The thermally rather stable complex hexahydrido(η5-pentamethylcyclopentadienyl)rhenium, (η5-C5Me5)ReH6 (1), undergoes a variety of thermal and photochemical reactions involving partial or complete loss of the hydrido ligands. Thus heating of 1 yields the dinuclear hydrido complex of composition [(η5-C5Me5)ReH3]2 (2, which displays two bridging and four terminal hydrido ligands (X-ray diffraction study of the derivative [(η5-C4Me4Et)ReH3]2 (2′)). The dinuclear μ-hydrido chloro complex [(η5-C5Me5)Re(H)Cl2]2 (3) is isolated in 20% yield from a solution of 1 in carbon tetrachloride, while treatment of 1 with either elemental iodine (thermal reaction) or methyl iodide (photochemical reaction) yields the novel iodo derivative 4 formulated as [(η5-C5Me5)ReI3]2. In all these reactions, rhenium(IV) compounds are formed. By way of contrast, rhenium(III) complexes are obtained from 1 upon treatment with a phosphane such as trimethylphosphane or dimethyl(phenyl)phosphane. In the absence of chlorinated solvents, the complexes (η5-C5Me5)ReH2(PMe2R)2 (5a: R = CH3; 5b: R = C6H5) are formed under UV irradiation (n-hexane as solvent), whereas the chloro complexes (η5-C5Me5)Re(H)Cl(PMe3)2 (6a) and (η5-C5Me5)ReCl2(PMe3)2 (7a) are formed under same conditions when these reactions are conducted in methylene chloride solutions. All three types of rhenium(III) complexes undergo one-electron oxidations as indicated by cyclovoltammetric measurements. On the basis of the oxidation potentials, which are in the order of 0.13 V (vs. NHE; CH2Cl2), chemical oxidation is possible by means of silver(I) compounds; the rhenium(IV) cations 8, 9, and 10 having the same composition as their precursors 7, 6, and 5, respectively, are formed in quantitative yields. Compound 5b has a pseudo-square-pyramidal structure (X-ray diffraction (-50°C)), with the two phosphane ligands adopting trans positions relative to each other. Finally, complete hydrogen elimination from 1 can be achieved with carbon monoxide under pressure at high temperatures (70 bar/160°C), with the known rhenium(I) compound (η5-C5Me5)Re(CO)3 (11) being formed in good yield.