Reactions of the dirhenium(II) complexes Re2X4(mu-dppm)(2) (X = Cl, Br; dppm = Ph2PCH2PPh2) with isocyanides. 16. Complexes of the type [Re-2(mu-Br)(2)(mu-dppm)(2)Br(CO)(CNXyl)(2)]Y (Y = O3SCF3, PF6) that exist in isomeric forms with boat and chair conformations for the [Re-2(mu-dppm)(2)] unit