Tris(glycinato)cobalt(III) describes
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. Many ...
es with the formula . Several isomers exist of these octahedral complexes formed between low-spin d6 Co(III) and the conjugate base of the amino acid
glycine
Glycine (symbol Gly or G; ) is an amino acid that has a single hydrogen atom as its side chain. It is the simplest stable amino acid (carbamic acid is unstable), with the chemical formula NH2‐ CH2‐ COOH. Glycine is one of the proteinogeni ...
.
Structures
Both a
meridional isomer and a
facial isomer are known. In the former the Co-O bonds share a plane, and in the facial isomer they do not. Each of these two isomers exists also as pairs of stereoisomers, termed Δ and Λ. This set of compounds are prototypes of many
tris(aminocarboxylate) complexes, with the notable distinction that the Co(III) derivatives do not isomerize readily and can thus be separated.
The violet isomer is obtained anhydrous, whereas the red derivative is the monohydrate.
[ X-ray crystallographic characterization of the mer isomer demonstrates the existence of a dihydrate, however.
]
Synthesis
The reaction of glycine with sodium tris(carbonato)cobalt(III)
Sodium tris(carbonato)cobalt(III) is the name given to the inorganic compound with the formula Na3Co(CO3)3•3H2O. The salt contains an olive-green metastable cobalt(III) coordination complex. The salt is sometimes referred to as the “Field-Du ...
produces both the violet meridional and red-pink facial isomers in approximately equal amounts. The compounds are separated by fractional crystallization Fractional crystallization may refer to:
* Fractional crystallization (chemistry), a process to separate different solutes from a solution
* Fractional crystallization (geology)
Fractional crystallization, or crystal fractionation, is one of the ...
.X-ray crystallography
X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles ...
.
Similar synthetic methods apply to the preparation of tris(chelates) of other amino acid
Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha am ...
s. With L-alanine
Alanine (symbol Ala or A), or α-alanine, is an α-amino acid that is used in the biosynthesis of proteins. It contains an amine group and a carboxylic acid group, both attached to the central carbon atom which also carries a methyl group side c ...
, L-leucine
Leucine (symbol Leu or L) is an essential amino acid that is used in the biosynthesis of proteins. Leucine is an α-amino acid, meaning it contains an α-amino group (which is in the protonated −NH3+ form under biological conditions), an α- ca ...
, and other amino acids, one obtains four stereoisomers. With cysteine, the amino acid binds through N and thiolate.[{{cite journal , doi=10.1021/ic00343a061, title=Stereospecificity in the Synthesis of the Tris((''R'')-Cysteinato-''N,S'')- and Tris((''R'')-Cysteinesulfinato-N,S)cobaltate(III) Ions , year=1990 , last1=Arnold , first1=Alan P. , last2=Jackson , first2=W. Gregory , journal=Inorganic Chemistry , volume=29 , issue=18 , pages=3618–3620 ]
The isomeric forms of tris(glycinato)cobalt(III) are poorly soluble in water. The solubility increases considerably in acidic solution.
References
Cobalt complexes
Cobalt(III) compounds
Octahedral compounds
Glycinates