Cyclopentadienyl anion
In chemistry, the cyclopentadienyl anion or cyclopentadienide is an aromatic species with a formula of and abbreviated as Cp. It is formed from the deprotonation of the molecule cyclopentadiene. Properties. The cyclopentadienyl anion is a planar, cyclic, regular-pentagonal ion; it has 6 π-electrons (4"n" + 2, where "n" = 1), which fulfills Hückel's rule of aromaticity. The structure shown is a composite of five resonance contributors in which each carbon atom carries part of the negative charge. Salts of the cyclopentadienyl anion can be stable, e.g., sodium cyclopentadienide. It can also coordinate as a ligand  to metal atoms, forming coordination compounds known as cyclopentadienyl complexes. Biscyclopentadienyl complexes are called metallocenes. Cyclopentadienyl, , and cyclopentadiene, , can substitute one or more hydrogens, forming derivatives having covalent bonds. (See Cyclopentadiene#Derivatives) Abbreviation. The abbreviation Cp for cyclopentadienyl played a role in the naming of element 112, copernicium, for which the originally proposed symbol was Cp. However, as Cp was already in use as the abbreviation for cyclopentadienyl (and Cp had been used for cassiopeium, a former name of lutetium), the symbol for copernicium was changed to Cn.