Nickelocene is the organonickel compound with the

formula In science, a formula is a concise way of expressing information symbolically, as in a mathematical formula or a ''chemical formula''. The informal use of the term ''formula'' in science refers to the general construct of a relationship betwee ...

Ni( ''η''⁵-C₅H₅)₂. Also known as bis(cyclopentadienyl)nickel or NiCp₂, this bright green

paramagnetic Paramagnetism is a form of magnetism whereby some materials are weakly attracted by an externally applied magnetic field, and form internal, induced magnetic fields in the direction of the applied magnetic field. In contrast with this behavior, d ...

solid is of enduring academic interest, although it does not yet have any known practical applications.

Structure and bonding

Ni(C₅H₅)₂ belongs to a group of organometallic compounds called

metallocene A metallocene is a compound typically consisting of two cyclopentadienyl anions (, abbreviated Cp) bound to a metallic element, metal center (M) in the oxidation state II, with the resulting general formula Closely related to the metallocenes are ...

s. Metallocenes usually adopt structures in which a metal

ion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...

is sandwiched between two parallel

cyclopentadienyl Cyclopentadienyl can refer to *Cyclopentadienyl anion, or cyclopentadienide, **Cyclopentadienyl ligand *Cyclopentadienyl radical, • *Cyclopentadienyl cation, See also *Pentadienyl In organic chemistry, pentadienyl refers to the organic radic ...

(Cp) rings. In the solid-state, the molecule has D_5d

symmetry Symmetry (from grc, συμμετρία "agreement in dimensions, due proportion, arrangement") in everyday language refers to a sense of harmonious and beautiful proportion and balance. In mathematics, "symmetry" has a more precise definit ...

, wherein the two rings are staggered. The Ni center has a formal +2 charge, and the Cp rings are usually assigned as

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 i ...

s (Cp⁻), related to cyclopentadiene by deprotonation. The structure is similar to

ferrocene Ferrocene is an organometallic compound with the formula . The molecule is a complex consisting of two cyclopentadienyl rings bound to a central iron atom. It is an orange solid with a camphor-like odor, that sublimes above room temperature, a ...

. 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: d_''xy'', d_{''x''²–''y''²}, d_''z''². The two remaining d-electrons each reside in the d_''yz'' and d_''xz'' orbitals, giving rise to the molecule's paramagnetism, as manifested in the unusually high field chemical shift observed in its ¹H

NMR spectrum Nuclear magnetic resonance spectroscopy, most commonly known as NMR spectroscopy or magnetic resonance spectroscopy (MRS), is a spectroscopic technique to observe local magnetic fields around atomic nuclei. The sample is placed in a magnetic fie ...

. With 20

valence electron In chemistry and physics, a valence electron is an electron in the outer shell associated with an atom, and that can participate in the formation of a chemical bond if the outer shell is not closed. In a single covalent bond, a shared pair forms ...

s, nickelocene has the highest electron count of the transition metal metallocenes.

Cobaltocene Cobaltocene, known also as bis(cyclopentadienyl)cobalt(II) or even "bis Cp cobalt", is an organocobalt compound with the formula Co(C5H5)2. It is a dark purple solid that sublimes readily slightly above room temperature. Cobaltocene was discovere ...

, Co(C₅H₅)₂, with only 19 valence electrons is, however, a stronger reducing agent, illustrating the fact that electron energy, not electron count determines redox potential.

Preparation

Nickelocene was first prepared by

E. O. Fischer Ernst Otto Fischer (; 10 November 1918 – 23 July 2007) was a German chemist who won the Nobel Prize for pioneering work in the area of organometallic chemistry. Early life He was born in Solln, a borough of Munich. His parents were Karl T. F ...

in 1953, shortly after the discovery of

, the first metallocene compound to be discovered. It has been prepared in a one-pot reaction, by deprotonating cyclopentadiene with ethylmagnesium bromide, and adding anhydrous

nickel(II) acetylacetonate Nickel(II) bis(acetylacetonate) is a coordination complex with the formula i(acac)2sub>3, where acac is the anion C5H7O2− derived from deprotonation of acetylacetone. It is a dark green paramagnetic solid that is soluble in organic solvents suc ...

. A modern synthesis entails treatment of anhydrous sources of NiCl₂ (such as

hexaamminenickel chloride Hexaamminenickel chloride is the chemical compound with the formula i(NH3)6l2. It is the chloride salt of the metal ammine complex i(NH3)6sup>2+. The cation features six ammonia (called ammines in coordination chemistry) ligands attached to the ...

) with

sodium cyclopentadienyl Sodium cyclopentadienide is an organosodium compound with the formula C5H5Na. The compound is often abbreviated as NaCp, where Cp− is the cyclopentadienide anion. Sodium cyclopentadienide is a colorless solid, although samples often are p ...

: :: i(NH₃)₆l₂ + 2 NaC₅H₅ → Ni(C₅H₅)₂ + 2 NaCl + 6 NH₃

Properties

Like many organometallic compounds, Ni(C₅H₅)₂ does not tolerate extended exposure to air before noticeable decomposition. Samples are typically handled with

air-free technique Air-free techniques refer to a range of manipulations in the chemistry laboratory for the handling of compounds that are air-sensitive. These techniques prevent the compounds from reacting with components of air, usually water and oxygen; less comm ...

s. 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 → i₂(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 nickel mirror upon contact with a hot surface, releasing the hydrocarbon ligands as gaseous coproducts. This process has been considered as a means of preparing nickel films. Nickelocene reacts with

nitric acid Nitric acid is the inorganic compound with the formula . It is a highly corrosive mineral acid. The compound is colorless, but older samples tend to be yellow cast due to decomposition into oxides of nitrogen. Most commercially available nitri ...

to produce

cyclopentadienyl nickel nitrosyl Cyclopentadienyl nickel nitrosyl is a highly toxic organonickel chemical. In its pure form, it is a diamagnetic, volatile, relatively air-stable liquid with a blood-red color. It has been reported to be the simplest mono- cyclopentadienyl metal co ...

, a highly toxic organonickel compound.

References

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External links

IARC Monograph "Nickel and Nickel compounds"
Metallocenes Organonickel compounds Cyclopentadienyl complexes Substances discovered in the 1950s