Rhodocene

Rhodocene, formally known as bis(η⁵-cyclopentadienyl)rhodium(II), is a chemical compound with the formula [Rh(C₅H₅)₂]. Each molecule contains an atom of rhodium bound between two planar systems of five carbon atoms known as cyclopentadienyl rings in a sandwich arrangement. It is an organometallic compound as it has (haptic) covalent rhodium–carbon bonds. The [Rh(C₅H₅)₂] radical is found above 150 °C or when trapped by cooling to liquid nitrogen temperatures (−196 °C). At room temperature, pairs of these radicals combine to form a dimer, a yellow solid in which two of these cyclopentadienyl rings are joined.

The history of organometallic chemistry includes the 19th century discoveries of Zeise's salt and Ludwig Mond's discovery of nickel tetracarbonyl. These compounds posed a challenge to chemists as the compounds did not fit with chemical bonding models as they were then understood. A further challenge arose with the discovery of ferrocene, the iron analogue of rhodocene and the first of the class of compounds now known as metallocenes. Ferrocene was found to be unusually chemically stable, as were analogous chemical structures including rhodocenium, the unipositive cation of rhodocene and its cobalt and iridium counterparts. The study of organometallic species including these ultimately led to the development of new bonding models that explained both their formation and their stability. Work on sandwich compounds, including the rhodocenium / rhodocene system, earned Geoffrey Wilkinson and Ernst Otto Fischer the 1973 Nobel Prize for Chemistry.