), a useful single-electron mild oxidant. Ferrocene is a superaromatic, starting point of many syntheses, for instance of Friedel-Crafts type. The α-ferrocenylcarbonium ions FcCR2 + (Fc = ferrocenyl) are stabilized by conjugation with Fe.
Cobaltocene
[CoCp2] (19e) is electron rich and air sensitive; the 18e [CoCp2]+ cation is robust.
Nickelocene
[NiCp2] (20e) is a practical source of the fragment CpNi and its protonation led to the first triple-decker sandwich [Ni2Cp3]+.
Bent metallocenes
derivatives with non-parallel rings (angle: about 130°), mostly known with early transition metals, are of the type MCp2(R), MCp2(R)2 or MCp2(R)2 (M = Sc, Lu, Ti, V, Cr, etc.; R = H, Cl, alkyl, etc.), and often are stabilized with Cp* (in particular for Sc and the rare earths).
Metal-bis-arene sandwich complexes
accessible by reactions of the Fischer type (MCl3 + arene + AlCl3 + Al) or by the metal-vapor condensation technique, have electronic structures and redox properties close to those of the metallocenes.
Cyclopentadienyl-metal-arene complexes
are mostly known with Fe and Ru. The CpM+ group (M = Fe or Ru) activates many aromatic syntheses (nucleophilic addition and substitution, benzylic deprotonation, etc.). With Fe, they are stable in the 17, 18 and 19e forms with permethylated rings, which provides electron-reservoir properties (stoichiometric and catalytic) and proton-reservoir functions (perfunctionalization of polymethylbenzene ligands).
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Chapter 11-Metallocenes and sandwich complexes
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(2007). Metallocenes and Sandwich Complexes. In: Organometallic Chemistry and Catalysis. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-540-46129-6_13
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