Ferrocene and Other Sandwich Compounds

This potassium salt, K4Ni2(CN)6, may be further reduced by potassium in liquid ammonia to yield a yellow substance, K4Ni(CN)4. This has nickel in the zero-valent state and is thus comparable to the metal carbonyls, Fe(CO)5 and Ni(CO)4 (p. 157), to cobalt nitrosyl carbonyl Co(CO)4NO, and to the metal ammoniates Ca(NH3)6 and Pt(NH3)2. However, K4Ni(CN)4, and the closely related acetylene derivative, K4Ni(C=CH)4, are especially unusual, for in them, the zero-valent metal has been incorporated into an anion, whereas in the carbonyls and metal ammoniates, the zerovalent metals are present as uncharged species. 

A very remarkable series of organometallic compounds, derived principally from the hydrocarbon cyclopentadiene, (III), and having sandwichlike structures, was first synthesized in the 1950 s, chiefly by G. Wilkinson. The first member of the series, ferrocene (biscyeiopentadienyl-iron), abbreviated Fe(cpd)2, may be made by heating cyclopentadiene 

It should not be supposed that all metal derivatives of cyclopentadiehe are sandwichlike molecules. In many cases, the metal is joined to just one of the carbons in the ring through a single (cr) bond, which, however, often has considerable ionic character. Metal-to-earbon bonds almost certainly exist in the cyclopentadiene derivatives of the alkali and alkaline-earth metals, the rare earths, and certain of the post-transitioii elements. 

Very recently a number of more complicated sandwichlike compounds 

Sidgwick, N. V. The Chemical Elements and Their Compounds, 1316-1543, Oxford University Press, London (1950). 

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