Cation Homopolar Bonding

In M0O2 and WO2, R < R and one d electron per metal atom is involved in cation-cation homopolar bonding through fn orbitals. The extra d electron partly fills the n band, rendering the oxides metallic (Fig. 6.16(d)). This simplified picture does... 

On the homopolar line between the A(P and the C(P regions, for example, the usual anionic chemisorption of the last section and the unusual cationic chemisorption of this section coalesce, and a homopolar bond is formed between the foreign atom and the lattice. One electron is lost from an impurity level for each foreign atom adsorbed, and this homopolar chemisorption is depletive. 

The amount of water, which is sorbed on zeolites, can be very significant, up to 25 wt% for Na and H-FAU samples with low Si/Al ratios. This amount depends on the partial pressure of water and on various characteristics of the zeolite, such as the pore system, which determines the micropore volume accessible to water, the framework Si/Al ratio, the nature of the cations and the crystallite size. Thus, a linear decrease was found in the amount of water adsorbed at low pressure (that is, strongly adsorbed) over various protonic high silica zeolite materials MOR,[28] bea[29] or MFI,[30] with a decrease in the framework A1 content. This can be attributed to the decrease in the number of partially ionic, hydrophilic centres associated with the tetrahedrally coordinated A1 atoms at the profit of nearly homopolar (hydrophobic) =Si-0-Si= bonds. A stoichiometry of four water molecules per H(A1) was found, suggesting the formation of H9O4 species. 

Electron-pair bonding is therefore more probable if R tt Rc, since the curvature of V(R) has its maximum positive value at R tt / c. For R near the equilibrium for d-electron bonding, the curvature is negative and close-packed structures are more stable. Further, if the cations are small relative to the anion interstice, Aei is smaller and the transition temperature will be higher. (Conversely, if pressure is applied, Ae 1 will generally increase faster than An, so that Tt decreases.) Electron-pair bonding is also more probable if homopolar, that is if between like cations, as there is then no ionic component, which favors close packing. 

Heteropolar semiconductors can be thought to form sp hybrid molecular orbitals exactly as do homopolar semiconductors. When we considered two atoms together in a homopolar semiconductor, bonding and antibonding states resulted from symmetric and antisymmetric mixtures of identical hybrid orbitals. The same combinations occur in a heteropolar semiconductor, but now the cation and anion hybrid orbitals sp c snd sp A are more distinguishable and have different electron densities. Furthermore, the symmetric and antisymmetric states now have different contributions from the cation and anion molecular orbitals. 

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