Two structural types of perrhenic anhydrides result from treatment of Re2O7 with carboxylic anhydrides (RCO)2O in acetonitrile or tetrahydrofuran solution. Strongly electron-withdrawing groups, cf. R = CF3, thus lead to complexes of type RC(O)-O-ReO3.2L (L = CH3CN, thf) exhibiting eta1-coordinated carboxylato groups and octahedral coordination at rhenium(VII), both in solution (IR, O-17 NMR) and in the solid state (X-ray crystal structure of CF3C(O)-O-ReO3.2CH3CN (crystal data: space group P2(1)/n (No. 14), a = 10.522(3) angstrom, b = 8.193(1) angstrom, c = 13.094(4) angstrom, beta = 90.59(1)degrees, Z = 4)). Analogous coordination is observed in the X-ray crystal structure of Re2O7.2CH3CN (space group P2(1)/m (No. 11), a = 5.363(1) angstrom, b = 11.763(2) angstrom, c = 8.273(2) angstrom, beta = 94.395(9)degrees, Z = 2)). By way of contrast, anhydrides of weak carboxylic acids, cf. R = CH3, yield mixed anhydrides of general formula RC(O)-OReO3.L (L = CH3CN, thf), the structures of which compounds display octahedral rhenium(VII) and eta2-coordinated carboxylato groups. These structures are again observed in solution and in the solid state (X-ray crystal structure of CH3C(O)-OReO3.CH3CN (space group C2/m (No. 12), a = 14.818(8) angstrom, b = 6.976(2) angstrom, c = 7.390(4) angstrom, beta = 94.64(2)degrees, Z = 4)). As a result of these structural features, only mixed anhydrides containing strongly electron-withdrawing substituents react cleanly with alkyltin compounds to yield the alkylrhenium(VII) oxides of the type alkyl-ReO3.
