The first ever cyclopentadienyl organometallic was synthesised by K. Thiele in 1901.
This was done by reacting C5H6 with potassium. Strictly speaking this was a cyclopentadienyl salt.
In 1948 3 scientists J.F. Tremaine, J.A. Tebboth and S.A. Miller were attempting to synthesise amines form olefins using and iron catalyst.
Instead the cyclopentadiene reacted with the catalyst to form what appeared to be an iron containing compound.
This exact same compound was remade by T.J. Kealy and P.L. Pauson in an attempt to synthesise "Fulvalene".
It was first sugested that the iron containing compound was the following molecule
The true nature of this compound was determined in 1952 independently by G. Wilkinson, R.B. Woodward at Havard and E.O. Fisher in Munich.
G. Wilkinson, R.B. Woodward sugested a "Sandwich structure" based on infrared spectroscopy,diamagnetism and dipole moment =0.
E.O Fisher suggested a "Double cone structure" based on X-ray structural analysis, diamagnetism and chemical behaviour.
This compound was to be named ferrocene due to its aromatic behaviour and has become one of the most studied organometallic compound. Soon many other derivatives were prepared such as ruthenocene, osmocene, manganocene etc. By analogy of there names these becmae know as metallocenes.
The discovery of ferrocene was in effect the discovery of sandwich compounds which has now formed the conerstone of organometallic chemistry.
The general formulae for such compounds is a metal sandwiched between two hydrocarbon ligands (CnHnX-) with a planar pie electron system.
Metallocenes have now becomes one of the magor reagents used in catalysis chemistry.
Unfortunately only the organotransition compounds follow the 18 electron rule and even with these metals there are exeptions.
The ligands in sanwich compounds do not nescesarily have to be equal and often we get a variety of different types.
