Tetrakis(1-norbornyl)cobalt(IV)

Tetrakis(1-norbornyl)cobalt(IV) is an air-sensitive organometallic compound of cobalt. 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 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 is prepared from the reaction between 1-chloro-norbornane and lithium metal in a solvent such as pentane:

2 CoCl₂•THF 4 norLi → [Co(nor)₄] Co 4 LiCl 2 THF

The compound can then be purified by recrystallization.

Properties

The complex is a thermally stable homoleptic tetraorganylcobalt(IV) complex with exclusively σ-bonding ligands. It was the first low-spin 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). 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 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 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 counterions, along with elemental cobalt.

{\ce {3{CoCl2.THF} 8{norLi} 5{THF}->[{} \atop {\ce {Et2O/THF}}]2{[Li(THF)4][Co(nor)4]} {Co} LiCl}}

The compound is air-sensitive, has a green color and is paramagnetic, 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[BF₄] in THF and crystallizes with tetrafluoroborate as the counterion.

Co(nor)₄ AgBF₄ → [Co(nor)₄]BF₄ Ag

This complex :[Co(nor)₄] is the first cobalt(V) complex to be isolated. Again the configuration is low-spin (d⁴, e⁴t₂⁰).