View source for Tetrakis(1-norbornyl)cobalt(IV)

{{Chembox
| ImageFile      = [[File:Tetrakis(1-norbornyl)cobalt(IV).png|200px|Structure of tetrakis(1-norbornyl)cobalt(IV)]]
| OtherNames     = (''T''-4)-Tetrakis(bicyclo[2.2.1]hept-1-yl)cobalt
| Section1       = {{Chembox Identifiers
| CASNo=36333-80-9
| CASNo_Ref= {{cascite|correct|CAS}}
| SMILES = C1CC2CCC1(C2)[Co](C(CC1)2CCC1C2)(C(CC1)2CCC1C2)C(CC1)2CCC1C2
| StdInChI=1S/4C7H11.Co/c4*1-2-7-4-3-6(1)5-7;/h4*6H,1-5H2;
| StdInChIKey=FULAMMPKLQBENQ-UHFFFAOYSA-N
}}
| Section2       = {{Chembox Properties
|C=28|H=44|Co=1
|Appearance = brown crystals
|MeltingPt = 100 °C (decomposes)
|SolubleOther = soluble in THF
}}
}}
'''Tetrakis(1-norbornyl)cobalt(IV)''' is an air-sensitive [[Organometallic chemistry|organometallic compound]] of [[cobalt]]. It was first synthesized by Barton K. Bower and Howard G. Tennent in 1972<ref name="Mnor4">{{Cite journal |last=B. K. Bower and H. G. Tennent |year=1972 |title=Transition metal bicyclo[2.2.1]hept-1-yls |journal=[[J. Am. Chem. Soc.]] |volume=94 |issue=7 |pages=2512–2514 |doi=10.1021/ja00762a056}}</ref> 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 CoCl<sub>2</sub>•THF with 1-norbornyllithium (norLi) in [[Pentanes|''n''-pentane]] under an [[inert atmosphere]].<ref name="Mnor4"/> The cobalt(II) chloride-THF adduct is prepared from [[Soxhlet extraction]] of anhydrous [[Cobalt(II) chloride|CoCl<sub>2</sub>]] with [[Tetrahydrofuran|THF]], and the [[organolithium reagent]] is prepared from the reaction between 1-chloro-[[norbornane]] and [[lithium]] metal.

:<chem>{2CoCl2.THF} + 4 norLi ->[\text{pentane}][] {[Co(nor)4]} + {Co} + 4 {LiCl} + 2 THF </chem>

:[[File:1-nobornyl-group.png|180x180px|The 1-norbornyl group]]

The compound can then be purified by [[Recrystallization (chemistry)|recrystallization]].

== Properties ==
The complex is a thermally stable [[homoleptic]] tetraorganylcobalt(IV) complex with exclusively [[Molecular orbital theory|''σ''-bonding]] [[ligand]]s. It was the first [[low-spin]] complex with [[tetrahedral geometry]] to be isolated.<ref name="Holleman 2007 1695">{{Cite book |last=Holleman |first=Arnold F. |url=https://www.worldcat.org/oclc/180963521 |title=Lehrbuch der anorganischen Chemie |last2=Wiberg |first2=E. |last3=Wiberg |first3=N. |date=2007 |isbn=978-3-11-017770-1 |edition=102nd |location=Berlin |pages=1695 |oclc=180963521}}</ref><ref name="Conor4-1986">{{Cite journal |last=E. K. Byrne, D. S. Richeson and K. H. Theopold |year=1986 |title=Tetrakis(1-norbornyl)cobalt, a low spin tetrahedral complex of a first row transition metal |journal=[[J. Chem. Soc., Chem. Commun.]] |issue=19 |pages=1491–1492 |doi=10.1039/C39860001491}}</ref><ref name="Conor345-2">{{Cite journal |last=E. K. Byrne, K. H. Theopold |year=1989 |title=Synthesis, characterization, and electron-transfer reactivity of norbornyl complexes of cobalt in unusually high oxidation states |journal=[[J. Am. Chem. Soc.]] |volume=111 |issue=11 |pages=3887–3896 |doi=10.1021/ja00193a021}}</ref>

=== Stability ===
The exceptional stability of the complex is in large part due to its inability to undergo either [[Alpha elimination|α-]] or [[Beta-Hydride elimination|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 (chemistry)|homolysis]].<ref name="Mnor4"/><ref>{{Cite book |last=Riedel |first=Erwin |url=http://www.degruyter.com/view/books/9783110190601/9783110190601/9783110190601.xml |title=Moderne Anorganische Chemie |last2=Alsfasser |first2=R. |last3=Janiak |first3=C. |last4=Klapötke |first4=T. M. |last5=Meyer |first5=H.-J. |date=2007 |publisher=Walter de Gruyter |isbn=978-3-11-020685-2 |location=Berlin • New York |pages=718 |doi=10.1515/9783110206852}}</ref>

The rare d<sup>5</sup> 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 t<sub>2</sub> orbitals is raised sufficiently to overcome the spin pairing energy. The resulting configuration is e<sup>4</sup>t<sub>2</sub><sup>1</sup>, with [[Magnetochemistry|magnetic measurements]] showing [[paramagnetism]] consistent with only one unpaired electron.<ref name="Mnor4"/><ref name="Conor4-1986"/><ref name="Conor345-2"/>

=== Cobalt(III) and cobalt(V) derivatives ===
The reaction between CoCl<sub>2</sub>•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 [[Solvation|solvated]] lithium [[counterion]]s, along with elemental [[cobalt]].<ref name="Conor345-2"/><ref name="Conor345">{{Cite journal |last=E. K. Byrne, K. H. Theopold |year=1987 |title=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 |journal=[[J. Am. Chem. Soc.]] |volume=109 |issue=4 |pages=1282–1283 |doi=10.1021/ja00238a066}}</ref>

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

The compound is air-sensitive, has a green color and is [[paramagnetic]], with two unpaired electrons, again indicating a low-spin tetrahedral configuration (d<sup>6</sup>, e<sup>4</sup>t<sub>2</sub><sup>2</sup>).<ref name="Conor345"/><ref name="Conor345-2"/>

The corresponding cobalt(V) complex is prepared by oxidizing tetrakis(1-norbornyl)cobalt(IV) with [[Silver tetrafluoroborate|Ag[BF<sub>4</sub><nowiki>]</nowiki>]] in THF and crystallizes with [[Non-coordinating anion|tetrafluoroborate]] as the counterion.<ref name="Conor345"/><ref name="Conor345-2"/>

:{{chem2|Co(nor)4 + AgBF4 -> [Co(nor)4]BF4 + Ag}}

This complex :{{chem2|[Co(nor)4]+}} is the first cobalt(V) complex to be isolated. Again the configuration is low-spin (d<sup>4</sup>, e<sup>4</sup>t<sub>2</sub><sup>0</sup>).<ref name="Holleman 2007 1695"/><ref name="Conor345-2"/><ref name="Conor345"/>

== See also ==

* [[Metal tetranorbornyl]]

== References ==
<references />

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

[[Category:Cobalt compounds]]
[[Category:Organometallic compounds]]
[[Category:Norbornanes]]