Bis cyclopentadienyl complexes - the metallocenes

Bis(cyclopentadienyl)complexes have been isolated for the elements of the First Transition Series from vanadium to nickel inclusive and also for ruthenium and osmium. The most general method of preparation is to treat an anhydrous halide of the metal with sodium cyclopentadienide in tetrahydrofuran under an atmosphere of nitrogen or argon. 

The dihalides of Mn, Fe, Co and Ni are readily available. Trihalides of vanadium and chromium can be used, but an excess of NaCgHg is then required as a reducing agent. Alternatively MX, can be reduced in situ to MX, using zinc or magnesium before NaC5Hg is added. 

Cyclopentadiene can be deprotonated by dimethylamine in the presence of anhydrous TiCl, FeCl, or NiCl, to yield Cp,TiCl, (p. 290), Cp,Fe (ferrocene) or Cp,Ni (nickelocene) respectively. Ferrocene has even been prepared in a rapidly stirred two phase system (benzene/water) from FeCl, 4H,0, cyclopentadiene and potassium hydroxide using a crown ether, 18-crown-6, as a phase-transfer catalyst. 

The metallocenes are coloured crystalline solids, all of which melt at about 173°C. They are all thermally robust and sublime readily under reduced pressure. Ferrocene is remarkably resistant to air, moisture and heat, surviving even up to 470°C. All the other metallocenes listed in Table 9.1 are oxidized in air, manganocene and cobaltocene being especially vulnerable. Manganocene is also very easily hydrolysed, making it a rather difficult substance to handle. 

The sandwich structure of ferrocene was suggested in 1952, soon after its discovery, by G. Wilkinson and independently by E.O. Fischer, and confirmed 

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