Diplatinum-(II) and -(III) complexes with bridging µ-diphosphonito-P,P′ or µ-methylenebis(phosphonito)-P,P′ ligands. Synthesis, structure, and nuclear magnetic resonance analysis of the ‘lantern’ complexes. Crystal structures of K4[Pt2{µ-CH2[P(O)OH]2}4]·6H2O and K4[Pt2{µ-CH2[P(O)OH]2}4Cl2]·8H2O