Solute-vacancy/-complexes

In spite of its rigid structure in solution and the trans disposition of the hydride and the coordination vacancy, complex 1 reacts with terminal alkynes to afford alkenyl derivatives, as a result of the addition of the Ru-H bond to the carbon-carbon triple bond of the alkynes [9]. In all cases, the alkenyl ligands have an -stereochemistry and lie in the apex of a square pyramid similar to that of 1. This property along with the presence of the chloride ligand allows the entry to C[, C2, C4, and SH fragments into the coordination sphere of the ruthenium. 

This conceptual link extends to surfaces that are not so obviously similar in stmcture to molecular species. For example, the early Ziegler catalysts for polymerization of propylene were a-TiCl. Today, supported Ti complexes are used instead (26,57). These catalysts are selective for stereospecific polymerization, giving high yields of isotactic polypropylene from propylene. The catalytic sites are beheved to be located at the edges of TiCl crystals. The surface stmctures have been inferred to incorporate anion vacancies that is, sites where CL ions are not present and where TL" ions are exposed (66). These cations exist in octahedral surroundings, The polymerization has been explained by a mechanism whereby the growing polymer chain and an adsorbed propylene bonded cis to it on the surface undergo an insertion reaction (67). In this respect, there is no essential difference between the explanation of the surface catalyzed polymerization and that catalyzed in solution. 

The important feature is the formation of a coordinatively unsaturated site (cus), permitting the reaction to occur in the coordinative sphere of the metal cation. The cus is a metal cationic site that is able to present at least three vacancies permitting, in the DeNOx process, to insert ligands such as NO, CO, H20, and any olefin or CxHyOz species that is able to behave like ligands in its coordinative environment. A cus can be located on kinks, ledges or corners of crystals [16] in such a location, they are unsaturated. This situation is quite comparable to an exchanged cation in a zeolite, as studied by Iizuka and Lundsford [17] or to a transition metal complex in solution, as studied by Hendriksen et al. [18] for NO reduction in the presence of CO. 

The chemical state of the metal can play a decisive role on the reaction mechanism. In TWC, Rh is thought to remain in the zero-valent state, which favors NO dissociation [77,78], However, the role of the OSC materials is complex, and it is not inert with respect to NO activation. Ranga Rao et al. [79] showed that, when bulk oxygen vacancies are formed in a reduced Ce06Zr04O2 solid solution, NO was efficiently decomposed on the support to give N20 and N2. Further studies by the same group... 

A Stern-Volmer plot obtained in the presence of donors for the stilbene isomerization has both curved and linear components. Two minimal mechanistic schemes were proposed to explain this unforeseen complexity they differ as to whether the adsorption of the quencher on the surface competes with that of the reactant or whether each species has a preferred site and is adsorbed independently. In either mechanism, quenching of a surface adsorbed radical cation by a quencher in solution is required In an analogous study on ZnS with simple alkenes, high turnover numbers were observed at active sites where trapped holes derived from surface states (sulfur radicals from zinc vacancies or interstitial sulfur) play a decisive role... 

The conductivity may be compared with that of a 35% aqueous solution of sulfuric acid. 0.8 fl-1 cm-1. The structure consists of a complex (not a simple closest-packed) arrangement of iodide ions with Rb ions in octahedral holes and Ag+ ions in tetrahedral holes. Of the 56 tetrahedral sites available to the Ag+ ions, only 16 are occupied, leaving many vacancies. The relatively small size of the silver ion (114 pm) compared with the rubidium (166 pm) and iodide (206 pm) ions give the silver ion more mobility in the relatively rigid latice of the latter ions. Furthermore, the vacant sites are arranged in channels, down which the Ag+ can readily move (Fig. 7.16). 

The presence of free electrons thus bears a certain similarity to solutions of these same metals in liquid ammonia. If the electrons are thought to be trapped in anion vacancies in the melt, an analogy to F-centers (see Chapter 7) may be made. Undoubtedly the situation is considerably more complex with the possibility of the electron being delocalized in energy levels or bands characteristic of several atoms, but a thorough discussion of this problem is beyond the scope of this book.-5... 

Diffusion of Solute Atoms by Vacancy Mechanism in Close-Packed Structure. Diffusion of substitutional solutes in dilute solution by the vacancy mechanism is more complex than self-diffusion because the vacancies may interact with the solute atoms and no longer be randomly distributed. If the vacancies are attracted to the solute atoms, any resulting association will strongly affect the solute-atom diffusivity. 

The vacancy diffusion field around the toroidal loop will be quite complex, but at distances from it greater than about 2RL, it will appear approximately as shown in Fig. 11.12a. A reasonably accurate solution to this complex diffusion problem may be obtained by noting that the total flux to the two flat surfaces in Fig. 11.12a will not differ greatly from the total flux that would diffuse to a spherical surface of radius d centered on the loop as illustrated in Fig. 11.126. Furthermore, when d > Rl, the diffusion field around such a source will quickly reach a quasi-steady state [20, 26], and therefore... 

Chemical dealumination involves reaction of the zeolite framework with any one of a variety of reagents(2). In this work, zeolites were reacted with ammonium hexafluorosilicate in aqueous solution(9-12) to prepare dealuminated products. Aluminum was extracted from the zeolite framework and removed from the crystal as a soluble fluoroaluminate complex the resulting lattice vacancies are believed to be filled by silicon in solution. Composition profiles of chemically dealuminated zeolites (AFS) are homogeneous and indicate the entire crystal is accessible for dealumination(13). Sorption data indicate that AFS zeolites do not possess a secondary pore system although pore blockage may occur due to occlusion of fluoroaluminate species(13). 

The obtained from the slope of vs. [cf. Eq. (2.95)] is near zero and it is inferred that this value indicates a value of zero for the translational entropy of the ion. A similar result was obtained for the solvated complex. Zero translational entropy for a solvated ion is a reasonable conclusion. Thus, for most of its time, the ion is still in a cell in the solution and only occasionally does it jump into a vacancy, or if it shuffles about, its movement is so constrained compared with that of a gas that it may approach zero. 

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