Bonding delocalized versus localized

The question of aromaticity versus antiaromaticity and delocalized versus localized double bonds in pentalene (2) dates back to 1922, when Armit and Robinson compared it with naphthalene and postulated that the former might be similarly aromatic [32, 33]. While the first synthesis of a non-fused hexaphenylpentalene (38) [30] provided only some clues as to the non-aromatic reactivity of the pentalene skeleton, the tri-tert-butyl derivative 39, prepared and studied by Hafner et al. in great detail [31], gave a better insight. The ring-proton signals of this alkyl-substituted pentalene 39 are shifted upfield compared to those of fulvene (27) and other cyclic polyenes. This observation led to the conclusion that the pentalene derivative 39 should be an antiaromatic species. However, the results did not permit a distinction... 

Table 4. Delocalized versus Localized Bonding and the Matching Rule (assumes three internal orbitals per vertex atom)...

Localized versus delocalized descriptions of transition-metal bonding and hyperbonding... 

Molecular-orbital- versus valence-bond-type descriptions Let us now examine the connection between localized and delocalized descriptions in somewhat more formal terms, using the actual numerical energy levels of PtH42 to illustrate the formal connections. Inevitably, this discussion entails somewhat more technical detail concerning computational MO methodology than required elsewhere in this book, and we assume a general familiarity with Sections 1.4 and 1.5, Appendix A, and other sections mentioned below. 

VSEPR theory indicates nothing about the nature of the chemical bond (localized Heitler-London versus delocalized Hund-Mulliken). It simply predicts the geometrical shape, specifically, the X-X, M-X and/or X-M-X bond angles in the molecule. 

Through its ups and downs, the concept of orbital hybridization is most significative for the perspectives opened out by Quantum Chemistry from about 1930. Let us only mention all the disputes in which it was involved during almost one century with regard to tricky questions of molecular structure (i.e., the supremacy of valence-bond pictures over molecular orbitals or vice-versa, the physical content of the resonance theory, the localization versus delocalization dilemma and so on...). Furthermore, its study gives us a good example of the specificity of the scientific explanation among chemists (i.e., the rejection of reductionism to Physics [1]). 

In contrast to all other rare earth metal compounds discussed so far, the hydride halides contain mobile interstitial atoms that can be added or (partly) removed at will in a topochemical reaction. These compounds therefore offer a unique possibility to study the delocalization of electrons in extended metal-metal bonds versus the localization at interstitial atoms. The known compounds are summarized in table 7. 

The weaker secondary NSD contributions of the nitroxide environment can also be qualitatively understood from leading (vicinal) donor-acceptor delocalizations of unique spin NBOs of structures (7.6a,p). Because the local nitroxide frame is essentially planar, the relevant spin NBOs ( n, o in a ttNO in P) nre all of local p-7i type, perpendicular to the C(3)C(4)N(2)0(1) plane. The leading hyper-conjugating candidates are therefore the vicinal C—C bonds of methyl substituents, distinguishable as being oriented strongly [C(3)—C(17) or C(4)—C(21) 82°] versus weakly [C(3)—C(13) or C(4)—C(21) 38°] out-of-plane with respect to the nitroxide moiety. 

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