Isomers diamminedichloroplatinum

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],... 

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 ... 

Given the same coordination polyhedron, diastereoisomerism can arise when the ligands are not all alike as with the as and irans - isomers of tetraamminedichloro-chromium(lll), diamminedichloroplatinum(ll), and bis(2-aminoeihanethiolato)nickel(ll) (Examples 1-6). 

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. 

Geometric isomers are isomers in which the atoms are joined to one another in the same way but differ because some atoms occupy different relative positions in space. Consider the complex diamminedichloroplatinum(II), [Pt(NH3)2Cl2]. Two compounds of this composition are known. One is an orange-yellow compound with solubility at 25°C of 0.252 g pct 100 g of water. The other compound is pale yellow and much less soluble (0.037 g per 100 g of water at 25°C). How do you explain the occurrence of these two isomCTs ... 

Geometric isomers of the square planar complex diamminedichloroplatinum(ll)... 

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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