Clusters carbonyl migration

Ligand fluxionality on metal clusters has been the subject of many studies, the majority of reports focusing on carbonyl migration on homometallic tri- and... 

The next exchange process to occur at slightly higher temperatures in each of these clusters involves migration of the carbonyls around the Fe-M-M triangle which possesses the bridging carbonyls. It seems reasonable to propose that the intermediates in this cyclic movement are the tautomers which have the semi-bridging carbonyls bound mainly to Ru or Os, instead of Fe, as indicated in Scheme 5. 

Metal atom clusters in the 26-hedra could (as with water molecules) contain many metal atoms. An example is the fourteen atom Agg+8Ag+ cluster in Y-irradiated zeolite Ag-A[25]. Saturation is achieved for mercury sorbed into silver-exchanged faujasites and other zeolites [26]. The Ag+ is reduced to Ag atoms and then at an approximate critical pressure of mercury vapour there is nucleation of mercury clusters which fill all the pore volume as the pressure of Hg vapour increases further. Mercury-zeolite systems are the oily ones in which sorption isotherms have been investigated quantitatively. Hcwever other metal atoms introduced into zeolites (by ion exchange and reduction, or as metal carbonyls and their decomposition) all show, on heating, a strong tendency to form clusters by migration of atoms, which can aggregate both within and outside the crystals. 

Although decarbonylation of supported metal carbonyl clusters sometimes occurs almost without changes in the metal frames, the chemistry is complex and only partially understood. When decarbonylation takes place at elevated temperatures (depending on the support), migration and aggregation of the metal inevitably occur, and these processes are less well understood than the decarbonylation with near retention of the metal frame. 

As an aside, we should mention that the same principles apply to the formation of bimetallic clusters on a support. In the case of Pt-Re on AI2O3 it has been shown that hydroxylation of the surface favors the ability of Re ions to migrate toward the Pt nuclei and thus the formation of alloy particles, whereas fixing the Re ions onto a dehydroxylated alumina surface creates mainly separated Re particles. As catalytic activity and selectivity of the bimetallic particles differ vastly from those of a physical mixture of monometallic particles, the catalytic performance of the reduced catalyst depends significantly on the protocol used during its formation. The bimetallic Pt-Re catalysts have been identified by comparison with preparations in which gaseous Re carbonyl was decomposed on conventionally prepared Pt/Al203 catalysts. ... 

When calcination is carried out at 5(X)°C and reduction is at 2(X)°C, the original nuclearity n is 1. For room temperature and zeolite Y the final nuclearity an is 13. The migration and coalescence of the mobile primary Pd carbonyl clusters leading to the formation of PdnfCO) , clusters, with concomitant release of zeolite protons, are schematically illustrated in Fig. 8. 

Fig. 8. Migration and coalescence of primary Pd carbonyl clusters in feujasite cages (schematic).

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