Optical activity - the Cotton effect

Optical activity in a molecule can occur when and only when it contains a chiral centre. This means that the molecule can exist in two diastereomeric forms, which are non-superimposable. Six-coordinate chelate complexes of the type M(bidentate ligand)3 (Fig. 3.1(a)) and ds-M(bidentate ligand)2X2 (Fig 3.1(b)) are common examples in metallic complexes. 

The Cotton effect can be used to correlate the configurations of related disym-metric molecules and the magnitudes of the ORD and CD effects are useful in determining or confirming the assignments of electronic absorption bands. Optical activity is likely to occur when transition metals are complexed by 

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Ring expansion of a- and fi-pinene. When either a- or jS-pinene, (1) or (2), is heated neat with an initial pressure of 30 psi CO at 160° for 68 hr. with an equimolar amount of iron pentacarbonyl, two ketones, (3) and (4), are obtained, formed by insertion of CO into the cyclobutane ring together with some a-pinene. Both ketones are optically active. The Cotton effects are of nearly the same... 

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