Diamminedichloroplatinum trans isomers

Two milestones in chemistry may be addressed to the geometric isomers of diamminedichloroplatinum(II). About 100 years ago Alfred Werner laid down the principles of modem coordination chemistry with the help of isomeric [PtCl2(NH3)2] [1]. About 70 years later Barnett Rosenberg and co-workers discovered that the cis isomer is antitumor active, whereas the trans isomer is not [2,3]. Today m-[PtCl2(NH3)2] (cisplatin) is routinely used as a drug, particularly effective against testicular and ovarian cancer [4],... 

In geometric isomers, or cis-trans isomers, of coordination compounds, the same ligands are arranged in different orders within the coordination sphere. Geometric isomerism occurs when atoms or groups of atoms can be arranged on two sides of a rigid structure. Cis means adjacent to and trans means on the opposite side of. Cis- and trans-diamminedichloroplatinum(II) are shown below. 

FIGURE 9.5 cis and trans Isomers of Diamminedichloroplatinum(ll), [PtCljtNHjlj]. The cis isomer, also known as cisplatin, is used in cancer treatment. 

The presence of large deposits of platinum ores in Russia led to an intensive study of the coordination compounds of platinum early in the development of coordination chemistry. As a result of these studies by the Russian school, the first stereospecific displacement reaction (and first example of the trans effect) was discovered. Consider two means of forming diamminedichloroplatinum(ll) (1) displacement of Cl ions from [PtCl4] by NH, (2) displacement of NH3 from [Pt(NH3)4]" by CP ions. It is found that two different isomers are formed ... 

Cisplatin, cw-diamminedichloroplatinum(II), is an extremely potent antitumor agent. Show how it can be prepared in high purity to the exclusion of the trans geometric isomer by employing the kinetic trans effect. 

It is important to underline the fact that a tetrahedral complex, which would have the same global formula as the preceding one, cannot exhibit this isomerism. Hence, for example, the isolation of two isomers of the diamminedichloroplatinum(II) complex [Pt (Cl)2(NH3)2] has been a very strong argument in favor of its planar stmcture. The octahedral hexacoordinated complex dichlorobis(ethylenediamine)cobalt(III) [Co (Cl2)(en)2]+ exists in the cis and trans forms (Fig. 23.4). 

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