Binuclear platinum diphosphite complexes. Crystal structures of tetrapotassium bromotetrakis(diphosphito)diplatinate trihydrate (K4[Pt2(pop)4Br].3H2O), a new linear chain semiconductor, and tetrapotassium dichlorotetrakis(diphosphito)diplatinate dihydrate (K4[Pt2(pop)4Cl2].2H2O)