Nickelocene

Nickelocene is the organonickel 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,[1] although it does not yet have any known practical applications.

Nickelocene
Nickelocene
Space-filling model of nickelocene
Names
IUPAC name
nickelocene
Other names
Bis(cyclopentadienyl) nickel(II)
Identifiers
3D model (JSmol)
ChEBI
ChemSpider
ECHA InfoCard 100.013.672
EC Number
  • 215-039-0
3412
RTECS number
  • QR6500000
UNII
UN number 1325 3082
Properties
C10H10Ni
Molar mass 188.88 g/mol
Appearance Green crystals
Density 1.47 g/cm3
Melting point 171 to 173 °C (340 to 343 °F; 444 to 446 K)
insoluble
Structure
D5h, D5d
0 D
Hazards
GHS pictograms
GHS Signal word Danger
H228, H302, H317, H350
P201, P202, P210, P240, P241, P261, P264, P270, P272, P280, P281, P301+312, P302+352, P308+313, P321, P330, P333+313, P363, P370+378, P405, P501
NFPA 704 (fire diamond)
Flammability code 4: Will rapidly or completely vaporize at normal atmospheric pressure and temperature, or is readily dispersed in air and will burn readily. Flash point below 23 °C (73 °F). E.g. propaneHealth code 2: Intense or continued but not chronic exposure could cause temporary incapacitation or possible residual injury. E.g. chloroformReactivity code 2: Undergoes violent chemical change at elevated temperatures and pressures, reacts violently with water, or may form explosive mixtures with water. E.g. white phosphorusSpecial hazards (white): no code
4
2
2
Lethal dose or concentration (LD, LC):
490 mg kg−1 (oral, rat)
600 mg kg−1 (oral, mouse)
Related compounds
Related compounds
 CoCp2, FeCp2
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
N verify (what is YN ?)
Infobox references

Structure and bonding

Ni(C5H5)2 belongs to a group of organometallic compounds called metallocenes. Metallocenes usually adopt structures in which a metal ion is sandwiched between two parallel cyclopentadienyl (Cp) rings. In the solid-state, the molecule has D5d symmetry, wherein the two rings are staggered.[2]

The Ni center has a formal +2 charge, and the Cp rings are usually assigned as cyclopentadienyl anions (Cp−), related to cyclopentadiene by deprotonation. The 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. The two remaining d-electrons each reside in the dyz and dxz orbitals, giving rise to the molecule's paramagnetism, as manifested in the unusually high field chemical shift observed in its 1H NMR spectrum. With 20 valence electrons, nickelocene has the highest electron count of the transition metal metallocenes. Cobaltocene, 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 in 1953, shortly after the discovery of ferrocene, the first metallocene compound.[3] It has been prepared in a one-pot reaction, by deprotonating cyclopentadiene with ethylmagnesium bromide, and adding anhydrous nickel(II) acetylacetonate.[4] A modern synthesis entails treatment of anhydrous sources of NiCl2 (such as hexaamminenickel chloride) with sodium cyclopentadienyl:[5]

[Ni(NH3)6]Cl2 + 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.[6] Samples are typically handled with air-free techniques.

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 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 to produce cyclopentadienyl nickel nitrosyl, a highly toxic organonickel compound.

References
  1. Elschenbroich, C. (2006). Organometallics. Weinheim: Wiley-VCH. ISBN 978-3-527-29390-2.
  2. P. Seiler; J. D. Dunitz (1980). "The structure of nickelocene at room temperature and at 101 K". Acta Crystallogr. B36: 2255–2260. doi:10.1107/S0567740880008539.
  3. Fischer, E. O.; Pfab, W. (1952). "Zur Kristallstruktur der Di-Cyclopentadienyl-Verbindungen des zweiwertigen Eisens, Kobalts und Nickels" [On the crystal structure of biscyclopentadienyl compounds of divalent iron, cobalt and nickel]. Z. Naturforsch. B. 7: 377–379.
  4. Wilkinson, G.; Pauson, P. L.; Cotton, F. A. (1954). "Bis-cyclopentadienyl Compounds of Nickel and Cobalt". J. Am. Chem. Soc. 76 (7): 1970–4. doi:10.1021/ja01636a080.
  5. Girolami, G. S.; Rauchfuss, T. B.; Angelici, R. J. (1999). Synthesis and Technique in Inorganic Chemistry. Mill Valley, CA: University Science Books. ISBN 0935702482.
  6. Jaworska-Augustyniak, Anna; Wojtczak, Jan (1979). "Charge-transfer complexes of cobaltocene and nickelocene with tetrachloromethane". Monatshefte für Chemie. 110 (5): 1113–1121. doi:10.1007/BF00910959.
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