The introduction to this chapter describes the important anticancer agent cisplatin, cis-[Pt(NH₃)₂Cl₂]. As outlined earlier, the reaction of 1 mol [PtCl₄]²⁺ with 2 mol NH₃ yields this cis complex, whereas the reaction of 1 mol [Pt(NH₃)₄]²⁺ and 2 mol Cl^- yields the trans isomer. The diff erence in these reactions has to do with the difference in the rate of reaction. Substitution occurs more rapidly for a ligand trans to a chloride ion than one trans to ammonia. Draw the structure of both [PtCl₄]²⁺ and [Pt(NH₃)₄]²⁺, and sequentially do the substitution reactions for each stepwise, one ligand at a time, always substituting a ligand trans to a Cl^- to see how the difference in rates produces the different isomers.