Tetrakis(1-norbornyl)cobalt(IV) explained

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[1] 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.[1] 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.

+ 4 norLi ->[\text{pentane}][] + + 4 + 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.[2] [3] [4]

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.[1] [5]

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 unparied electron.[1] [3] [4]

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.[4] [6]

3 + 8 + 5 ->[{}\atop\ce{Et2O/THF}] 2 + + 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).[6] [4]

The corresponding cobalt(V) complex is prepared by oxidizing tetrakis(1-norbornyl)cobalt(IV) with Ag[BF<sub>4</sub><nowiki>]]] in THF and crystallizes with tetrafluoroborate as the counterion.

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

See also

External links

Notes and References

  1. B. K. Bower and H. G. Tennent . 1972 . Transition metal bicyclo[2.2.1]hept-1-yls . . 94 . 7 . 2512–2514 . 10.1021/ja00762a056.
  2. Book: Holleman, Arnold F. . Lehrbuch der anorganischen Chemie . Wiberg . E. . Wiberg . N. . 2007 . 978-3-11-017770-1 . 102nd . Berlin . 1695 . 180963521.
  3. E. K. Byrne, D. S. Richeson and K. H. Theopold . 1986 . Tetrakis(1-norbornyl)cobalt, a low spin tetrahedral complex of a first row transition metal . . 19 . 1491–1492 . 10.1039/C39860001491.
  4. E. K. Byrne, K. H. Theopold . 1989 . Synthesis, characterization, and electron-transfer reactivity of norbornyl complexes of cobalt in unusually high oxidation states . . 111 . 11 . 3887–3896 . 10.1021/ja00193a021.
  5. Book: Riedel, Erwin . Moderne Anorganische Chemie . Alsfasser . R. . Janiak . C. . Klapötke . T. M. . Meyer . H.-J. . 2007 . Walter de Gruyter . 978-3-11-020685-2 . Berlin • New York . 718 . 10.1515/9783110206852.
  6. E. K. Byrne, K. H. Theopold . 1987 . Redox chemistry of tetrakis(1-norbornyl)cobalt. Synthesis and characterization of a cobalt(V) alkyl and self-exchange rate of a Co(III)/Co(IV) couple . . 109 . 4 . 1282–1283 . 10.1021/ja00238a066.