Metallocenes and Sandwich Complexes

Abstract

{M}etallocenes 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. {W}ith the {MCp2} structure, they are sandwich complexes in which the two {C}p rings are parallel (ferrocene). {T}he sandwich {MCp2} structure is very robust with {NVE} = 18 ({F}e, {R}u, {O}s). {T}he neutral metallocenes of the 1st-row with 15 to 20 e− ({V}, {C}r, {M}n, {F}e, {C}o, {N}i) 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 {C}p ligands. {T}he metallocenes are all the more fragile as their {NVE} is further from 18, but they are stabilized by the {C}p* ({η5-C5Me5}) ligand. {F}errocene 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. {I}t readily oxidizes to blue ferrocenium [{FeCp2}]+, (most often synthesized as the {PF6} − salt), a useful single-electron mild oxidant. {F}errocene is a superaromatic, starting point of many syntheses, for instance of {Friedel-Crafts} type. {T}he α-ferrocenylcarbonium ions {FcCR2} + ({F}c = ferrocenyl) are stabilized by conjugation with {F}e. {C}obaltocene [{CoCp2}] (19e) is electron rich and air sensitive; the 18e [{CoCp2}]+ cation is robust. {N}ickelocene [{NiCp2}] (20e) is a practical source of the fragment {CpNi} and its protonation led to the first triple-decker sandwich [{Ni2Cp3}]+. {B}ent 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} = {S}c, {L}u, {T}i, {V}, {C}r, etc.; {R} = {H}, {C}l, alkyl, etc.), and often are stabilized with {C}p* (in particular for {S}c and the rare earths). {M}etal-bis-arene sandwich complexes accessible by reactions of the {F}ischer type ({MCl3} + arene + {AlCl3} + {A}l) or by the metal-vapor condensation technique, have electronic structures and redox properties close to those of the metallocenes. {C}yclopentadienyl-metal-arene complexes are mostly known with {F}e and {R}u. {T}he {CpM}+ group ({M} = {F}e or {R}u) activates many aromatic syntheses (nucleophilic addition and substitution, benzylic deprotonation, etc.). {W}ith {F}e, 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).