Chirality and optical isomerism

Key points: A number of ligand arrangements at an octahedral centre give rise to chiral compounds; isomers are designated or depending on their configuration. In addition to the many examples of geometrical isomerism shown by octahedral compounds, many are also chiral. A very simple example is [Mn(acac)₃] (66), where three bidentate

acetylacetonato (acac) ligands result in the existence of enantiomers. One way of looking at the optical isomers that arise in complexes of this nature is to imagine looking down one of the threefold axes and see the ligand arrangement as a propellor or screw thread. Chirality can also exist for complexes of formula [MA₂B₂C₂] when the ligands of each pair are cis to each other (62). In fact, many examples of optical isomerism are known for octahedral complexes with both monodentate and polydentate ligands, and we must always be alert to the possibility of optical isomerism. As a further example of optical isomerism, consider the products of the reaction of cobalt(III) chloride and ethylenediamine in a 1:2 mole ratio. The product includes a pair of dichlorido complexes, one of which is violet (67) and the other green (68); they are, respectively, the cis and trans isomers of dichloridobis(ethylenediamine)cobalt(III), [CoCl₂ (en)₂] . As can be seen from their structures, the cis isomer cannot be superimposed on its mirror image. It is therefore chiral and hence (because the complexes are long-lived) optically active. The trans isomer has a mirror plane and can be superimposed on its mirror image; it is achiral and optically inactive. The absolute configuration of a chiral octahedral complex is described by imagining a view along a threefold rotation axis of the regular octahedron and noting the handed ness of the helix formed by the ligands (Fig. 7.7). Clockwise rotation of the helix is then designated ∆ (delta) whereas the anticlockwise rotation is designated (lambda). The designation of the absolute configuration must be distinguished from the experimentally determined direction in which an isomer rotates polarized light: some compounds rotate in one direction, others rotate in the opposite direction, and the direction may change with wavelength. The isomer that rotates the plane of polarization clockwise (when viewed into the oncoming beam) at a specified wavelength is designated the d-isomer, or the (+)-isomer; the one rotating the plane anticlockwise is designated the l-isomer, or the (-)-isomer. Box 7.1 describes how the specific isomers of a complex might be synthesized and Box 7.2 describes how enantiomers of metal complexes may be separated. Complexes with coordination numbers of greater than six have the potential for a great number of isomers, both geometrical and optical. As these complexes are often stereochemically nonrigid, the isomers are usually not separable and we do not consider them further.

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مواضيع ذات صلة

The atom and its ions

Oxidation states

Occurrence

Enthalpies of atomization

Electronegativity

Ionization energies and electron affinities

Computational techniques

Electrochemical techniques

Thermal analysis

X-ray fluorescence elemental analysis

CHN analysis

Atomic absorption spectroscopy