Reactions of the dirhenium(II) complexes Re2X4(μ-dppm)2 (X=Cl, Br; dppm =Ph2PCH2PPh2) with isocyanides: Part XX. Oxidation of the complexes Re2X4(μ-dppm)2(L) (X=Cl or Br; dppm=Ph2PCH2PPh2; L=t-BuNC, XylNC or CO) to the edge-sharing bioctahedral dirhenium(III) cations [Re2(μ-X)2(μ-dppm)2X3(L)]+