trans-Dichlorodiammineplatinum(II) is the trans isomer of the coordination complex with the formula trans-PtCl<sub>2</sub>(NH<sub>3</sub>)<sub>2</sub>, sometimes called transplatin. It is a yellow solid with low solubility in water but good solubility in dimethylformamide. The existence of two isomers of PtCl<sub>2</sub>(NH<sub>3</sub>)<sub>2</sub> led Alfred Werner to propose square planar molecular geometry. It belongs to the molecular symmetry point group D<sub>2h</sub>.
The complex is prepared by treating [Pt(NH<sub>3</sub>)<sub>4</sub>]Cl<sub>2</sub> with hydrochloric acid.
Many of the reactions of this complex can be explained by the trans effect. It slowly hydrolyzes in aqueous solution to give the mixed aquo complex trans-[PtCl(H<sub>2</sub>O)(NH<sub>3</sub>)<sub>2</sub>]Cl. Similarly it reacts with thiourea (tu) to give colorless trans-[Pt(tu)<sub>2</sub>(NH<sub>3</sub>)<sub>2</sub>]Cl<sub>2</sub>. In contrast, the cis isomer gives [Pt(tu)<sub>4</sub>]Cl<sub>2</sub>. Oxidative addition of chlorine gives trans-PtCl<sub>4</sub>(NH<sub>3</sub>)<sub>2</sub>.
trans-Dichlorodiammineplatinum(II) has had far less impact on medicinal chemistry compared to its cis isomer, cisplatin, which is a major anticancer drug. Nonetheless, replacement of the ammonia with other ligands has led to highly active drugs that have attracted much attention.