Local coordinative saturation structure

Surface states can arise simply because the atomic bonding at a semiconductor surface is necessarily different from that in the bulk. For example, in a Si lattice, the bonds at the Si surface are not ftilly coordinatively saturated. To relieve this unsaturation, either a surface reconstruction can occur and/or bonds to the metallic material can be formed. This distinct type of surface bonding results in a localized electronic structure for the surface which is different from that in the bulk. The energies of these localized surface orbitals are not restricted to reside in the bands of the bulk material, and can often be located at energies that are inside the band gap of the semiconductor. Orbitals that reside in this forbidden gap region are particularly important, because they will require modifications of our ideal model of charge equilibration at semiconductor/metal interfaces. ... 

The surface of a metal oxide consists of exposed cations, oxide ions, and hydroxyl groups. It is clear that these cations and anions in the surface of an oxide cannot be coordinatively saturated and, hence, that they must develop characteristic properties. The degree of unsaturation of individual surface atoms will be determined by the requirement for retaining stoichiometry in the crystal, and the type of crystal lattice will determine the local symmetry of surface vacancies. A detailed knowledge of the properties, structural and electronic, at an atomic level would be required for an in-depth understanding of the surface chemistry of oxides at a molecular level. This information, however, is almost impossible to obtain experimentally for high-surface-area materials of practical importance in adsorption and catalysis. The descriptions of these surfaces at an atomic level are based almost exclusively on model surfaces and the assumption that certain well-defined crystal planes (preferentially those providing the lowest... 

INNER COORDINATION COMPOUNDS A particular type of non-localized bond occurs in the so-called inner coordination compounds. This group of compounds may be illustrated by reference to the copper salt of aminoacetic acid (glycine). This compound dissociates only weakly in solution, it possesses a colour similar to the ammino complexes of copper and does not react with ammonia. These properties all tend to show that copper atom has a saturated valency and may be explained by considering the copper as bound to both the hydroxyl and the amino groups. In accordance with our concept of the nature of the bond between a metal and an amino group in such a compound, the structure of the complex will be represented by resonance amongst the forms I to IX X... 

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