HYDROCARBON-BRIDGED COMPLEXES .15. MOLECULAR AND ELECTRONIC-STRUCTURES OF (MU-ETHYNEDIYL)BIS(PENTACARBONYLRHENIUM), (OC)5REC=CRE(CO)5

The structure of (OC)5ReC≡CRe(CO)5 has been determined by single-crystal X-ray analysis. Crystal data: space group P1, Z = 1, a = 652.8 (2), b = 651.6 (2), c = 987.2 (2) pm, α = 90.40 (2), β = 96.77 (3), γ = 98.62 (3)°. The two Re(CO)5 groups are arranged in the eclipsed conformation. The C≡C bond length (119 (3) pm) parallels that in ethyne. The Re‒C bond (214 (2) pm) is significantly shorter than that in (OC)5ReCH2CH2Re(CO)5 (230 (1) pm). The UV‒visible spectrum of (OC)5ReC≡CRe(CO)5 in THF shows absorptions at 319 (12000 M-1 cm-1), 242 (50000 M-1 cm-1), and 223 nm (68000 M-1 cm-1). Cyclic voltammetry measurements in acetonitrile reveal two irreversible oxidations at +1.02 and +1.55 V vs an Ag wire reference. SCF-Xα-DV calculations for D4h (OC)5ReC≡CRe(CO)5 show that the highest occupied molecular orbital, 13eu, derives from the C≡C π bond. Little mixing between the C≡C π, π*, or σ orbitals occurs, so that the bonding is similar to ethyne. The predominant bonding between Re and the acetylide carbons involves mixing of pz-dz2 hybrid orbitals on rhenium with the a1g and a2u combinations of the acetylide lone pairs. Although the C≡C triple bond is relatively unperturbed, a high negative charge density predicted for this fragment results in a lower oxidation potential for (OC)5ReC≡CRe(CO)5 as compared to Re2(CO)10. Also, the lowest energy-allowed electronic transition at 319 nm is assigned to the 13eu (C≡C π) → 13eg (CO π*) one-electron excitation (1A1g →1A2u). © 1990, American Chemical Society. All rights reserved.