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| Nickelocene | |
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| IUPAC name | nickelocene |
| Other names | Bis(cyclopentadienyl)nickel(II) |
| Identifiers | |
| CAS number | 1271-28-9] |
| RTECS number | QR6500000 |
| Properties | |
| Molecular formula | C10H10Ni |
| Molar mass | 188.88 g/mol |
| Appearance | Green crystals |
| Density | ? g/cm3, solid |
| Melting point |
171-173 °C |
| Solubility in water | insoluble |
| Structure | |
| Coordination geometry |
D5h, D5d |
| Dipole moment | 0 D |
| Hazards | |
| Main hazards | flammable |
| R-phrases | 49-11-22-43 |
| S-phrases | 53-36/37/39-45 |
| Related compounds | |
| Related compounds | CoCp2, FeCp2 |
| Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox references |
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Nickelocene is the organometallic 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 has no practical applications.
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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 the solid-state, the molecule has D5h symmetry, wherein the two rings are eclipsed.
The Ni center has a formal 2+ charge, and the Cp rings are usually assigned as cyclopentadienyl anions (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. With 20 valence electrons, nickelocene 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.[2] It has been prepared in a one-pot reaction, by deprotonating cyclopentadiene with ethylmagnesium bromide, and adding anhydrous nickel(II) acac.[3] A modern synthesis entails treatment of anhydrous sources of NiCl2 with sodium cyclopentadienyl:[4]
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- [Ni(NH3)6Cl2 + 2 NaC5H5 → Ni(C5H5)2 + 2 NaCl + 6 NH3
Chemical properties
Like many organometallic compounds, Ni(C5H5)2 can tolerate only brief exposure to air before noticeable decomposition. 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:
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- 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.
References
- ^ Elschenbroich, C. ”Organometallics” (2006) Wiley-VCH: Weinheim. ISBN 978-3-29390-6
- ^ E. O. Fischer, W. Pfab (1952). "Zur Kristallstruktur der Di-Cyclopentadienyl-Verbindungen des zweiwertigen Eisens, Kobalts und Nickels". Z. Naturforsch. B 7: 377–379.
- ^ Wilkinson, G.; Pauson, P. L.; Cotton, F. A. (1954). "Bis-cyclopentadienyl Compounds of Nickel and Cobalt". J. Am. Chem. Soc. 76. doi:.
- ^ Girolami, G. S.; Rauchfuss, T. B. and Angelici, R. J., Synthesis and Technique in Inorganic Chemistry, University Science Books: Mill Valley, CA, 1999.ISBN: 0935702482
External links
- IARC Monograph "Nickel and Nickel compounds"
- National Pollutant Inventory - Nickel and compounds Fact Sheet
