Resonance between valence-bond structures

The bond valence of Re4(/u-3 -H)4(CO) 12 is 8, and the tetrahedral Re4 skeleton can be described in two ways (I) resonance between valence-bond structures, leading to a formal bond order of lj, and (II) four 3c-2e ReReRe bonds. Since there are already four 1x3 -11 capping the faces, description (II) is not as good as (I). 

Large deviations from constancy of bond energies are found in compounds for which two or more equally acceptable valence bond structures can be written. Such compounds usually have higher heats of atomization than those calculated on the basis of the sum of bond energy terms for any one of these structures. This extra stability is usually ascribed to resonance between valence bond structures, and the excess heat of atomization above that calculated for the most stable valence bond structure is referred to as the resonance energy. 

In Section 2-2, we have indicated that the standard Lewis representation for a 4-electron 3-centre bonding unit involves resonance between valence-bond structures of the general types (1) and (2), i.e. 

If we use the result that a 1-electron bond between a pair of atoms A and B summarizes resonance between valence-bond structures (A B) and (A B) (i.e. 

As does resonance between valence-bond structures (l)-(4), these Pauling 3-electron bond structures accoimt qualitatively for the distribution of the odd electron of NOj, and for the equahty of the N-0 bond-lengths. They are also in accord with the observation that the N-0 lengths of 1.19 A are intermediate between those for NO (1.15 A)" and NOj (1.24 A), as are the bond-angles ( NO2, 134° no , 180° NO , 115°). The NO and NO ions have, respectively, 16 and 18 valence-shell electrons, and standard Lewis structures for these ions are those of (11) and (12)... 

When O o Cl o O is used to represent the five n-electrons (cf. Figure 6-2), valence-bond structure (50) is obtained (without electron spins). It is equivalent to resonance between valence-bond structures (48) and (49). In Figure 11-6, another procedure is used to construct valence-bond structures that are analogous to structures (48) and (49). 

It may be that it is necessary in compounds containing tctravalent carbon atoms, to take into account the resonance with valence bond structures in which carbon is in the divalent, ue. state. If this be so, then for methane it is necessary to consider the resonance between the orms H H H H... 

Three years ago it was pointed out2 that observed values of interatomic distances provide useful information regarding the electronic structures of molecules and especially regarding resonance between two or more valence bond structures. On the basis of the available information it was concluded that resonance between two or more structures leads to interatomic distances nearly as small Us the smallest of those for the individual structures. For example, in benzene each carbon-carbon bond resonates about equally between a single bond and a double bond (as given by the two Kekul6 structures) the observed carbon-carbon distance, 1.39 A., is much closer to the carbon-carbon double bond distance, 1.38 A., than to the shrgle bond distance, 1.54 A. 

The electronic structure corresponds to two valence-bond structures resonance not only between the... 

Each of these tautomers in its normal state is represented not by the conventional valence-bond structure shown above, but by a resonance hybrid of this structure and others. For tautomer A, with the hydrogen atom attached to the nitrogen atom 1, the principal resonance is between structures A I and A II, with A I the more important smaller contributions are made also by other structures such as A III. Similar resonance occurs for tautomer B. Thus for both tautomers the principal resonance... 

It is often asked whether or not the constituent structures of a resonating system, such as the Kekul4 structures for the benzene molecule, are to be considered as having reality. There is one sense in which this question may be answered in the affirmative but the answer is definitely negative if the usual chemical significance is attributed to the structures. A substance showing resonance between two or more valence-bond structures does not contain molecules with the configurations and properties usually associated with these structures. The constituent structures of the resonance hybrid do not have reality in this sense. 

The investigation of methyl azide, methyl nitrate, and fluorine nitrate by electron diffraction is shown to lead to configurations of the molecules corresponding in each case to resonance between two important valence-bond structures. The unimportance of a third otherwise reasonable structure for these molecules as well as for nitrous oxide is ascribed to instability due to the presence of electric charges of the same sign on adjacent atoms. It is shown that the differ-... 

Several structural features, including electron transfer between atoms of different electronegativity, oxygen deficiency, and unsynchronized resonance of valence bonds, as well as tight binding of atoms and the presence of both hypoelectronic and hyperelectronic elements, cooperate to confer metallic properties and high-temperature superconductivity on compounds such as (Sr.Ba.Y.LahCuO,-,. 

It is wrong (but common) to see a reversible reaction written with a double-headed arrow, as A B. Such an arrow implies resonance, e.g. between the two extreme valence-bond structures of Kekule benzene. 

The r-delocalisation in the parent phospholide anion I (Fig. 3, R =R =H) can be expressed in the valence bond picture by resonance between the canonical structures lA-IC (and their mirror images). Phosphonio-sub-stituents (R =R =PH3 ) increase the weight of the 1,2-dipolaric canonical structure IB and induce thus, in essence, a partial r-bond localisation and a shift of r-electron density from the phosphorus to the adjacent carbon atoms [16]. Consequences of this effect are the decrease in delocalisation energy for reaction (1) depicted in Fig. 4, and lower C2-C3/C4-C5 and higher C3-C4 bond orders which are reproduced in concomitant variations of computed bond distances [16]. 

The difference between the observed heat of formation and that calculated for a single valence-bond structure for a molecule with use of the table of bond energies is an empirical value of the resonance energy of the molecule relative to the assumed valence-bond structure. 

If the radical were restricted to resonance between the KekulA structures A and B, with the free valence on the methyl carbon, resonance would stabilize the radicals to just the same extent as the undissociated molecules, which would then have only the same tendency to dissociate as a hexaalkylethane. But actually the five structures A, B, C, D and E (each with three double bonds) contribute about equally to the structure of the radical, which thus resonates among five structures instead of two and is correspondingly stabilized by the additional resonance energy. 

The theory of resonance has been applied to many problems in chemistry. In addition to its use in the discussion of the normal covalent bond (involving the interchange of two electrons, with opposed spins, between two atoms) and to the structure of molecules for which a single valence-bond structure does not provide a satisfactory description, it has rendered service to chemistry by leading to the discovery of several... 

It is convenient for us to draw a rather broad line between resonance (in the sense of resonance of a molecule among alternative valence-bond structures) and tautornerism by the use of the ratio of the resonance... 

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