Bond-character modification

Titanium dioxide differs from silica mainly in two respects (1) the Ti + ions are octahedrally coordinated in all three modifications of TiOji (2) the Ti—0 bond is more pronouncedly ionic than the Si—O bond. Using Pauling s electronegativity values (297), one calculates a 63% ionic character for the Ti—0 single bond versus 50% for Si—O. In SiOj, there is certainly some double bond character involving 3d orbitals of the Si atom, causing lowered ionic character. Therefore, characteristic differences should be expected regarding the surface chemistry. 

The X-ray structure also revealed that the tyrosine bonded in the equatorial plane, Tyr 272, has undergone a remarkable modification. A covalent bond has formed between a carbon atom ortho to the hydroxyl group and a cysteinyl sulfur atom from Cys 228. The cysteinyl carbon atom bonded to the sulfur atom lies in the plane of the tyrosine phenyl ring. This planarity suggests partial double-bond character in the C-S... 

A recent example demonstrated that modification of the central atom of the pincer with acidic and basic functionality was possible with a mononuclear PPP pincer iridium(I) complex (Figure 3.18) [76]. In contrast to a meridional geometry typical of pincer ligands, the phosphide ligand in 30 is pyramidalized and, accordingly, affords a facial coordination mode. The geometry about the central phosphoms atom is consistent with predominantly a-bonding character of the Ir-P unit. 

It is now easier to understand why the two-electron reduction is reversible in the tungstic polyanion electrons occupy the empty d . orbital. Its non-bonding character has little influence on the structure. However, in the cis-dioxo configuration, electrons must place themselves in the available orbitals, which arc antibonding, causing important structural modifications and, in most cases, irreversibility of the reduction [86,87]. 

Next, we will treat cycloaddition reactions of alkenes with the bare diamond surface, after high temperature vacuum annealing, which results in the formation of carbon carbon bonds [39 4l]. For example, if the diamond (100) surface is heated in vacuum to 1000°C, hydrogen desorbs, leaving surface C-C dimers. These have appreciable double-bond character and can react with alkenes under conditions appropriate for the Diels Alder cycloaddition reaction. Either the [2-h2] or the [2-1-4] product can be formed, with the latter being the energetically favored pathway. This type of modification can also be carried out photochemically, and this approach is the one that is used more commonly, as discussed in the next section. 

From the inspection of the data in Table 2.4, it is clear that NO changes its original molecular character after adsorption. In general, coordination of nitric oxide leads to a pronounced redistribution of the electron and spin densities, accompanied by modification of the N-0 bond order and its polarization. Thus, in the case of the (MNO 7 10 and ZnNO 11 species, slender shortening of the N-0 bond is observed, whereas for the MNO 6 and CuNO 11 complexes it is distinctly elongated. Interestingly, polarization of the bound nitric oxide assumes its extreme values in the complexes of the same formal electron count ( NiNO 10 and CuNO 10) exhibiting however different valence. 

Extension of the model to take into account the bond bending effect and the interactions within the product cluster leads to the following modification of equations (6.209) and (6.210), in which the diabatic character of the surfaces is emphasized in the subscript notations ... 

Modifications of the chemical reactivity generated by the presence of fluorine atoms in a molecule are connected to three main factors the strength of the C— F bond, the electron-withdrawing character of the fluorinated substituents, and the possible loss of a fluoride ion or of HF in the processes of )S-elimination. On these bases and taking into account the ability of fluoro-substituents to sterically or electronically mimic other... 

---