Surface-bound coordinatively unsaturated metal

Surface-Bound Coordinatively Unsaturated Metal Clusters in Catalysis... 

The picture that emerges is that the bonding within the majority of thiophene molecules adsorbed on the catalyst surfaces is hardly perturbed, and this contrasts sharply with the situation in the thiophene complexes. The thiophene molecule parallel to the surface does not correspond to a metal f/ -bound thiophene. Rather, it is suggestive of a weakly chemisorbed precursor state of thiophene that lies parallel to the surface. In this state, the molecule interacts indiscriminately with the alumina, the basal or edge planes, or both. Moreover, the weakness of this binding enhances the surface mobility of thiophene and allows molecules to move across the surface to the catalytic site for reaction with hydrogen atoms. The few sulfur-bound thiophene molecules, no more than 5-10%, would then correspond to thiophene at the coordinatively unsaturated Mo (or Co) atoms. 

The predominance of the cis isomers in the products of the Pd- or Ni-catalyzed hydrogenations of di-substituted alkynes suggests that the main reaction path involves the alkyne bound to a single metal atom as in catalysis by the mononuclear complexes ClRhLs or HRhL4- Surface sites which can furnish the required coordinative unsaturation correspond to the designations and — M. The latter designa-... 

In conclusion, partially dehydroxylated oxide surfaces exhibit a large inventory of surface OH groups and water molecules together with Lewis acidic and Lewis basic sites with coordinative unsaturation (structures II and III of Scheme 1). The hydroxyl population is the souree of protons that cause enhanced surface electrical conductivity and catalytic activity. It is significant that the increase in the conductivity value is paralleled by increases in either the amount of weakly bound protons or their mobility [48]. Almost all metal oxides are active in catalytic isomerization of alkenes, which is one of the least demanding reactions in terms of the requirements for the acid strength of active sites [34]. Studies on several oxide systems show that the activity is lost after extensive dehydration and is partially restored by... 

The adsorption of adatoms, and also other strongly bound surface intermediates, weakens the metal-metal bonds in the surface layer and between surface and subsurface. The bond strength between the adatom (A) and the surface-metal atom (M) is sensitive to the coordinative unsaturation of the surface metal atoms. The combination of these two effects can result in a rearrangement of the surface metal atoms to increase the surface energy when the surface is covered by an overlayer of adsorbed atoms. 

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