User:Doodlepuppy123/Cyclopentadienyl anion

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In chemistry, the cyclopentadienyl anion or cyclopentadienide is an aromatic species with a formula of and abbreviated as Cp&#x2212;. Deprotonation of cyclopentadiene forms the cyclopentadienyl anion. The cyclopentadienyl anion is a ligand which binds to a metal in organometallic chemistry.

Resonance and aromaticity
The cyclopentadienyl anion is a planar, cyclic, regular-pentagonal ion; it has 6 π-electrons (4n + 2, where n = 1), which fulfills Hückel's rule of aromaticity. Each double bond and lone pair contains 2 π-electrons because the lone pair delocalizes into the ring, shown by moving to different positions in its resonance structures. The cyclopentadienyl anion is a conjugated system because there are alternating π and 𝜎 bonds. All carbons in the planar ring are sp2 hybridized and contain a p orbital. Two p orbitals overlap to form a π bond.

The structure shown is a composite of five resonance contributors in which each carbon atom carries part of the negative charge.

Cyclopentadiene has a pKa of about 16 which is acidic since the conjugate base, cyclopentadienyl anion, has resonance stabilization of the lone pair and negative formal charge.

Ligand Characteristics
The cyclopentadienyl anion is a polydentate ligand because it has multiple attachment points to a metal. The carbon with the lone pair and negative formal charge is an X-type ligand (anionic electron donor) and both double bonds are L-type (neutral electron donors). Since the cyclopentadienyl anion has two L-type ligands and one X-type ligand, it is an L2X ligand.

The cyclopentadienyl anion has 5 atoms in an uninterrupted π system that bind to a metal, so it has a maximum hapticity of 5.

Cyclopentadienyl groups are not reactive with nucleophiles or electrophiles, making them good spectator ligands.

Bonding to form complexes
The cyclopentadienyl anion can bond as a ligand to metal atoms, forming coordination compounds known as cyclopentadienyl complexes. Examples of cyclopentadienyl complexes are ferrocenium, which is an oxidant, and cobaltocene, which is a soluble reductant.



Cyclopentadienyl anions bonded to metals form shorter bonds than neutral aromatic hydrocarbons because they are more electron-rich and therefore better electron donors.

When two cyclopentadienyl anions bind to a metal, they form bis(cyclopentadienyl) complexes.

Bis(cyclopentadienyl) complexes with an oxidation state of two are called metallocenes. An X-type ligand causes the oxidation state to increase by one, so two L2X cyclopentadienyl ligands would increase the oxidation state by two. If there are no other X-type ligands and no overall charge then the oxidation state of the metal would be two. The cyclopentadienyl anions can bind to the metal in different ways. For example, cyclopentadienyl anions bind linearly in ferrocene, either eclipsed or staggered, so the complex will have D5h or D5d symmetry, respectively. If the cyclopentadienyl anions bind in a bent shape (less than 180 degrees apart), the symmetry will be C2v or Cs for eclipsed or staggered, respectively. Cyclopentadienyl zinc complexes show that the cyclopentadienyl anion can bind to the zinc metal center in different ways such that it can have hapticities of 1 to 5. The complex is symmetric when the cyclopentadienyl anion has a hapticity of 5 and is asymmetric when there are intermediate hapticities of 2 or 3.

To synthesize cyclopentadienyl complexes, salts of the cyclopentadienyl anion are reagents. Lithium and sodium cations are commonly used, but potassium cations can also form cyclopentadienide salts. Sodium cyclopentadienide and potassium cyclopentadienide are stable because of the aromatic anion.

Cyclopentadienyl,, and cyclopentadiene, , can substitute one or more hydrogens, forming derivatives having covalent bonds. (See Cyclopentadiene)

Applications
Cyclopentadienyl complexes are a potential anticancer therapy. Organometallic C,N-bound Ir(III) cyclopentadienyl complexes exhibit cytotoxicity. The cytotoxicity was increased by substituting biphenyl and phenyl groups to the Cp ligand.

Cyclopentadienyl anion influenced Copernicium's symbol
It was proposed that element 112, copernicium, have the symbol Cp, but Cp- was already the abbreviation for the cyclopentadienyl anion, so copernicium’s elemental symbol is Cn.