Bonding supported metal complexes

The band at 1600 cm-1 due to a double-bond stretch shows that chemisorbed ethylene is olefinic C—H stretching bands above 3000 cm-1 support this view. Interaction of an olefin with a surface with appreciable heat suggests 7r-bonding is involved. Powell and Sheppard (4-1) have noted that the spectrum of olefins in 7r-bonded transition metal complexes appears to involve fundamentals similar to those of the free olefin. Two striking differences occur. First, infrared forbidden bands for the free olefin become allowed for the lower symmetry complex second, the fundamentals of ethylene corresponding to v and v% shift much more than the other fundamentals. In Table III we compare the fundamentals observed for liquid ethylene (42) and a 7r-complex (43) to those observed for chemisorbed ethylene. Two points are clear from Table III. First, bands forbidden in the IR for gaseous ethylene are observed for chemisorbed ethyl-... 

II. Supported Metal Complexes—Molecular Analogues Bonded to Surfaces... 

In the following paragraphs, methods of preparation and characterization of structurally simple supported metal complexes are summarized, and examples are presented that illustrate characterization data and support general conclusions about structure, bonding, reactivity, and catalysis. 

In summary, the theoretical results representing the rhenium carbonyls on MgO are in very good agreement with the symmetry indicated by the vibrational spectra and with the coordination numbers and bond distances indicated by EXAFS spectroscopy. Thus, the rhenium carbonyls are regarded as prototype supported metal complexes. They reaffirm the strong analogy between surface-bound metal complexes and molecular metal complexes. The MgO surface is clearly identified as a polydentate ligand. 

The results summarized here support the following generalizations about structure and bonding of metal complexes bonded to metal oxide and zeolite supports ... 

The methods of structure determination of supported nanoclusters are essentially the same as those mentioned previously for supported metal complexes. EXAFS spectroscopy plays a more dominant role for the metal clusters than for the complexes because it provides good evidence of metal-metal bonds. Combined with density functional theory, EXAFS spectroscopy has provided much of the structural foundation for investigation of supported metal clusters. EXAFS spectroscopy provides accurate determinations of metal-metal distances ( 1-2%), but it gives only average structural information and relatively imprecise values of coordination numbers. EXAFS spectroscopy provides structure data that are most precise when the clusters are extremely small (containing about six or fewer atoms) and nearly uniform (Alexeev and Gates, 2000). 

The theoretical parameters characterizing Ir4 in zeolite NaX (Fig. 3) indicate Ir-O distances of about 2.2 A, in good agreement with EXAFS data (Ferrari et al., 1999) and approximately equal to the metal-oxygen bond distances found experimentally and theoretically for supported metal complexes, as discussed above. When the structure of Fig. 3 is rotated 60°, the theory indicates an Ir-O distance of about 2.7 A, in agreement with the longer distances observed by EXAFS spectroscopy (but this agreement may be fortuitous). 

Chemists have prepared metal complexes containing metal atoms/ions as a means to understand better the structure, chemical bonding, and properties of metals and metal ions. One of the first efforts to affix these metal complexes to a surface as a means to create a supported catalyst was reported by Ballard followed by reports collected by Yermakov, et al. and Basset et al. We distinguish here between metal complexes that contain zero-valent metals and those that show metal cations and we limit this review to complexes containing metal ions as others have published extensive reviews of zero-valent, metal clusters and their chemistryIn our previous three reviews on the chemistry of supported, polynuclear metal complexes, we described efforts to synthesize and characterize oxide-supported, metal complexes as adsorbents, catalysts and precursors to supported metal oxides. In one application of this technology, efforts were... 

Some of the most thoroughly characterized supported metal complexes are zeolite-supported metal carbonyls. These have been prepared, for example, by the adsorption of Rh(CO)2(acac) on zeolites (e.g., the faujasite zeolite NaY [26] or dealuminated zeolite Y [27]) followed by CO treatment of the resultant material (Fig. 19.3). The IR spectra (not shown, but found in [26, 27]) of the rhodium dicarbonyl represented in Fig. 19.3 are consistent with a square-planar complex (formally Rh(I)) with the Rh atom bonded to two zeolite oxygen atoms. 

The most important advantage of this method is that molecular species are bonded via chemical bonds and experience almost no leaching from the support, as far as the metal complex ligation and all the bonds are adequately stable in catalytic media. Nevertheless, an important drawback of this method is the large preparative effort, since it involves a multistep procedure that usually includes not only the functionalization of the ligands coordinated to the metal, but also the grafting of spacers onto the support. Furthermore, the covalent bonding of metal complexes, directly or via spacers, can alter, to different extents, the electronic density within the metal, which in turn, can alter the catalytic performance of the catalysts in a way that may be difficult to foresee [3-10]. 

Supported metal complexes and clusters with well-defined structures offer the advantages of catalysts that are selective and structures that can be understood in depth. Such catalysts can be synthesized precisely with organometallic precursors, as illustrated in this review. Synthetic methods are illustrated with examples, including silica-supported chromium and titanium complexes for alkene polymerization rhodium carbonyls bonded predominantly at crystallographically specific sites in a zeolite and metal clusters, including Ir4, Rhg, OsjC, and bimetallics. 

The alkane metathesis with the W-alkyhdyne/alkyl complexes supported on alumina or sihca-alumina yielded a different distribution of branched, neopentyl alkane derivatives, albeit in lower amounts. These results suggest a different mechanism than the alkyhdene-supported metal complexes depicted above. A chrect, C - H bond activation of propane on the alkyhdyne species [91,92] or, alternatively, the formation of bisalkylidene intermediates [93] have been proposed [71]. 

F. R. Hartley, Chem. Met. Carbon Bond, 1987, 4, 1164-1225. Supported Metal Complex Catalysts. 

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