NORBORNADIENE ON PT(111) IS NOT BOUND AS AN ETA(2)ETA(2) DIENE - CHARACTERIZATION OF AN UNEXPECTED ETA(2)ETA(1) BONDING MODE INVOLVING AN AGOSTIC PT-CENTER-DOT-CENTER-DOT-CENTER-DOT-H-C INTERACTION

The structure and reactivity of norbornadiene (NBD) adsorbed on Pt(lll ) has been studied by several physical techniques, including reflectio n-absorption infrared spectroscopy (RAIRS), high-resolution electron e nergy loss spectroscopy (HREELS), temperature-programmed reaction spec troscopy (TPRS), integrated desorption mass spectrometry (IDMS), and A uger electron spectroscopy(AES). At 130 K, NBD is bound to the surface in an unusual way: through only one C=C double bond and through an ag ostic interaction involving one of the C-H bonds of the bridging CH2 g roup. The latter interaction is characterized by an intense, broad ban d centered at 2670 cm(-1) (fwhm approximate to 50 cm(-1)) in the RAIR spectra. The data strongly suggest that the other C=C double bond of N BD does not interact with the surface. This unusual binding mode, whic h may be adopted because steric factors favor it over the alternative eta(2):eta(2) geometry, persists to 220-260 K, whereupon the agostic C -H bond is cleaved to give a norbornadienyl intermediate that is stabl e to 400-450 K. In this latter temperature range the norbornadienyl in termediate decomposes, probably via a retro cyclization reaction, to g ive benzene and dihydrogen (which desorb) and a fractional monolayer o f carbon (which remains on the surface). Comparisons of the low-freque ncy C-H stretching mode seen for NBD with those seen for other adsorbe d hydrocarbons suggest that the line width of these modes is largely d ue to inhomogeneous broadening, not to dispersion or lifetime effects.