CO NMR Survey of Results

Among the bulk transition metals of group VlllB (Fe to Pt), only Ni reacts very rapidly with CO (in fact, the reaction is used in the preparation of ultrapure Ni metal). At the other extreme, no free, pure carbonyl derivatives of zerovalent Pd or Pt have been reported (but sec Section IV.B), although it is thought that they are stable (99). Cycles of adsorption at RT and desorption at 673 K of CO on very small Pd particles lead to slight disproportionation, 2CO CO2 + C 100). On Ru and Rh particles some disproportionati on occurs even upon adsorption at RT. In some cases the 002(g) is detected by NMR. 

According to X-ray photoelectron spectroscopy data, the adsorption of CO on supported Ru of rather low-dispersion (0.27 by hydrogen chemisorption) at 310 K leads to the formation of some oxidized Ru, which can be re-reduced by hydrogen treatment 101). Similarly, an EXAFS study of the effect of RT chemisorption of CO on very highly dispersed Rh (1.70 

The shielding tensors for O and C in the CO molecule are anisotropic with zero asymmetry, reflecting the axial symmetry of the molecule. The isotropic shift of C with respect to TMS is 185 ppm, and the anisotropy in the solid is 406 ppm. Upon linear coordination in transition metal carbonyl molecules, these values typically change by tens of ppm 103). In the few bridge-coordinated compounds for which data are available the isotropic shifts are around 230 ppm and the anisotropies 175 ppm. (No data for solid-state platinum carbonyls have been reported, but there is no reason to believe that they are an exception.) In the CO molecule, the isotropic shift of O is 350 ppm with respect to H2O, and the anisotropy is 675 ppm. Again, these values change by a few tens of ppm in solid molecular metal carbonyls with linear coordination 104). 

A coexistence of metallic and molecular metal carbonyls has been found in CO/Rh catalysts 106) and for Pt in L zeolites 107). When the CO adsorbs on large particles, the resulting carbonyls have metallic character, 

Torr of CO at. 500 K. The line marked P, at about 350 ppm, has been attributed to CO on metallic copper the line N, in the 110-130 ppm range, is believed to be related to some form of adsorbed C02. These data have been presented as preliminary, but they still seem to be the only ones possibly related to CO on copper. [Reproduced with permission from Dennison ei al. (P.5). Copyright 1989 Royal Society of Chemistry.] 

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