Synthesis and spectroscopic characterization (IR, 1H and 31P NMR, electrospray ionization mass) of mono-, di-, tetra- and poly-meric complexes of silver(I) with diphosphine ligands: X-ray crystal structures of AgNO2:(Ph2PCH2PPh2) (1:1)2, AgNO2:(Ph2P(CH2)3PPh2) (1:1)2, AgNO2:(Ph2PCHCHPPh2) (2:1)2 and AgNO2:{p-tolyl)2P(C10H6)2P(p-tolyl)2} (1:1)