Crystal structures of linear-chain halogen-bridged binuclear platinum complexes, dihydrate forms of K₄[Pt₂X(pop)₄] ·nH₂O (X=Cl and Br)

Crystal structures of the potassium catena-μ-halogenotetrakis(μ-diphosphonato-P,P)-diplatinum(4-) dihydrate, K₄[Pt₂X(pop)₄]·2H₂O (X=Cl (1) and Br (2); pop=P₂O₅H₂^2-), have been determined by the single crystal X-ray diffraction method. The complexes are isomorphous to each other and crystallize in orthorhombic, space group Pbnm, Z=4: for 1, a=9.553(2), b=15.440(3), c=17.123(3) Å, V=2525.6(8) ų at room temperature; for 2, a=9.510(1), b=15.338(2), c=17.125(2) Å, V=2497.9(6) ų at 125 K. The structures consist of linear chains with a repeating unit of ··· Pt^IIPt^II ··· XPt^IIIPt^IIIX ··· along the c axes. The linear chains are not straight but a little bent to form zigzag chain structures, the bent angles defined by the PtPt bonds and the c axes being 3.34(1)° for 1 and 3.11(1)° for 2, respectively. The PtPt separations, which are bridged by four pyrophosphato ligands (P₂O₅H₂^2-), are 2.835(1) Å for 1 and 2.834(1) Å for 2. The bridging halogen atoms are disordered over two sites in the chains, giving short PtX and long Pt ··· X separations: 2.406(4) and 3.362(4) Å for 1 and 2.539(1) and 3.217(1) Å for 2, respectively. The deviations of the bridging halogen atoms from the midpoints between two Pt dimers are significantly larger than those of the trihydrate modifications.