Reactions of the dirhenium(II) complexes Re2X4(mu-dppm)(2) (X = Cl, Br; dppm = Ph2PCH2PPh2) with isocyanides. Part XVII. Isomerizations of triply bonded dirhenium complexes of the type [Re2X3(mu-dppm)(2)(CO)(CNXyl) (CNR)](+)(R = Xyl or t-Bu) with open bioctahedral structures to di-mu-chloro edge-sharing bioctahedral structures with no Re-Re bond

The diamagnetic dirhenium(Ir) complexes [Re2X3(mu-dppm)(2)(CO)(CNXyl)(2)]O3 SCF3, where dppm = Ph2PCH2PPh2, XylNC = xylyl isocyanide and X = Cl (3a) or Br(3b), in which the cations possess the Re-Re triply bonded, open bioctah edral structure [(XylNC)(2)XRe(mu-dppm)(2)ReX2(CO)]O3SCF3, convert to the p aramagnetic di-mu-halo bridged complexes [(XylNC)(CO)Re(mu-X)(2)(mu-dppm)(2 )ReX(CNXyl)]O3SCF3, X = Cl (4a) or Br (4b), in very high yield when their s olutions in 1,2-dichloroethane are heated to reflux for ca. 24 h. This isom erization involves a switch from a Re=Re bonded structure (Re-Re distance = 2.383(1) Angstrom in 3a) to one in which no Re-Re bond is present (Re-Re d istance = 3.321(1) Angstrom in 4a). The complex cations in 4a and 4b are fo rmulated as mixed-valence Re(I)-Re(III) species, and their formation repres ents the first example of the isomerization of a multiply bonded dimetal co mplex to one in which no metal-metal bond is present. The related thermal i somerizations of the Re-Re triply bonded, mixed-isocyanide ligand complexes [Re2X3(mu-dppm)(2)(CO)(CNXyl)(CN-t-Bu)]O3SCF3, X = Cl or Br, have also bee n examined. (C) 1999 Elsevier Science S.A. All rights reserved.