One-Electron Oxidation of ReCp(CO)₂L (L = PPh₃, η²‑2-Butene, η‑Diphenylacetylene): Electrochemical, Spectroscopic, and Computational Studies of the Electronic Properties and Dimerization Tendencies of 17-Electron Rhenium Complexes

Electrochemical oxidation of ReCp­(CO)₂L (Cp = η⁵-C₅H₅; L = PPh₃ (<b>2</b>), η²-2-C₂Me₂H₂ (<b>3</b>), η-C₂Ph₂ (<b>4</b>)) has been studied in CH₂Cl₂/[NBu₄]­[B­(C₆F₅)₄]. All three complexes undergo quasi-Nernstian one-electron oxidations with <i>E</i>_1/2 values (vs ferrocene) of 0.49 V (<b>2</b>/<b>2</b>⁺), 0.45 V (<b>3</b>/<b>3</b>⁺), and 0.15 V (<b>4</b>/<b>4</b>⁺). A second reversible one-electron oxidation is observed for <b>4</b> at 0.97 V. The radical cation <b>2</b>⁺ efficiently forms the Re–Re-bonded dimer dication <b>2</b>_<b>2</b>²⁺, which was isolated after chemical oxidation of <b>2</b> by [thianthrene]­[B­(C₆F₅)₄]. The 17-electron complex <b>3</b>⁺ shows no tendency to dimerize. On the basis of the <i>E</i>_1/2 value of <b>3</b> and expected potential shifts when replacing a methyl group by a hydrogen, a ligand electronic parameter, <i>E</i>_L, of 0.38 was calculated for π-bonded ethylene, which is much lower than the existing literature value of 0.76. IR, ESR, and UV–vis spectra were recorded for <b>4</b>⁺. DFT calculations were performed on the neutral complexes, the monocations <b>2</b>⁺, <b>3</b>⁺, and <b>4</b>⁺, and the dimer dication <b>2</b>_<b>2</b>²⁺. The SOMO of <b>2</b>⁺ is similar to that of the parent piano-stool complex [ReCp­(CO)₃]⁺ (<b>1</b>⁺) in being metal-based (55%) and directionally disposed to formation of a metal–metal bond with another radical cation. A Re–Re bond distance of 3.315 Å was calculated for the resulting dimer <b>2</b>_<b>2</b>²⁺. Although the SOMO of <b>3</b>⁺ is also predominantly (60%) metal-based, the orbital lacks the geometry necessary to form a metal–metal-bonded dimer. The HOMO of <b>4</b> is highly delocalized, having only about one-third metal character and two-thirds coming from the Cp and diphenylactylene ligands. One-electron oxidation of <b>4</b> brings about a significant change in electronic structure, with the SOMO of <b>4</b>⁺ seeing increased contributions from diphenylacetylene and the two carbonyls (14%), along with a decreased metal contribution (27%). The diphenylacetylene contributions are large enough to justify π-C₂Ph₂ being designated as a “non-innocent” ligand in this system