Two key compounds of organometal oxides, methyltrioxorhenium(VII) (1) and trioxo(η 5-pentamethylcyclopentadienyl)rhenium(VII) (2), have been structurally characterized by means of electron diffraction techniques, showing that the ReO3 fragments of these compounds have trigonal-pyramidal structures in the gas phase. The rhenium-carbon distance of the 14e complex 1 amounts to 206.0 (9) pm, which is the shortest Re-C(sp3) bond so far recorded. The pentamethyl-cyclopentadienyl derivative 2 has the longest known rhenium-carbon bond (240.5 (6) pm) due to the size of this particular ir-bonded ligand and the σ/π-donor properties of the oxo ligands (“trans influence”). Infrared and Raman spectra show a much higher triple-bond contribution in the rhenium-oxygen bonds of 1 (force constant k = 8.16 mdyn/Å) compared with 2 (k = 6.99 mdyn/Å). The π-donor qualities of the ring ligand of 2 are considered the major effect to reduce the rhenium-oxygen bond order of this 18e compound since the σ-aryl complex (σ-C6H2Me3)ReO3 (3) has a force constant of k = 8.08 mdyn/Å. According to cyclovoltammetric data, the methyl derivative 1 is more easily reduced (Epc = −0.84 V vs Ag/AgCl, THF, 20 °C) than the half-sandwich congener 2 (Epc = −1.72 V), again reflecting the electronic situation of the two compounds (14e vs 18e, respectively). The first vertical PE ionization energies of 1 and 2, 11.8 and 8.6 eV, differ by 3.2 eV due to their different radical-cation ground states, X(a2n) and X(e,πCp*). The equivalent oxygen lone pair type ionization of 2, IE2v(a2,n0) = 9.9 eV, is lower by 1.9 eV and gives proof of the electron donation from the η5-bonded π-ligand C5Me5. In addition, the PE spectrum of trioxo(η1-mesityl)rhenium(VII) (3) has been recorded: Its first ionization energy of 9.00 eV exceeds the corresponding one of mesitylene by 0.6 eV, thus demonstrating the considerable acceptor effect of the ReO3 substituent group toward an η1-bonded π ligand. The high electric dipole moment of 2 (µ = 6.2 D; benzene, 25 °C) appears reasonable in light of the high polarizability of the C5Me5-Re bond (approximately 4 D); the ReO3 unit has a dipole increment of ca. 2.2 D in 1 and 2. NMR and PE spectra clearly show that the ReO3 functionality is a strong electron-withdrawing substituent, stereoelectronically comparable with the SO3H substituent in organic compounds.