Ethylene ligand structures of Os(CO)4(C2H4) and Os2(CO)8(C2H4) determined by 1H NMR in liquid crystal solvents

The ethylene complexes Os-2(CO)(8)(mu-eta(1),eta(1)-C2H4) (1) and Os(CO)(4)(eta-C2H4) (2) have been studied by H-1 NMR in liquid crystal (nematic phase) solvents. For 1 or 2, three dipolar couplings were observed and assigned to intramolecular geminal, cis, and trans H-1-H-1 dipolar couplings. The H-1 NMR spectrum of Os(CO)(4)((CH2CH2)-C-13)(2-C-13) has also been analyzed and two additional C-13-H-1 dipolar couplinps have allowed determination of the absolute bond angles and relative bond lengths of the ethylene portion of 2 C-13. The observed dipolar couplings for 2-C-13 have been corrected for harmonic vibrations. A comparison of the ethylene geometry of 2 with that of other transition metal ethylene complexes and with free ethylene shows that the (C-2-H-4)Os unit of 2 is best described as a metallacyclopropane. Deuterium substitution is used to demonstrate liquid crystal NMR as a stereochemical probe. The H-1 NMR spectrum of Os-2(CO)(8)(mu-eta(1),eta(1)-(CH2CH2)-C-13) (1-C-13) has also been analyzed in a nematic phase solvent. The dimetallacyclobutane ring of 1-C-13 is best described as two rapidly interconverting ring-puckered conformers of C-2-symmetry. The liquid crystal NMR derived structural parameters for 1-C-13 are compared with those previously determined by neutron diffraction. The solution and solid-state structures are very similar. The ethylene complexes 1 and 2 are proposed as structural and spectroscopic models for ethylene chemisorbed on metal surfaces.