Structure of tetracarbonylethyleneosmium: ethylene structure changes upon complex formation

Area of Science:

• Organometallic Chemistry
• Spectroscopy
• Inorganic Chemistry

Background:

• Olefin-transition metal complexes are crucial in organic synthesis.
• These complexes serve as models for transition states in metal-mediated alkene transformations.

Purpose of the Study:

• To investigate the structure of tetracarbonylethyleneosmium (Os(CO)4(eta2-C2H4)) using rotational spectroscopy.
• To determine the nature of the bonding between ethylene and the osmium center.

Main Methods:

• Measured rotational spectra of seven isotopomers of Os(CO)4(eta2-C2H4) using a pulsed-beam Fourier transform microwave spectrometer.
• Analyzed approximately 140 assigned b-type transitions using a Watson S-reduced Hamiltonian.
• Determined the gas-phase structure via a least-squares fit of 21 measured rotational constants.

Main Results:

• The experimental ethylene C-C bond length is 1.432(5) Å, significantly longer than free ethylene.
• The angle between the CH2 plane and the C-C bond (angleout-of-plane) is 26.0(3)°, indicating metallacyclopropane character.
• Ethylene carbon atoms exhibit near sp3 hybridization in the complex.

Conclusions:

• Tetracarbonylethyleneosmium is better described as a metallacyclopropane.
• Structural changes in ethylene upon coordination are larger for osmium than for iron.
• This work enhances understanding of alkene coordination in organometallic chemistry.