Synthesis of cis-trans isomers

There are two approaches to the preparation of cis-trans isomers preparation of a mixture of isomers (neeessitating subsequent separation) and stereospecific synthesis that yields a single product. The second approach has been most successful for the synthesis of isomers of platinum(ll) complexes by taking advantage of the irons effect (Section 4.8). Reactions of cobalt(in) complexes often yield a mixture of cis-trans isomers which must then be separated. 

Complexes of platinum(IV) can readily be prepared as the trans isomer, whereas the cis form is much more difficult to obtain. The oxidation of square planar platinum(II) complexes generally yields the corresponding octahedral platinum(rV) complex. The preparation of tratw-[PtCl2(NH3)4] is achieved in this manner (42). Oxidizing agents such as Br2 and H2O2 yield the 

Synthesis of czs-[PtCl2(NH3)4]Cl2 requires the more tedious approach given in reaction (43). 

The replacement of one or more ligands in a particular isomer by other groups is sometimes used to prepare new compounds which have a desired stereochemistiy. This is not a reliable method, however, because in many cases the stereochemistty of the starting material is not preserved during reaction. 

Structural assignments are now usually made on the basis of physicochemical techniques such as X-ray diffraction and spectroscopy. Another relatively simple technique involves measurements of dipole moments. The dipole moments of cis and trans isomers are often markedly different. This seems to be particularly true for certain square planar complexes, structure (45). 

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