respectively. From Figure 2 we see that when the clusters are small &/Ic < I), the proportion of Sz-is essentially independent of temperature, but the temperature dependence becomes more marked with the increase in the cluster size. This observation suggests that on small Ni clusters, H,S dissociates to the maximum extent possible even at the lowest temperature, whereas in the case of larger clusters, the extent of dissociation is dependent on temperature.We have studied the interaction of carbon monoxide on Ni clusters deposited onto an A1,0, substrate by means of He I1 UPS. Figure 3 shows the difference spectra of CO absorbed on Ni clusters of different sizes. INi 'I0 0.5 0.9 2.0 I I I , 11 16 BE [eVI -0 3.5x 10 14 1 1 6.3 x10 14 1 4 . 0~ 10 Fig. 3. He11 difference spectra ofCO adsorbed (at 50L) on Ni clusters (AI,O, substrate) for various cluster sizes. The asymmetry of the hands arises because of the low intensity of the signal and the background signal due to the AI,O, support.A shift in the Co valence band closer to the EF is evident with increasing cluster size. This shift is due to the increasing relaxation of the highest excited states of the CO molecule which depends on the density of Ni clusters and therefore on their size. More importantly, the size of the separation between the (ln + 5a) and the 40 bands of the adsorbed CO varies with the cluster size. On bulk nickel this is around 3 eV. The separation increases with the decrease in the cluster size reaching a value of x 4.0eV for the smallest cluster size we studied (ZNJ0 x 0.5). We consider this to be due to the weakening of the C-0 bond as a result of the increasing interaction between CO and Ni with decreasing cluster size.[*] The present investigation shows that small clusters of Ag and Ni are more reactive than the bulk metal, a conclusion that finds support from an earlier study on the chemisorption of C1, on Cu, Ag, and Pt clusters[g9 and of CO on Pt clusters.["] What is noteworthy in the present study is that the dependence of reactivity on cluster size becomes marked at a stage where metallic screening becomes weak, signifying the onset of a metal-insulator transition. The cluster regime where the reactivity is enhanced seems to correspond to one where the clusters are not really metallic. This observation is of relevance to heterogeneous catalysis in which the metals dispersed on oxide supports generally consist of clusters of 100-200 atoms.

Acyl‐Ylide und Hexamethyldisiloxan entstehen bei der Eintopfreaktion von freien. Carbonsäuren mit Bis(trimethylssilyl)methylentriphenylphosphoran [GI. (a)]. Auch Dicarbonsäuren und Aminosäuren lassen sich auf diese Weise umsetzen, nur O‐geschützte Weinsäuren und Weinsäuremonoester führen zu undefinierten Eliminierugsprodukten.

A Simple Method for Replacement of the OH Group in Free Carboxylic Acids by the Phosphonium Ylide Group.-In a one-pot reaction, the methylenetriphenylphosphorane (I) and mono-and dicarboxylic acids lead, besides formation of hexamethyldisiloxane, to acyl ylides such as (III) and (V) (nearly 20 examples). It is demonstrated that amino acids (e.g. (IIc)) also undergo this reaction. The method allows, for the first time, access to α-aminoacyl ylides which can be used as synthetic potentials in subsequent reactions. -(BESTMANN, H. J.; DOSTALEK, R.; BAUROTH, B.; Angew.