Structure of tetracarbonylethyleneosmium: Ethylene structure changes upon complex formation

Rotational spectra of seven isotopomers of tetracarbonylethyleneosmium, Os(CO)₄(η²-C₂H₄), were measured in the 4-12 GHz range using a Flygare-Balle-type pulsed-beam Fourier transform microwave spectrometer system. Olefin-transition metal complexes of this type occur extensively in recent organic syntheses and serve as important models for transition states in the metal-mediated transformations of alkenes. Three osmium (¹⁹²Os, ¹⁹⁰Os, and ¹⁸⁸Os) and three unique ¹³C isotopomers (¹³C in ethylene, axial, and equatorial positions) were observed in natural abundance. Additional spectra were measured for a perdeuterated sample, Os(CO)₄(η²-C ₂D₄). The measured rotational constants for the main osmium isotopomer (¹⁹²Os) are A = 929.3256(6), B = 755.1707(3), and C = 752.7446(3) MHz, indicating a near-prolate asymmetric top molecule. The ∼140 assigned b-type transitions were fit using a Watson S-reduced Hamiltonian including A, B, C, and five centrifugal distortion constants. A near-complete r₀ gas-phase structure has been determined from a least-squares structural fit using eight adjustable structural parameters to fit the 21 measured rotational constants. Changes in the structure of ethylene on coordination to Os(CO)₄ are large and well-determined. For the complex, the experimental ethylene C-C bond length is 1.432(5) Å, which falls between the free ethylene value of 1.3391 (13) Å and the ethane value of 1.534(2) Å. The angle between the plane of the CH₂ group and the extended ethylene C-C bond (Zout-of-plane) is 26.0(3)°, indicating that this complex is better described as a metallacyclopropane than as a π-bonded olefin-metal complex. The Os-C-C-H dihedral angle is 106.7(2)°, indicating that the ethylene carbon atoms have near sp ³ character in the complex. Kraitchman analysis of the available rotational constants gave principal axis coordinates for the carbon and hydrogen atoms in excellent agreement with the least-squares fit results. The new results on this osmium complex are compared with earlier work on the similar complex, tetracarbonylethyleneiron (Fe(CO)₄(η²-C ₂H₄)). The ethylene structural changes upon coordination to the metal are found to be larger for the ethylene-osmium complex than for the analogous ethylene-iron complex, consistent with the expected greater π donation for the osmium atom.