Changes of bond-type

Interpretations of the crystal chemistry of transition metal ions by crystal field theory are based on a predominatly ionic model of the chemical bond. As 

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An abrupt change in properties in a series of compounds, as in the melting points or boiling points of halides, is often taken as indicating an abrupt change in bond type. Thus of the fluorides... 

Discontinuous Change in Bond Type.5—In molecules and complex ions of certain types continuous transition from one extreme bond type to another is not possible. In order for continuous transition to be possible between two extreme bond types the conditions for resonance between the corresponding structures must be satisfied. The most important of these conditions is that the two structures must involve the same numbers of unpaired electrons. If the two structures under consideration involve different numbers of unpaired electrons, then the transition between the two must be discontinuous, the discontinuity being associated with the pairing or unpairing of electrons ... 

There is a variety of disparate binary oxide compounds. The change of physical properties is attributable to the range of bond types from essentially ionic to essentially covalent. 

The second type of atomic polarization is attributable to intramolecular vibrations of the bonds between atoms and the change of bond length. It occurs in the near-infrared region with the frequency V of the order of lO -10 s . ... 

The dissociations described above have mostly been identified as interface advance reactions for which the nucleation step occurs relatively readily. The dominant kinetic feature is the progress of the reaction zone inwards to the particle centres. The Polanyi-Wigner reaction model (Chapter 7) was developed to account for the rates of such processes. Shannon [83] identified 29 different chemical changes of this type and found that only one-third of the reported kinetic parameters were within an order of magnitude of the theoretically expected values. From these, the dissociations of CaCO, and MgCOj were selected for more quantitative application of the absolute reaction rate theory. The known crystal structure and physical properties of the participating bonds were used to represent the transition state as follows ... 

Figure 9.23 Properties of the Period 3 chlorides. Samples of the compounds formed from each of the Period 3 elements with chlorine are shown in periodic table sequence in the photo. Note the trend in properties displayed in the bar graphs as AEN decreases, both melting point and electrical conductivity (at the melting point) decrease. These trends are consistent with a change in bond type from ionic through polar covalent to nonpolar covalent.

For the effect of cation of metal salt of polymer complex, it was observed that [Figs. 7.7b and 7.8b], CMC-Zn[H] complex provided paper sheet with greater dielectric properties than that produced by CMC-Fe[IH] complex. Both type of complexes produced relatively high insulating paper than paper made from untreated wood pulp. The results obtained are probably due to the change of the type of combination between metal and cellulose. However, in case of Zn[H], the ionical bond is formed, while chelated covalent coordinated bond is formed in case of Fe[IH] [47]. 

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