Coordination Properties of Silver Diphosphine Complexes. Crystal and Molecular Structures of Ag2X2(m-PP)2 (X = I, Cl, NO3; m-PP = 1,3-Bis[(diphenylphosphino)methyl]benzene) and Ag2X2(o-PP)2 (X = I, Cl; o-PP = 1,2-Bis[(diphenylphosphino)methyl]benzene). Synthesis and 1H- and 31P-NMR Spectroscopy of S