Tris(ethylenediamine)cobalt(III) chloride

Tris(ethylenediamine)cobalt(III) chloride is a coordination complex with the formula [Co(en)₃]Cl₃ where en is the abbreviation for ethylenediamine. This complex 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.^[1]

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Synthesis and structure

The compound is prepared from an aqueous solution of ethylenediamine and virtually any cobalt(II) salt, such as cobalt(II) chloride. 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 [Co(en)₂Cl(H₂NCH₂CH₂NH₃]Cl₃, which contains a rare monodentate ethylenediamine ligand (protonated).^[2]

The cation [Co(en)₃]³⁺ is octahedral 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.^[3]

Stereochemistry

The complex can be resolved into enantiomers that are described as Δ and Λ. Usually the resolution entails use of tartrate salts.^[4] The optical resolution is a standard component of inorganic synthesis courses.^[5] 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 diastereomeric 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.^[6]

Hydrates

Cationic coordination complexes of ammonia and alkyl amines typically crystallize with water in the lattice, and the stoichiometry can depend on the conditions of crystallization and, in the cases of chiral complexes, the optical purity of the cation. Racemic [Co(en)₃]Cl₃ is most often obtained as the di- or trihydrate. For the optically pure salt (+)-[Co(en)₃]C1₃^.1.5H₂0, (+)-[Co(en)₃]Cl₃^.0.5NaCl^.3H₂O, and (+)-[Co(en)₃]C1₃^.H₂O are also known.^[3]

References

1. ^ A. Werner (1912). "Zur Kenntnis des asymmetrischen Kobaltatoms. V". Chemische Berichte 45: 121-130. doi:10.1002/cber.19120450116.
2. ^ Jack M. Harrowfield, Mark I. Ogden, Brian W. Skelton, Allan H. White (2005). "Alfred Werner revisited: Some subtleties of complex ion synthesis and isomerism". C. R. Chimie 8: 121–128. doi:10.1016/j.crci.2004.10.013.
3. ^ ^{a b} D. Witiak, J. C. Clardy, and D. S. Martin, Jnr. (1972). "The Crystal Structure of (+)-D-tris(ethylenediamine)cobalt(III) nitrate". Acta Crystallographica B28: 2694-2699. doi:10.1107/S056774087200679X.
4. ^ J. A. Broomhead, F. P. Dwyer, J. W. Hogarth (1960). "Resolution of the Tris(Ethylenediamine)Cobalt(III) Ion". Inorganic Syntheses VI: 183-186. doi:10.1002/9780470132371.
5. ^ Girolami, G. S.; Rauchfuss, T. B. and Angelici, R. J., Synthesis and Technique in Inorganic Chemistry, University Science Books: Mill Valley, CA, 1999.ISBN: 0935702482
6. ^ von Zelewsky, A. "Stereochemistry of Coordination Compounds" John Wiley: Chichester, 1995. ISBN 047195599.