STRUCTURE OF OS(CO)4(C2H4), AN OSMACYCLOPROPANE

1J(C-13-C-13) for an ethylene coordinated to Os(CO)4 has been determined as 39.0 (2) Hz from the H-1 NMR spectrum of Os(CO)4(eta2-(C2H4)-C-13). The molecular structure of Os(CO)4(eta2-C2H4) (1) has been determined by single-crystal X-ray diffraction; 1 crystallizes in space group C2/c with a = 12.271 (5) angstrom, b = 6.778 (1) angstrom, c = 12.345 (4) angstrom, beta = 129.49 (2)-degrees, V = 792.4 (2) angstrom3, and Z = 4. The geometry of 1 is approximately trigonal bipyramidal, with the ethylene carbons twisted out of the equatorial plane by 4.0-degrees. The axial carbonyls of 1 tilt toward the coordinated ethylene, with a C1-Os-C1' angle of 171.3 (5)-degrees. The optimal geometries of Os(CO)4, H2Os(CO)4, and 1 have been calculated by Hartree-Fock methods. The orbital amplitude contour diagrams of a GVB(6/12) wave function for 1 show that the axial bending occurs in order to mix in virtual p, character and improve back-bonding to the equatorial carbonyls; they also show that 1 is best described as a metallacyclopropane.