Circular dichroism spectra chelates

Optical activity in metal complexes may also arise either if one of the ligands bound to the metal in the first co-ordination sphere is itself optically active or if the complex as a whole lacks a centre of inversion and a plane of symmetry. Thus all octahedral cts-complexes of the tris-or bis-chelate type have two isomeric forms related by a mirror plane, the d- and /-forms. These species have circular dichroism spectra of identical intensities but opposite in sign. The bands in the circular dichroism spectrum are, of course, modified if ligand exchange occurs but they are also exceedingly sensitive to the environment beyond the first co-ordination sphere. This effect has been used to obtain association constants for ion-pair formation. There also exists the possibility that, if such compounds display anti-tumour activity, only one of the mirror isomers will be effective. 

The circular dichroism spectra in the region of the first absorption band of the tris-bidentate complex ions having six-membered chelate rings sure known to be particularly sensitive to experimental conditions. For example, the CD spectrum of A-lel -[Co(R,R-ptn) ]C1, in an aqueous solution shows two peaks A = -O.589, 522 nm A = - 0.104, 462.5 nm, whereas that of A-lel,-[Co(R,R-ptn) ](CIO,) in an aqueous solution gives a negative peak (A , = -O.587T at 518 nm (41). The solid state CD differ from the solution CD and the solution CD are sensitive to the temperature of measurement and are affected by the presence of oxo anions (42, 43, 44). Table V lists the lowest frequency CD spectra of tris-diamine cobalt(III) complexes in the CT region. 

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