[AlexRM]

We have been learning about cistplatin, and our lecturer told us that we always get the cis-complex, rather than the trans complex when substituting two equivalents of ammonia onto tetrachloroplatinate.  This is due the trans effect, whereby the stability of a ligand's bond to the metal ion can be reduced by the ligand trans to it.  Chlorine's trans effect is stronger than amine's, so the second ammonia is substituted trans to chlorine rather than to amine.  This makes sense to me.

However, then when he described cisplatin's believed mechanism, he said than when cisplatin enters a cell, chloride ligands will be substituted with aqua ligands.  That is the aqua ligand will substitute trans to amine, not trans to chlorine.  This contradicts the above.  Why is chloride substituted rather than amine?

[Schrödinger]

If you're talking about the compound which has 2 ammonia ligands and 2 chlorine ligands (cis), then look at which ligand is more labile. Which is a better leaving group? Cl^- or NH₃?