Nickelocene

Nickelocene is the chemical compound with the formula Ni(η5-C5H5)2. Also known as bis(cyclopentadienyl)nickel or NiCp2 this bright green paramagnetic solid is of enduring academic interest, although it has few real applications.

Structure and bonding
Ni(C5H5)2 belongs to a group of organometallic compounds called metallocenes. Metallocenes usually adopt sandwich structures in which a metal ion is sandwiched between two parallel cyclopentadienyl rings, a structure which not only confers stability but also solubility in organic solvents and volatility (nickelocene readily sublimes under vacuum). In its crystalline form the molecule has D5h symmetry, in which the two rings adopt an eclipsed conformation.

The Ni center can be considered to have a 2+ charge, and the Cp rings are each considered to be a cyclopentadienyl anion (Cp-), related to cyclopentadiene by deprotonation (this structure is similar to ferrocene). In terms of its electronic structure, three pairs of d electrons on nickel are allocated to the three d orbitals involved in Ni - Cp bonding: dxy, dx2–y2, dz2. One of the two remaining d-electrons resides in each of the dyz and dxz orbitals, giving rise to the molecule's paramagnetism. Nickelocene, with 20 valence electrons, has the highest electron count of the transition metal metallocenes. Cobaltocene, Co(C5H5)2, is, however, a stronger reducing agent.

Preparation
Nickelocene was first prepared by E. O. Fischer in 1953, shortly after the discovery of ferrocene, the first metallocene compound. The modern synthesis entails treatment of anhydrous sources of NiCl2 with sodium cyclopentadienyl:
 * [Ni(NH3)6]Cl2 +  2 NaC5H5  →  Ni(C5H5)2  +  2 NaCl  +  6 NH3

Chemical properties
Like many organometallic compounds, Ni(C5H5)2 decomposes in air. Thus samples are typically stored cold and protected from air, although samples can tolerate exposure for several minutes without noticeable decomposition.

Most chemical reactions of nickelocene are characterized by its tendency to yield 18-electron products with loss or modification of one Cp ring.
 * Ni(C5H5)2 +  4 PF3  →  Ni(PF3)4   +  organic products

The reaction with secondary phosphines follows a similar pattern:
 * 2 Ni(C5H5)2 +  2 PPh2H  →  [Ni2(PPh2)2(C5H5)2]    +  2 C5H6

Nickelocene can be oxidized to the corresponding cation, which contains Ni(III).

Gaseous Ni(C5H5)2 decomposes to a Ni mirror upon contact with a hot surface, releasing the hydrocarbon ligands as gaseous coproducts. This process has been considered as a means of preparing Ni films.