Surface Organometalic Species

2 Surface Organometal1ic Species. - Often relatively simple materials can be made by grafting organometallic complexes onto oxide supports, and having the metal in one predominant form often allows more precision in describing the local structure. Several studies have dealt with the initial chemisorption product of silica, and on alumina (Os only).  

Empirically derived phase shifts and back-scattering 

This provided a more closely defined estimate of the Os-O 

This oxidative reaction with the oxide surface has been observed even when the triosmium clusters are first bound to anchored ligands, such as PPh2 (0112)2 SIL or 118(0112)3 SIL. Analysis of the osmium L(III) edge EXAFS of the product of Os (00) j 2 with the latter functionalised oxide was entirely in accord with the structure Os H (00) j q(M-S(CH2) 3SIL) with Os-Os bond lengths of 2.848. On thermal decomposition at 48 h, a new component was 

Re H (CO) 2/ 90 also afforded small metal particles, but in this case agglomeration of the trimeric units to a mean of ca 15 atom particles was indicated by the Re-Re coordination number of 5.5. In this case the metal-metal distance was the same as that of the bulk metal. 

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Alkylidene complexes are generally considered to be reactive intermediates but the actual surface organometallic species have never been fully characterized. However, the synthesis of silica-supported tantalum(V) carbene complexes and their characterization have been reported.332... 

Structural evidences of the nature of surface organometallic species were confirmed in a few cases by the synthesis and structural characterization of molecular models (in particular using silanolate ligands) of surface organometallic... 

In addition, aided by profound knowledge of the nature and reactivity of some surface organometallic species, it was possible to identify the various steps and the nature of intermediates involved in the nucleation processes occurring on the surface in the selective growth of very large clusters such as for instance in the case of [Os5C(CO)i4] and [OsioC(CO)24] [52]. As this subject is treated in detail elsewhere in this book it is not covered here. 

Stable rhenium tricarbonyls bonded to the surface of MgO have been prepared and characterized by EXAFS. Heating under He, O2 or vacuum of a sample obtained by impregnation of Re2(CO)io produced the oxidative fragmentation of the initial surface organometallic species [39-41]. These types of supported well-characterized species can be used as models in the study of reaction mechanisms [42]. 

Tuning the Catalytic Activity of Surface Organometallic Species 429... 

The last decade has seen enormous developments in SS NMR, and techniques that were used only in soluhon have become available also for solid samples. Nowadays, even conformers can be identified by solid-state NMR [22]. Therefore, this section outlines these improvements as well as their application to the characterization of selected surface organometallic species. An application of EXAFS to the characterization of surface organometallic tantalum species is also shown. Structural data obtained through EXAFS (such as that in Table 11.1) are often essential for comparisons with data obtained by DFT calculations. 

Surface organometallic species have also been used for the olefin metathesis reaction [10, 13]. In the case of molybdenum, the molecular complex N=Mo[CH2C(CH3)3]3 showed very little activity for the metathesis of simple olefins, presumably because the catalytically active carbene complex did not form under reaction conditions. The reaction of this complex with silica, however, proceeds by the addition of the silanol O-H bond over the Mo N triple bond leading first to a trisalkyl Mo complex which undergoes a-elimination of neopentane to produce a carbene complex which was found to exhibit a significant activity for the metathesis of internal olefins according to eq. (2) [10]. 

The material balance of tin anchoring indicated that under monolayer coverage of platinum by PSC the average value of x, is around 1.5. This fact pointed out that Pt nanoclusters are covered by -SnRs and -SnR2 moieties formed in 1 1 ratio. It has been suggested that all anchored surface organometallic species with general formula of -SnR(4.x) and x> 1 can be considered as Coordinatively Unsaturated Primary Surface Complex (CUPSC). 

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