Reacting ReO2I(PPh(3))(2) with imidazole (ImH) and its methylated derivatives 1-MeIm, 1,2-Me(2)Im, 2-MeImH, and 4(5)-MeImH yielded salts of the trans-dioxo cation [ReO(2)L(4)](+). Pure stable compounds were isolated for X(-) = I- and L = ImH, 1,2-Me(2)Im, and 1-MeIm and X(-) = B(C6H5)(4)(-) and L = 1-MeIm, 2-MeImH and 5-MeImH. The v(as) (O=Re=O) IR bands were observed between 765 and 794 cm(-1), whereas the v(s) Raman band appeared in the 900-925 cm(-1) range. The compounds were also characterized by H-1 and C-13 NMR and UV-vis spectroscopies. Crystal structures were determined for three compounds : [ReO2(2-MeImH)(4)][B(C6H5)(4)]. 3CH(3)-OH, triclinic, , a = 10.696(3) Angstrom, b = 15.128(4) Angstrom, c = 15.397(4) Angstrom, alpha = 113.57(2)degrees, beta = 97.25(2)degrees, gamma = 95.94(2)degrees, Z = 2, R = 0.030; [ReO2(1-MeIm)(4)][B(C6H5)(4)]. H2O . O.SCHjOPI, monoclinic, P2(1)/n, a = 15.619(2) Angstrom, b = 9.486(2) Angstrom, c = 27.387(4) Angstrom, beta = 97.09(1)degrees, Z = 4, R = 0.057; [ReO2(5-MeImH)(4)] [B(C6H5)(4)], orthorhombic, P2(1)2(1)2(1), a = 10.199(2) Angstrom, b = 13.441(5) Angstrom, c = 28.798(10) Angstrom, Z = 4, R = 0.043. The complexes adopt the trans-dioxo geometry, and the octahedra are remarkably regular. Protonation of [ReO(2)L(4)]I was studied in methanol and in water. Acid dissociation constants were determined in aqueous solution for the [ReO(OH)L(4)](2+) (K-a1) and [ReO(OH2)L(4)](3+) (K-a2) species of 1-MeLm (pK(a1) = 2.0, pK(a2) = similar to-4.0) and 1,2-Me(2)Im (pK(a1), = similar to 3.8, pK(a2) = similar to-4.1). Iodide salts of [ReO(OH)L(4)](2+) were not stable enough to be isolated, but the B(C6H5)(4)(-) salt of [ReO(OH)(1,2-Me(2)Im)(4)](2+) was obtained in pure form. The crystal structure of [ReO(OH)(1,2-Me(2)Im)(4)](I-3)(0.5)-(ReO4)(1.5) (monoclinic, I2/a, a = 23.290(8) Angstrom, b = 10.863(7) Angstrom, c = 25.709(8) Angstrom, beta = 94.36(3)degrees, Z = 8, R = 0.060) showed the presence of octahedral dications in which the oxo and hydroxo groups are trans to one another and the Re-N bonds are displaced to the Re-OH side by the steric effect of the multiple Re-ore bond. The sigma-donor ability of imidazole, greater than that of pyridine, leads to more stable [ReO(2)L(4)](+) compounds in which the Re=O double bond character is lower and the oxo groups are better nucleophiles.