Chiral Complexes

Many ligands are chiral as a result of asymmetric centres which mean they exist as two different optical isomers. These ligands may be present as a racemate (a 50: 50 mixture of the two optical forms) or else as a chiral form (that is as one optical isomer). The presence of chirality in a ligand is 'felt' by the complex and seen in its chiroptical properties but there are rarely any particular special requirements for synthesis involving introduction of a chiral ligand as opposed to its racemic form. However, in some circumstances the complex itself can be chiral as a result of the disposition of ligands. As a general rule, the presence of at least two chelate rings is sufficient for some octahedral complexes to be resolvable into their enantiomeric forms. The classical chiral metal complex is the octahedral [Co(en)3]3+ ion which can be separated into Δ and A optical forms through successive recystallization of its salt formed with an optically active anion such as d-tartrate (6.23). This approach is based on the (Δ.d) salt forming a more stable crystalline lattice than the (A.d) salt meaning their solubility differs and one is preferentially precipitated.

It is also possible to separate the optical isomers through chromatography on a chiral cation-exchange resin (such as SP-Sephadex® C-25 resin) by using a chiral eluate such as a d-tartrate solution. Differential binding to the resin, which is itself chiral, means the complex separates on a sufficiently long column into two bands comprising the two optical forms of the complex.

Chirality is often met with polydentate ligands at a number of different centres in the complex, and separation of all optical isomers is either impractical or, in effect, impossible. In many cases, working with a racemate has no significant influence on the chemistry, and optical resolution of complexes is attempted on only very limited occasions, such as where researchers wish to record the chiroptical properties or where a chiral complex is required to assist in achieving a chiral reaction, such as use of a chiral complex as a catalyst in synthesis of organic molecules where a particular optical isomer is sought.

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مواضيع ذات صلة

Reactions Involving the Metal Oxidation State

Catalysed Reactions

Substitution Reactions without using a Solvent

Substitution Reactions in Nonaqueous Solvents

Reactions Involving the Coordination Shell

Block f : Inner Transition Metals (Lanthanoids and Actinoids)

Block d: Transition Metals

Block p: Main Group Metals

= Block s : Alkali and Alkaline Earth Metals

A New Face - Oxidation-Reduction

A New Body - Stereochemical Change

Lability and Inertness in Octahedral Complexes