The oxo group of [ReO2(1-MeIm)(4)](+) can be methylated with excess methyl trifluoromethanesulfonate in CH2Cl2 under mild conditions, leading to [ReO(OCH3)(1-MeIm)(4)](CF3SO3)(2), which is converted to the B(C6H5)(4)(-) and PF6- salts. When the reaction is carried out in methanol in the presence of excess PF6-, the phosphate ester complex [ReO{OP(O)(OCH3)(2)}(1-MeIm)(4)](PF6)(2) is isolated. [Reo(2)(1-MeIm)(4)](+) in the presence of acid, 2,2-dimethoxypropane, and excess BF4-transforms to the oxo-boron adduct [ReO(OBF3)(1-MeIm)(4)](+). The presence of the trans-O=Re-OR core was demonstrated by crystallographic studies on one compound of each type: [ReO-(OCH3)(1 -MeIm)(4)](PF6)(2), tetragonal, P4/ncc, Z = 4, a = 13.022(3) Angstrom, c = 17.218(5) Angstrom, R = 0.0271; [ReO-{OP(O)(OCH3)(2)}(1- -MeIm)(4)](PF6)(2).toluene, monoclinic, P2(1)/c, Z = 4, a = 14.165(4) Angstrom, b = 13.052(6) Angstrom, c 20.931(6) Angstrom, beta = 95.83(2)degrees, R = 0.0373; [ReO(OBF3)(1-MeIm)(4)](I-3), monoclinic, P2(1)/c, Z = 4, a = 13.373(4) Angstrom, b = 13.237(3) Angstrom, c = 16.689(6) Angstrom, beta = 103.67(3)degrees, R = 0.0445. The IR spectra show v(Re=O) vibrations near 960 cm(-1) in all cases. The solution UV-vis spectra of the CH3O- and BF3O2- compounds are similar to that of the parent oxo-hydroxo [ReO(OH)(1-MeIm)(4)](2+) species, whereas the visible spectrum of the blue phosphate ester compound resembles that of the oxo-aquo [ReO(OH2)(1-MeIm)(4)](3+) ion. The compounds were also characterized by solution H-1, C-13, and P-31 NMR spectroscopy. This work confirms earlier conclusions that ReO2L4+ units with good sigma-donor imidazole ligands are more resistant to ligand loss than the pyridine analogues.