Electronic, communication delocalization

One of the rules mentioned above and following from the LFER treatment is that the higher slope (higher /o-value) indicates either a shorter distance between the substituent and the reaction center, or, in the case of the same distance, a higher level of electron communication-delocalization. 

The degree of delocalization of the charge is directly related to what is called the extent of electronic communication between the redox centres. 

Despite the structural equivalence of the twelve metal centers in the o -PWi204o Keggin anion, a stepwise reduction of the tungsten centers is observed [30f]. EPR experiments on this species show that the added electron is delocalized (type II in the scheme proposed hy Rohin and Day) [4]. Over all 12 tungsten centers suggesting that there is communication... 

Cyclic voltammetry revealed that the N-atoms of 86a, 88a and 91a are oxidized at lower potentials than the trivalent P atoms. Comparison of these data with those observed with model compounds shows a very weak electronic delocalization via the P centers for copolymers 86a and 91a. In contrast, the low first oxidation potential observed for 88a (Table 4.5) is assumed to result from an electronic communication between the N moieties through the connecting P centers [59b]. The equivalence of the oxidation potentials for the oxidized polymers (Table 4.5) suggests the presence of electronically isolated triarylamine fragments in these derivatives. Note that the involvement of the P lone pair in jt-delocalization... 

M"-M but have some new properties that can be attributed to the mixed-valence species. Class II compounds typically have IT transitions that can be characterized using the Hush theory. Class III systems are compounds that have such strong electronic communication between the metal centers that they are completely delocalized systems. The oxidation state of the metal centers in a class ni system can best be described as Class III complexes typically do... 

Grimes and co-workers have reported the synthesis of polymetallacarborane staircase oligomers containing cobalt, nickel, and nitheniiun (124,125). Oligomers with up to 17 metal atoms were prepared, and electrochemical analysis of these materials indicated that although there was evidence for electron delocalization within the tetradecker stacks, there was very little intersandwich electronic communication (124). 

In Eq. 48.1, the value of the reaction constant p reflects the extent of interaction (electronic communication) between the substituent and the reaction center, that is, how much the measured potential depends on electron pushing/withdrawing ability of the substituent. In this way, a level of tt-delocalization along the molecule or displacement of oxidation and reduction centers, respectively, can be experimentally followed and correlated with quantum chemical calculations. Since the energy (potential) needed for oxidation or reduction of the studied molecule is proportional to the energy of the respective frontier molecular orbitals (and can be determined experimentally as and by an appropriate electrochemical method), their difference (E - E ) correlates well with the HOMO-LUMO gap obtained spectrophotometrically or using calculations. 

After removing a hydride of the cycloheptatriene and a proton of cyclopentadiene the bis-substituted p-carborane can be compared with the analogous organic [5.6.7]quinarene derivative shown in Figure 13.14 (left side), but there is one major difference between than the bridging benzene unit allows electronic communication between the tropylium and the cyclopentadienide unit and thus a fully electron-delocalized system exists, while the boron-based polyhedral bridge restricts the electron transport, which leads to a highly polarized molecule (Taylor et al., 2001). 

Driven by these results, other ligands have been successively developed, bearing a Jt-delocalized aromatic linker between the two carbenes (17—19) and whose main interesting feature relies on the possibility to enable electronic communication between the two carbene moieties, and as a consequence between the two coordinated metal centers. 

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