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 K 4 [Pt 2 {µ-CH 2 [P(O)OH] 2 } 4 ]·6H 2 O and K 4 [Pt 2 {µ-CH 2 [P(O)OH] 2 } 4 Cl 2 ]·8H 2 O