Localized cr-bonds

The azide ion (N3) is a weakly bound molecular ion. Formulate its MO structure for localized cr bonds and de-local-ized 77 bonds. Do you expect N3 and N3 to be bound as well Which of the three species do you expect to be paramagnetic ... 

A set of hybrid orbitals provides a bonding picture for a molecule in terms of localized cr-bonds. 

Localized electron model The central oxygen atom is sp hybridized, which is used to form the two cr bonds and hold the lone pair of electrons. An unchanged (unhybridized) p atomic orbital forms the tt bond with the neighboring oxygen atoms. The actual structure of O3 is an average of the two resonance structures. Molecular orbital model There are two localized cr bonds and a tt bond that is delocalized over the entire surface of the molecule. The delocalized TT bond results from overlap of a p atomic oribtal on each oxygen atom in O3. 59. 

Butadiene is known to be a substance in which the double bonds can react simultaneously as, for example, in the Diels-Alder reaction and in 1,4 additions of halogens. In the simple molecular orbital treatment, butadiene is treated as a system with localized cr bonds and delocalized u bonds. 

Calculate separate DE, valuep for the configurations (land II) of the proposed tricyclobutonium cation assuming that the CH carbon in each forms only localized cr bonds, that the C-C-C angles at the apex are 90°. and that the bond lengths are 1.54 A. 

An MO correlation diagram for the reaction in equation 11.36 is shown in Figure 11.79. As in the case of the Diels-Alder reaction (Figure 11.66), it is necessary to take combinations of the localized cr bond MOs to produce a set of MOs for the ethene component that are either symmetric or antisym-... 

The metal cyclopentadienyls represent the largest group of the non-classical organometallics, i.e. of those compounds which do not have localized cr bonds between metals and organic entities, but a or tt bonds between the metals and systems of delocalized electrons. It is surprising therefore that no complete vibrational spectrum of the parent compound of the entire class, the cyclopentadienyl anion (CsHg), has as yet been... 

Figure 19 compares experimental data with calculated curves (80). In the random Co—Cr alloy the Cr atoms are not distributed ia the most suitable way for reduciag the Af of the alloy. Therefore the maximum local content of Cr for this distribution is much higher than ia the case where Cr—Cr bonds are not present. The curve for no Cr—Cr bonds present shows that the Af becomes zero at 25 at. % Cr, based on the fact that for bulk material the measured Af for this composition is zero. Consequently 4 Cr nearest neighbor ia an hep lattice makes the final Af zero. 

In diamond, carbon is sp hybridized and forms a tetrahedral, three-dimensional network structure, which is extremely rigid. Graphite carbon is sp2 hybridized and planar. Its application as a lubricant results from the fact that the two-dimensional sheets can slide across one another, thereby reducing friction. In graphite, the unhybridized p-electrons are free to move from one carbon atom to another, which results in its high electrical conductivity. In diamond, all electrons are localized in sp3 hybridized C—C cr-bonds, so diamond is a poor conductor of electricity. 

FIGURE 29.3 Localization pattern for obtained via AdNDP procedure. Here and hereafter 2c-2e peripheral cr-bonds are placed at the cluster framework. 

The predominant 1,4-addition selectivity with lithium in nonpolar solvents is explained by the localization of Li+ to the terminal carbon. Morton et al. found197,198 that lithium is essentially cr-bonded to the terminal carbon (23). In polar solvents the negative charge is delocalized (24), generating a Jt-allyl structure that enhances the reactivity of the y-carbon, affording 1,2 addition ... 

In constructing a localized MO for the bond A—B it is necessary to specify an orbital centred on A (tpA) and an orbital centred on B (y ). In principle, provided symmetry about the bond axis is preserved (we are still considering only cr-bonded systems), our choice of tpA and pB is not restricted and we could use any well-defined mathematical function or combination of functions. Common sense, however, dictates that the most sensible functions to use for this purpose are the AOs of the free atoms A and B. There are three reasons why this is a sensible choice one mathematical, one chemical, and one practical. 

Valence bond theory thus gives a good description of the 0-0 cr bonds but a poor description of the tt bonding among p atomic orbitals, whose four electrons are spread out, or delocalized, over the molecule. Yet this is exactly what MO theory does best—describe bonds in which electrons are delocalized over a molecule. Thus, a combination of valence bond theory and MO theory is used. The cr bonds are best described in valence bond terminology as being localized between pairs of atoms, and the tt electrons are best described by MO theory as being delocalized over the entire molecule. 

There is little evidence for the operation in reactions of the inducto-meric effect, the time-dependent analogue of the inductive effect. This may be so because the electrons of the cr-bonds are more localized and more tightly bound than the electrons of the delocalized system, and are thus not so susceptible to the demands of the reagent. 

The organometallic compound shown below is represented with a Cr=Cr triple bond in the literature. Describe it as a cluster compound. The measured Cr-Cr distance is 2.34 A consistent with a triple bond. Comment on factors that favor localized Cr-Cr multiple bonding vs. delocalized cluster bonding as a mechanism for accommodating the electronic unsaturation. 

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