Synthesis and Structural Characterization of Trioxo(η⁵-ethyltetramethylcyclopentadienyl)rhenium, a Half-Sandwich Complex of Heptavalent Rhenium

A single-crystal X-ray structure determination of trioxo(η⁵-ethyltetramethylcyclopentadienyl)rhenium(VII), (η⁵-C₅Me₄Et)ReO₃ (1), has been performed. A three-legged piano-stool structure with the five-membered-ring ligand being centrally η⁵-bonded to the trioxorhenium fragment is found. While the Re-O bond lengths (166.4 (4)-171.6 (4) pm) are consistent with double bonds, the Re-C(ring) distances (238.5 (4)-243.0 (4) pm) are unusually long in comparison to those in any low-valent half-sandwich complexes of rhenium described in the literature. This effect stems from the pronounced trans influence of the three strongly π-donating terminal oxo ligands. Compound 1 crystallizes from dichloromethane/n-hexane in the orthorhombic space group P2₁2₁2₁ with cell dimensions a = 849.6 (1) pm, b = 1310.2 (2) pm, c = 1075.3 (2) pm, V= 1197 × 10⁶ pm³, and ρ(calcd) = 2.128 g·cm^-3 (Z = 4).