Platinum complexes geometric isomers

Platinum(ll) complexes are very stable and slow to react among them are numerous examples of square planar geometrical isomers. The best known of these are cis- and tra 5-[PtCl2(NH3)2], structures V and VI. In recent years,... 

Bipyridine and phenanthroline are employed as auxiliary ligands of the alkylplatinum(iv) complexes, for example, 921-923, and methyl(iodo)bis(thiolato)-platinum(iv) complexes are obtained as a mixture of the two geometrical isomers 924 and 925. The dimethyl(iodo)phenoxoplatinum(iv) complex 926 has also been prepared. Halogenation of the five-coordinated olefin-platinum(ii) complex with a phen ligand affords the Pt(iv) complex with three halides and a halogenoalkyl ligand. 

By a judicious application of this principle it has been possible to prepare many different geometric isomers of platinum(II) complexes, including the three possible isomers of complexes containing four different ligands such as nPtNH3pyN02Cl]. 

Pairs of geometric isomers differ from each other in their standard free energies of formation. However, the differences are typically not large. In the case of labile complexes, one isomer will be transformed into the thermodynamically favored form as equilibrium is reached. However, in the case of inert complexes, the conversion of one isomer into the other can be inhibited. In such cases, it may prove possible to study redox equilibria without the problem of an accompanying isomerization. Platinum and ruthenium chemistry has provided many examples of such systems. 

The synthesis and antitumour properties of DACH platinum(II) complexes containing a variety of different types of leaving ligand have been reported [38, 39, 60-67]. Although early studies with the compounds were content to utilize mixtures of geometric and optical isomers, the realization that the stereochemistry of the DACH ligand influenced antitumour activity underscored the importance of working with isomerically pure forms of the diamine [68-70],... 

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