[Os ₂ (CO) ₈ (μ ₂ -η ¹ ,η ¹ -propene)] and Related Complexes as Vibrational Models for Alkenes Chemisorbed on Single-Crystal Metal Surfaces

The FTIR spectra of [Os 2 (CO) 8 (μ 2 -η 1,η 1 -C 2 H 3 CH 3 )] and of its methyl- d 3 and d 6 isotopologues have been measured and assigned. Comparison of these vibrational data with previously published EELS and RAIRS studies of propene chemisorbed on Pt(111) and Ni(111) at low temperatures indicates that, on these surfaces, the propene species are chemisorbed via a (μ 2 -η 1,η 1 -C 2 H 3 CH 3 ) bonding mode. However, differences in the intensity patterns between the spectra of the adsorbed species compared with that of the model compound imply additional twisting or tilting with respect to the surface. Assignment of the FTIR spectra of the corresponding 1-butene and trans -2-butene complexes [Os 2 (CO) 8 (μ 2 -η 1,η 1 -C 2 H 3 C 2 H 5 )] and [Os 2 (CO) 8 (μ 2 -η 1,η 1 -CH 3 C 2 H 2 CH 3 )] indicates similar bonding modes for chemisorbed 1-butene and trans -2-butene on Pt(111). Infrared data for the mononuclear propene complex [Os(CO) 4 (η 2 -C 2 H 3 CH 3 )], on the other hand, are in good agreement with published EEL data for propene on Ru(0001) and Rh(111), indicating that at low temperatures on these surfaces, propene is chemisorbed as a methyl-substituted “metallacyclopropane-like” species. These bonding modes are analogous to those established for low-temperature ethene chemisorption on these surfaces.