Nickelocene is the organonickel compound with the
formulaA chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound....
Ni(
η5The term hapticity is used to describe how a group of contiguous atoms of a ligand are coordinated to a central atom. Hapticity of a ligand is indicated by the Greek character 'eta', η. A superscripted number following the η denotes the number of contiguous atoms of the ligand that are bound to...
-C
5H
5)
2. Also known as bis(cyclopentadienyl)nickel or NiCp
2, this bright green
paramagneticParamagnetism is a form of magnetism whereby the paramagnetic material is only attracted when in the presence of an externally applied magnetic field. In contrast with this, diamagnetic materials are repulsive when placed in a magnetic field...
solid is of enduring academic interest, although it yet has no practical applications.
Structure and bonding
Ni(C
5H
5)
2 belongs to a group of organometallic compounds called
metalloceneA metallocene is a compound typically consisting of two cyclopentadienyl anions bound to a metal center in the oxidation state II, with the resulting general formula 2M. Closely related to the metallocenes are the metallocene derivatives, e.g. titanocene dichloride, vanadocene dichloride...
s. Metallocenes usually adopt structures in which a metal
ionAn ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
is sandwiched between two parallel
cyclopentadienylA cyclopentadienyl complex is a metal complex with one or more cyclopentadienyl groups . Based on the type of bonding between the metals and the cyclopentadienyl]] moieties, cyclopentadienyl complexes are classified into the following three categories: a) π-complexes, b) σ-complexes, and c) ionic...
rings. This structure is relevant to the solubility in organic solvents and volatility. In the solid-state, the molecule has D
5h symmetryThe symmetry group of an object is the group of all isometries under which it is invariant with composition as the operation...
, wherein the two rings are eclipsed.
The Ni center has a formal 2+ charge, and the Cp rings are usually assigned as [cyclopentadienyl anion]]s (Cp
-), related to cyclopentadiene by deprotonation. The structure is similar to
ferroceneFerrocene is an organometallic compound with the formula Fe2. It is the prototypical metallocene, a type of organometallic chemical compound consisting of two cyclopentadienyl rings bound on opposite sides of a central metal atom. Such organometallic compounds are also known as sandwich compounds...
. 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
x2–y2, d
z2. One of the two remaining d-electrons resides in each of the d
yz and d
xz orbitals, giving rise to the molecule's paramagnetism. With 20
valence electronIn chemistry, valence electrons are the electrons of an atom that can participate in the formation of chemical bonds with other atoms. Valence electrons are the "own" electrons, present in the free neutral atom, that combine with valence electrons of other atoms to form chemical bonds. In a single...
s, nickelocene has the highest electron count of the transition metal metallocenes.
CobaltoceneCobaltocene, known also as biscobalt or even "bis Cp cobalt", is an organocobalt compound with the formula Co2. It is a dark purple solid that sublimes readily slightly above room temperature. Cobaltocene was discovered shortly after ferrocene, the first metallocene...
, Co(C
5H
5)
2, is, however, a stronger reducing agent.
Preparation
Nickelocene was first prepared by E. O. Fischer in 1953, shortly after the discovery of
ferroceneFerrocene is an organometallic compound with the formula Fe2. It is the prototypical metallocene, a type of organometallic chemical compound consisting of two cyclopentadienyl rings bound on opposite sides of a central metal atom. Such organometallic compounds are also known as sandwich compounds...
, the first metallocene compound. It has been prepared in a one-pot reaction, by deprotonating cyclopentadiene with ethylmagnesium bromide, and adding anhydrous
nickel(II) acetylacetonateNickel acetylacetonate is a coordination complex with the formula [Ni2]3. This dark green solid reacts with water to give the dihydrate Ni2·2H2O....
. A modern synthesis entails treatment of anhydrous sources of NiCl
2 with sodium cyclopentadienyl:
-
- [Ni(NH3)6]Cl2 + 2 NaC5H5 → Ni(C5H5)2 + 2 NaCl + 6 NH3
Properties
Like many organometallic compounds, Ni(C
5H
5)
2 does not tolerate extended exposure to air before noticeable decomposition. Samples are typically handled with
air-free techniqueAir-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 commonly carbon dioxide and nitrogen...
s.
Most chemical reactions of nickelocene are characterized by its tendency to yield
18-electronThe 18-electron rule is a rule of thumb used primarily for predicting formulas for stable metal complexes. The rule rests on the fact that valence shells of a transition metal consists of nine valence orbitals, which collectively can accommodate 18 electrons either as nonbinding electron pairs or...
products with loss or modification of one Cp ring.
- Ni(C5H5)2 + 4 PF3
Phosphorus trifluoride , is a colorless and odorless gas. It is highly toxic and it reacts slowly with water. Its main use is as a ligand in metal complexes...
→ Ni(PF3)4 + organic products
The reaction with secondary phosphines follows a similar pattern:
-
- 2 Ni(C5H5)2 + 2 PPh2H
Diphenylphosphine, also known as diphenylphosphane, is organophosphorus compound with the formula 2PH. This foul smelling, colorless liquid is easily oxidized in air...
→ [Ni2(PPh2)2(C5H5)2] + 2 C5H6
Nickelocene can be oxidized to the corresponding cation, which contains Ni(III).
Gaseous Ni(C
5H
5)
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.
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