Tetrakis(1-norbornyl)cobalt(IV) is an air-sensitive

organometallic compound Organometallic chemistry is the study of organometallic compounds, chemical compounds containing at least one chemical bond between a carbon atom of an organic molecule and a metal, including alkali, alkaline earth, and transition metals, and so ...

cobalt Cobalt is a chemical element with the symbol Co and atomic number 27. As with nickel, cobalt is found in the Earth's crust only in a chemically combined form, save for small deposits found in alloys of natural meteoric iron. The free element, pr ...

. It was first synthesized by Barton K. Bower and Howard G. Tennent in 1972 and is one of few compounds in which cobalt has a formal oxidations state of +4.

Preparation

Tetrakis(1-norbornyl)cobalt(IV) is formed the reaction of CoCl₂•THF with 1-norbornyllithium (norLi) in ''n''-pentane under an

inert atmosphere An inert gas is a gas that does not readily undergo chemical reactions with other chemical substances and therefore does not readily form chemical compounds. The noble gases often do not react with many substances and were historically referred to ...

. The cobalt(II) chloride-THF adduct is prepared from Soxhlet extraction of anhydrous CoCl₂ with

THF Tetrahydrofuran (THF), or oxolane, is an organic compound with the formula (CH2)4O. The compound is classified as heterocyclic compound, specifically a cyclic ether. It is a colorless, water- miscible organic liquid with low viscosity. It is ...

, and the

organolithium reagent In organometallic chemistry, organolithium reagents are chemical compounds that contain carbon–lithium (C–Li) bonds. These reagents are important in organic synthesis, and are frequently used to transfer the organic group or the lithium atom ...

is prepared from the reaction between 1-chloro-

norbornane Norbornane (also known as bicyclo .2.1eptane) is an organic compound and a saturated hydrocarbon with chemical formula C7H12. It is a crystalline compound with a melting point of 88 °C. The carbon skeleton is derived from cyclohexane ring with a ...

and

lithium Lithium (from el, λίθος, lithos, lit=stone) is a chemical element with the symbol Li and atomic number 3. It is a soft, silvery-white alkali metal. Under standard conditions, it is the least dense metal and the least dense solid el ...

metal. : + 4 norLi ->

text Text may refer to: Written word * Text (literary theory), any object that can be read, including: **Religious text, a writing that a religious tradition considers to be sacred **Text, a verse or passage from scripture used in expository preachin ...

] + + 4 + 2 THF : 1-nobornyl-group

The compound can then be purified by Recrystallization (chemistry), recrystallization.

Properties

The complex is a thermally stable

homoleptic In inorganic chemistry, a homoleptic chemical compound is a metal compound with all ligands identical. The term uses the " homo-" prefix to indicate that something is the same for all. Any metal species which has more than one type of ligand is he ...

tetraorganylcobalt(IV) complex with exclusively ''σ''-bonding

ligand In coordination chemistry, a ligand is an ion or molecule (functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's electr ...

s. It was the first

low-spin Spin states when describing transition metal coordination complexes refers to the potential spin configurations of the central metal's d electrons. For several oxidation states, metals can adopt high-spin and low-spin configurations. The ambiguity o ...

complex with

tetrahedral geometry In a tetrahedral molecular geometry, a central atom is located at the center with four substituents that are located at the corners of a tetrahedron. The bond angles are cos−1(−) = 109.4712206...° ≈ 109.5° when all four substituents are ...

to be isolated.

Stability

The exceptional stability of the complex is in large part due to its inability to undergo either α- or

β-hydride elimination β-Hydride elimination is a reaction in which an alkyl group bonded to a metal centre is converted into the corresponding metal-bonded hydride and an alkene. The alkyl must have hydrogens on the β-carbon. For instance butyl groups can undergo th ...

. The α-position of the metal (corresponding to the 1-position of the norbornyl ligand) has no more hydrogen atoms, while hydride elimination from the β-position would yield an energetically unfavorable double bond on a bridgehead atom (

Bredt's rule Bredt's rule is an empirical observation in organic chemistry that states that a double bond cannot be placed at the bridgehead of a bridged ring system, unless the rings are large enough. The rule is named after Julius Bredt, who first discussed i ...

). Moreover, the bulky norbornyl ligands sterically shield the central atom, hindering ligand substitutions as well as homolysis. The rare d⁵ low-spin configuration in a tetrahedral ligand field is possible because the ligand is so strongly ''σ''-donating that the gap between the e und t₂ orbitals is raised sufficiently to overcome the spin pairing energy. The resulting configuration is e⁴t₂¹, with magnetic measurements showing

paramagnetism 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, ...

consistent with only one unparied electron.

Cobalt(III) and cobalt(V) derivatives

The reaction between CoCl₂•THF and 1-norbornyllithium (norLi) also allows the formation of a cobalt(III) complex: if a mixture of

diethyl ether Diethyl ether, or simply ether, is an organic compound in the ether class with the formula , sometimes abbreviated as (see Pseudoelement symbols). It is a colourless, highly volatile, sweet-smelling ("ethereal odour"), extremely flammable liq ...

and THF is used as the solvent in place of ''n''-pentane, the resulting disproportionation reaction affords the complex tetrakis(1-norbornyl)cobaltate(III), which crystallizes out of solution with solvated lithium

counterion 160px, Polystyrene sulfonate, a cation-exchange resin, is typically supplied with as the counterion.">cation-exchange_resin.html" ;"title="Polystyrene sulfonate, a cation-exchange resin">Polystyrene sulfonate, a cation-exchange resin, is typical ...

s, along with elemental

. :3 + 8 + 5 -> atop\ce2 + + LiCl The compound is air-sensitive, has a green color and is

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 ...

, with two unpaired electrons, again indicating a low-spin tetrahedral configuration (d⁶, e⁴t₂²). The corresponding cobalt(V) complex is prepared by oxidizing tetrakis(1-norbornyl)cobalt(IV) with Ag F₄/nowiki> in THF and crystallizes with

tetrafluoroborate Tetrafluoroborate is the anion . This tetrahedral species is isoelectronic with tetrafluoroberyllate (), tetrafluoromethane (CF4), and tetrafluoroammonium () and is valence isoelectronic with many stable and important species including the perchl ...

as the counterion. : This complex : is the first cobalt(V) complex to be isolated. Again the configuration is low-spin (d⁴, e⁴t₂⁰).

References

External link

* {{cite web , last1=Bower , first1=Barton K. , title=Tetra(bicycloheptyl) transition metal compounds , url=https://patents.google.com/patent/US3705916 , date=12 December 1972 Cobalt compounds Organometallic compounds Norbornanes