Multicenter nature

The multicentered nature of adsorption on catalysts surfaces was discussed in section 2.5.6 and it was indicated that the number of adsorption sites on which a molecule (especially in organic catalysis) is adsorbed is not equal to unity as is frequently (and tacitly) assumed. Moreover, in some reactions (for example hydrogenation) there could be a different number of adsorption sites for different substances (for hydrogen and say benzene). This results in a competition for sites between participating molecules. 

Polymers with relatively high levels of stereoregularity can be synthesized using heterogeneous Ziegler-Natta catalysts, but the multicentered nature of the reactive sites makes the reaction difficult to control. 

This sum over pairs of atoms is quite general and results from the multicenter nature of the interaction. To obtain more detailed results we shall proceed to discuss the theoretical calculations on the scattering of Li". ... 

The scattering of atomic ions by polyatomics and solid surfaces is considered at hyperthermal collision energies, where the ions probe the multicenter nature of interaction potentials. Expressing these interactions as sums over target atoms, atom pairs, etc., a theory is developed for the double differential cross section by... 

A polymeric structure is exhibited by "beryllium dimethyl," which is actually [Be(CH3)2] (see the structure of (BeCl2) shown earlier), and LiCH3 exists as a tetramer, (LiCH3)4. The structure of the tet-ramer involves a tetrahedron of Li atoms with a methyl group residing above each face of the tetrahedron. An orbital on the CH3 group forms multicentered bonds to four Li atoms. There are numerous compounds for which the electron-deficient nature of the molecules leads to aggregation. 

Lyles, R.H. et al., Prognostic value of plasma HIV RNA in the natural history of Pneumocystis carinii pneumonia, cytomegalovirus and Mycobacterium avium complex. Multicenter AIDS Cohort Study, AIDS, 13, 341, 1999. 

In this respect, a classification was reported whose basis is the nature of ligands together with the characteristic bonding and structural peculiarities of the metal complexes [112,113], This allowed the identification of four types of coordination compounds molecular complex compounds (MCC) metal-cyclic complex compounds, metal chelates (MC) complexes with multicenter coordination bonds (CMCB) and di- and polynuclear coordination compounds (DPCC). The expediency of such a classification is based on the following considerations. 

A wide range of cyclometallation reactions have been attempted [685]. Depending on the nature of the metal, these reactions were separated into three categories nucleophilic (Co, Fe, Ru, Rh, Pd, Os, Ir, Pt), electrophilic (Ti, Zr, Hf), and multicentered (Th) cyclometallation [685]. 

It may be seen from this superficial summary that there is a wide variety of potentially active single or multicenter sites on oxide catalysts. There will be sites that are distinct with respect to their chemical nature, and for chemically equivalent sites there is a certain energy distribution. 

The catalysis of symmetry-forbidden reactions is currently of broad interest and a number of workers have expressed different points of view on the metal s role. Katz, for example, seems to take the position that these catalytic processes are very likely stepwise in nature, and, therefore, for-bidden-to-allowed arguments do not apply i5,40). This criticism is perhaps more broadly directed toward the concerted, "n complex multicenter reaction mechanism first postulated by Schrauzer to explain metal-assisted cycloaddition chemistry 54). A preference for nonconcerted mechanisms in this chemistry is also shared by Cassar, Eaton, and Halpem... 

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