Cationic with Localized Charge

Fig. 5.3. Formation of a sigma complex (with delocalized positive charge) instead of an ammonium cation (with localized positive charge) upon protonation of l,3,5-tris(pyrro-lidinyl)benzene.

Experimental observations,23 supported by high-level ab initio calculations, 24 indicate that two extreme resonance forms contribute to the general energy of the benzyl cation the aromatic form A, in which the positive charge is concentrated at the methylene group, and the nonaromatic, methylene arenium form B with a sp2 ipso-carbon atom and ring-localized charge (Scheme 3.13). Unlike benzyl cations of the form A, which were isolated and studied, especially by Olah and coworkers,23 compounds represented by the form B remained elusive. Thus, metal complexation... 

The required local charge balance between cations and anions which is expressed in Pauling s rule causes the distribution of cations and anions among the octahedral and tetrahedral interstices of the sphere packing. Other distributions of the cations are not compatible with Pauling s rule. 

Nucleophilic Trapping of Radical Cations. To investigate some of the properties of Mh radical cations these intermediates have been generated in two one-electron oxidant systems. The first contains iodine as oxidant and pyridine as nucleophile and solvent (8-10), while the second contains Mn(0Ac) in acetic acid (10,11). Studies with a number of PAH indicate that the formation of pyridinium-PAH or acetoxy-PAH by one-electron oxidation with Mn(0Ac)3 or iodine, respectively, is related to the ionization potential (IP) of the PAH. For PAH with relatively high IP, such as phenanthrene, chrysene, 5-methyl chrysene and dibenz[a,h]anthracene, no reaction occurs with these two oxidant systems. Another important factor influencing the specific reactivity of PAH radical cations with nucleophiles is localization of the positive charge at one or a few carbon atoms in the radical cation. 

The overall conclusion from the reaction of BP and 6-substituted BP radical cations with nucleophiles of various strengths is that weak nucleophiles display higher selectivity toward the position of highest charge localization. Thus another important factor in the chemical reactivity of radical cations is represented by the strength of the nucleophile. 

How then, can one recover some quantity that scales with the local charge on the metal atoms if their valence electrons are inherently delocalized Beyond the asymmetric lineshape of the metal 2p3/2 peak, there is also a distinct satellite structure seen in the spectra for CoP and elemental Co. From reflection electron energy loss spectroscopy (REELS), we have determined that this satellite structure originates from plasmon loss events (instead of a two-core-hole final state effect as previously thought [67,68]) in which exiting photoelectrons lose some of their energy to valence electrons of atoms near the surface of the solid [58]. The intensity of these satellite peaks (relative to the main peak) is weaker in CoP than in elemental Co. This implies that the Co atoms have fewer valence electrons in CoP than in elemental Co, that is, they are definitely cationic, notwithstanding the lack of a BE shift. For the other compounds in the MP (M = Cr, Mn, Fe) series, the satellite structure is probably too weak to be observed, but solid solutions Coi -xMxl> and CoAs i yPv do show this feature (vide infra) [60,61]. 

Fig. 10. Molecular and electronic structure of cation 74. (a) Perspective view on the cation 74. (b) Coordination sphere of the Pb atom, selected interatomic distances (pm) and angles (deg) Pb-(center CC, C7), 281.7 Pb-(center C6 CT), 280.7 Pbl-C2, 227(2) Pbl-C2 221.1(15) Pbl-C8, 231.0(19) C6-C7, 133(2) CC-CT 132(2) (center C6, C7)-Pb-(center C6, C7 163.6. (c) Contour plots of the Laplacian distribution [V p(r)] in the plane containing the atoms Pb, C6 and C7. Solid and dotted lines designate regions of local charge concentration and depletion, respectively. The bond paths are indicated by the solid back lines, bond critical points are marked with a black square. (Reprinted with permission from Ref. 53. Copyright 2003, Wiely-VCH.)...

Natural population analysis was carried out on 273 and 282. As shown in Figure 1, the positive charge of both cations is localized on the carbon atoms of the three-membered ring. This is consistent with the conclusion, based on C chemical shifts of benzocyclopropenyl cations, that about 45% of the positive charge resides in the cyclopropene moiety. ... 

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