tris(ethylenediamine)cobalt(III)
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Tris(ethylenediamine)cobalt(III) chloride is an inorganic compound with the formula o(en)3l3 (where "en" is the abbreviation for ethylenediamine). It is the chloride salt of the
coordination complex A coordination complex consists of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of bound molecules or ions, that are in turn known as '' ligands'' or complexing agents. ...
o(en)3sup>3+. This trication was important in the history of coordination chemistry because of its stability and its stereochemistry. Many different salts have been described. The complex was first described by Alfred Werner who isolated this salt as yellow-gold needle-like crystals.


Synthesis and structure

The compound is prepared from an aqueous solution of ethylenediamine and virtually any cobalt(II) salt, such as
cobalt(II) chloride Cobalt(II) chloride is an inorganic compound of cobalt and chlorine, with the formula . The compound forms several hydrates ·''n'', for ''n'' = 1, 2, 6, and 9. Claims of the formation of tri- and tetrahydrates have not been confirmed.M. T. Saug ...
. The solution is purged with air to oxidize the cobalt(II)-ethylenediamine complexes to cobalt(III). The reaction proceeds in 95% yield, and the trication can be isolated with a variety of anions. A detailed product analysis of a large-scale synthesis revealed that one minor by-product was o(en)2Cl(H2NCH2CH2NH3)l3, which contains a rare monodentate ethylenediamine ligand (protonated). The cation o(en)3sup>3+ is
octahedral In geometry, an octahedron (plural: octahedra, octahedrons) is a polyhedron with eight faces. The term is most commonly used to refer to the regular octahedron, a Platonic solid composed of eight equilateral triangles, four of which meet a ...
with Co-N distances in the range 1.947–1.981 Å. The N-Co-N angles are 85° within the chelate rings and 90° between nitrogen atoms on adjacent rings.


Stereochemistry

The point group of this complex is D3. The complex can be resolved into enantiomers that are described as Δ and Λ. Usually the resolution entails use of
tartrate A tartrate is a salt or ester of the organic compound tartaric acid, a dicarboxylic acid. The formula of the tartrate dianion is O−OC-CH(OH)-CH(OH)-COO− or C4H4O62−. The main forms of tartrates used commercially are pure crystalline ta ...
salts. The optical resolution is a standard component of inorganic synthesis courses. Because of its nonplanarity, the MN2C2 rings can adopt either of two conformations, which are described by the symbols λ and δ. The registry between these ring conformations and the absolute configuration of the metal centers is described by the nomenclature lel (when the en backbone lies parallel with the C3 symmetry axis) or ob (when the en backbone is obverse to this same C3 axis). Thus, the following
diastereomer In stereochemistry, diastereomers (sometimes called diastereoisomers) are a type of stereoisomer. Diastereomers are defined as non-mirror image, non-identical stereoisomers. Hence, they occur when two or more stereoisomers of a compound have di ...
ic conformations can be identified: Δ-(lel)3, Δ-(lel)2(ob), Δ-(lel)(ob)2, and Δ-(ob)3. The mirror images of these species of course exist also.von Zelewsky, A. "Stereochemistry of Coordination Compounds" John Wiley: Chichester, 1995 .


Hydrates

Cationic coordination complexes of
ammonia Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous wa ...
and
alkyl In organic chemistry, an alkyl group is an alkane missing one hydrogen. The term ''alkyl'' is intentionally unspecific to include many possible substitutions. An acyclic alkyl has the general formula of . A cycloalkyl is derived from a cycloa ...
amines typically crystallize with water in the
lattice Lattice may refer to: Arts and design * Latticework, an ornamental criss-crossed framework, an arrangement of crossing laths or other thin strips of material * Lattice (music), an organized grid model of pitch ratios * Lattice (pastry), an orna ...
, and the stoichiometry can depend on the conditions of crystallization and, in the cases of chiral complexes, the optical purity of the cation. Racemic o(en)3l3 is most often obtained as the di- or trihydrate. For the optically pure salt (+)- o(en)3l3·1.5H2O, (+)- o(en)3l3·0.5NaCl·3H2O, and (+)- o(en)3l3·H2O are also known.


References

{{reflist Cobalt complexes Cobalt(III) compounds Chlorides Metal halides Ethylenediamine complexes