Bonds representation

Valence bond representation of the hyperconjugation effect which leads to a lengthening of the C—H bond icetaldeyde. 

On the basis of these values one can conclude that, with increasing bond orders, the force constants rise, suggesting that the S—O bond of sulphoxides should have more semipolar character than that of sulphones. Furthermore, molecular diffraction measurements20 and Parachors21 for sulphoxides also suggest that the S—O bond in sulphoxides should have a semipolar single-bond representation while the S—O bond in sulphones is described by double bonds or better as the resonance hybride shown in Scheme 1. 

Figure 1-1. Limiting valence bond representations of the cation [Fe(H20)6]. ...

In triethylaluminum, each A1—C bond can be visualized as an. y p hybrid on aluminum overlapping with an S p hybrid on a carbon atom. Figure 10-13 shows this bonding representation, with three equivalent A1—C bonds and the unused 3 p orbital on the aluminum atom. 

Figure 3.19 Bent-bond representation of the double bond in ethene. The overlap of sp3 orbitals on each carbon atom produces to bend bond (r) orbitals.

In our laboratory we have examined the reactivity pattern of [0s3(y-H)2(C0)10], an unsaturated cluster which can be represented as possessing an osmium-osmium double bond in its classical valence bond representation. We find (2,3) that this compound undergoes a number of reactions with metal carbonyls which in some cases can be formulated as proceeding through intermediates analogous to metal olefin complexes ... 

Valence-bond representations of the (C, Hfchelating ( agostic ) bonding mode in a two-metal-atom site... 

Figure 2.9 Polymer bond representations for a PE resin segment. All carbon atoms are fixed in the plane of the diagram while half of the hydrogen atoms are in front of the plane and half are behind the plane...

Many other methods, of course, take advantage of invariances with respect to orbital transformations to obtain alternative representations of wavefunctions, such as for example the commonly employed localization procedures for the doubly-occupied MOs from Hartree-Fock calculations [11-15]. We give here a brief account of procedures that particularly seek a valence bond representation of MO wavefunctions. 

For these reasons, a permissive definition of nitrenium ions is chosen. A nitre-nium is any species that can, through valence bond representations, be depicted as having a dicoordinate positively charged nitrogen. Such a definition has the advantage of being readily applied to any species for which a valence bond representation can be depicted. On the other hand, such a definition does encompass a number of species with rather different properties. 

The SnI activation free energies and transition-state stractnre for the series t-bntyl chloride, -bromide, and -iodide in several solvents over a wide polarity range have been examined theoretically. The analysis is accomplished by nsing a two-state valence bond representation for the solute electronic stractnre, in combination with a two-dimensional free energy formalism in terms of the alkyl halide nuclear separation... 

However, the normal convention is to use the double-single bond representation. 

A satisfactory theoretical model for ethylene oxide should take into account as many as possible of the physical properties discussed above, but should be able to predict or explain its chemical properties as well. Three such ogodels have been proposed which are based on molecular-orbital theory s 1-3W.1 ° and two more which conform rather to tho valence-bond representation of chemical structure.1M, W 7 The relative merits of all these models have been discussed in recent reviews.8 7 1301... 

Valence bond representations of the ground and excited states of ethylene... 

These complexes are considered according to the mode of bonding of the hydrazido fragment. The known coordination types are shown in valence bond representations in Figure 15 together with the formal charge, geometry and numbers of valence electrons donated to the metal. 

Although various procedures are available for the model analysis of fibrous polymers, methods based on the virtual bond representation of the asymmetric residue may be of advantage in many cases. In the following, we describe one such method that began with simple procedures applied to polysaccharides, but has now been refined into a flexible and powerful model analysis tool that is simple to use with any class of polymer. Its use in the present case, however, is illustrated with examples drawn from the structure analysis of polysaccharides. 

Valence bond theory is somewhat out of favour at present a number of the spectroscopic and magnetic properties of transition-metal complexes are not simply explained by the model. Similarly, there are a number of compounds (with benzene as an organic archetype) which cannot be adequately portrayed by a single two-centre two-electron bonding representation. Valence bond theory explains these compounds in terms of resonance between various forms. This is the origin of the tautomeric forms so frequently encountered in organic chemistry texts. The structures of some common ligands which are represented by a number of resonance forms are shown in Fig. 1-11. 

Figure 2-12. Valence bond representation of the formation of a complex. The left hand form is the ionic representation with no covalent interaction between metal and ligand. The right-hand form shows the charge distribution which results from equal sharing of the lone pair.

Figure 2-13. Valence bond representations of the [Cr(H20)6]3+ ion showing the charge distributions.

Figure 8-16. A valence bond representation of a co-ordinated pyridine. The positive charge is delocalised and the 2- and the 4-positions of the ligand develop electrophilic character.

In azole N-oxides the bond between the nitrogen and the oxygen atom is formed by an overlap of a lone pair orbital at the N-atom with an empty p-orbital at the oxygen atom. In the literature the N-O bond has been depicted as a dipolar single bond, a double bond, or as an arrow as shown in Scheme 2. The dipolar representation is used here. The double bond representation is usually applied in literature search engines. 

---