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 ...

cobalt Cobalt is a chemical element; it has Symbol (chemistry), 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. ...

. 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

oxidation state In chemistry, the oxidation state, or oxidation number, is the hypothetical Electrical charge, charge of an atom if all of its Chemical bond, bonds to other atoms are fully Ionic bond, ionic. It describes the degree of oxidation (loss of electrons ...

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

and

lithium Lithium (from , , ) is a chemical element; it has chemical symbol, symbol Li and atomic number 3. It is a soft, silvery-white alkali metal. Under standard temperature and pressure, standard conditions, it is the least dense metal and the ...

metal in a solvent such as pentane: : : 1-nobornyl-group

The compound can then be purified by recrystallization.

Properties

The complex is a thermally stable homoleptic tetraorganylcobalt(IV) complex with exclusively ''σ''-bonding

ligand In coordination chemistry, a ligand is an ion or molecule with a 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 el ...

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 ...

complex with tetrahedral geometry 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. 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 In organic chemistry, an anti-Bredt molecule is a Bridged compound, bridged molecule with a double bond at the Bicyclic molecule, bridgehead. Bredt's rule is the empirical observation that such molecules only form in large ring systems. For exam ...

). 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 unpaired 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 with the chemical formula , sometimes abbreviated as . It is a colourless, highly Volatility (chemistry), volatile, sweet-smelling ("ethereal odour"), extremely flammable liquid. It belongs ...

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, cation-exchange_resin.html" ;"title="Polystyrene sulfonate, a cation-exchange resin">Polystyrene sulfonate, a cation-exchange resin, is typically supplied with as the counterion. In chemistry, a counterion (sometimes written as "counter ...

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, ...

, 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 perc ...

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

References

External links