The geometric structure of the confacial bioctahedral [Re2Cl9](z-) anion has been determined by single-crystal X-ray diffraction in two distinct oxidation states, Re-2(IV) and (ReReIV)-Re-III. [Bu(4)N][Re2Cl9] crystallizes in the monoclinic space group P2(1)/m [a/Angstrom = 10.6363(3), b/Angstrom = 11.420(1), c/Angstrom = 13.612(1), beta/deg = 111.18(1), Z = 2], while [Et(4)N](2)[Re2Cl9] crystallizes in the orthorhombic space group Pnma [a/Angstrom = 15.82(1), b/Angstrom = 8.55(2), c/Angstrom = 22.52(3), Z = 4]. The Re-Re separation contracts from 2,704(1) Angstrom in [Bu(4)N][Re2Cl9] to 2.473(4) Angstrom in [Et(4)N](2)[Re2Cl9] (or, equivalently, from 2.725 to 2.481 Angstrom after standard corrections for thermal motions), while the formal metal-metal bond order falls from 3.0 to 2.5. SCF-X alpha-SW molecular orbital calculations show that, despite the {d(3)d(3)} configuration, the single sigma bond in [Re2Cl9](-) dominates the observed structural properties. For [Re2Cl9](2-), the 0.23 Angstrom contraction in Re-Re is attributed jointly to radial expansion of the Re 5d orbitals and to diminished metal-metal electrostatic repulsion, which act in concert to make both sigma and delta(pi) bonding more important in the reduced species. Computed transition energies and oscillator strengths for the two structurally defined anions permit rational analysis of their ultraviolet spectra, which involve both sigma --> sigma* and halide-to-metal change-transfer absorptions. The intense sigma --> sigma* band progresses from 31 000 cm(-1) in [Re2Cl9](-) to 36 400 cm(-1) in [Re2Cl9](2-), according to the present assignments. For electrogenerated, highly reactive [Re2Cl9](3-) (where conventional X-ray structural information is unlikely to become available), the dominant absorption band advances to 40 000 cm(-1), suggesting further strengthening of the metal-metal sigma bond in the Re-2(III) species.