Studies directed towards the isolation and characterization of the paramagnetic cations [Re2X4(LL)2]+ (X = CL or Br; LL = Ph2PCH2PPh2, Ph2AsCH2AsPh2 or Ph2PNHPPh2), and the crystal structure of Re2Cl4(Ph2PNHPPh2)2·(Ch3)2CO