Valence-Dependent Metal−Metal Bonding and Optical Spectra in Confacial Bioctahedral [Re2Cl9]z- (z = 1, 2, 3). Crystallographic and Computational Characterization of [Re2Cl9]- and [Re2Cl9]2-

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Abstract
The geometric structure of the confacial bioctahedral [Re2Cl9]z- anion has been determined by single-crystal X-ray diffraction in two distinct oxidation states, ReIV2 and ReIIIReIV. [Bu4N][Re2Cl9] crystallizes in the monoclinic space group P21/m [a/Å = 10.6363(3), b/Å = 11.420(1), c/Å = 13.612(1), β/deg = 111.18(1), Z = 2], while [Et4N]2[Re2Cl9] crystallizes in the orthorhombic space group Pnma [a/Å = 15.82(1), b/Å = 8.55(2), c/Å = 22.52(3), Z = 4]. The Re−Re separation contracts from 2.704(1) Å in [Bu4N][Re2Cl9] to 2.473(4) Å in [Et4N]2[Re2Cl9] (or, equivalently, from 2.725 to 2.481 Å after standard corrections for thermal motions), while the formal metal−metal bond order falls from 3.0 to 2.5. SCF−Xα−SW molecular orbital calculations show that, despite the {d3d3} configuration, the single σ bond in [Re2Cl9]- dominates the observed structural properties. For [Re2Cl9]2-, the 0.23 Å 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 σ and δπ 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 σ → σ* and halide-to-metal change-transfer absorptions. The intense σ → σ* 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 σ bond in the ReIII2 species.

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