Electrons delocalisation

Mixed valence molecules electronic delocalisation and stabilisation. D. E. Richardson and H, Taube, Coord. Chem. Rev., 1984, 60,107 (40). 

According to these consideration the diamino-substituted phosphenium (an alternative suggestion for its nomenclature is phosphanylium) cation, 5, and the phosphanetriylammonium (iminophosphenium) cation, 6, possess the largest intrinsic (gas phase) stabihties. Since in the X-ray structures the molecules are to a first-order isolated, this theoretical stability scale determined for the gas phase should also mimic the various trends of the stabilities of the cations and their chelation behaviour. The methylenephosphenium, 7, and the PjH cations, 8, suffer from poor stabihties. On the other hand the phosphirenium cation, 11, is considered to be fairly well stabilized. It is due to n-electron delocalisation of the positive charge in the phosphirenium cation. Intermediate cases in stabihty are the PO+ (9) and PS+ cations (10). Of further interest are the frontier orbital considerations, as shown in Fig. 2. 

The operation of (d) is seen in cyclopentadiene (14) which is found to have a pKa value of 16 compared with 37 for a simple alkene. This is due to the resultant carbanion, the cyclopentadienyl anion (15), being a 6n electron delocalised system, i.e. a 4n + 2 Hiickel system where n = 1 (cf. p. 18). The 6 electrons can be accommodated in three stabilised n molecular orbitals, like benzene, and the anion thus shows quasi-aromatic stabilisation it is stabilised by aromatisation ... 

The reaction of the diene with a free radical produces an allyl radical having unpaired electron delocalised over more than one carbon atom. The allyl free radical can undergo 1, 2, or 1, 4 addition. 

Intensive research interest in peripherally functionalized porphyrazines dnring the last decade has shown that these tetrapyrol derivatives shonld be considered as alternatives to the phthalocyanines that have fonnd extensive apphcations in mat r fields. These include material science and the photodynamic therapy of tnmors as well as pigments and dyes [1]. These compounds have been nsed electrophotography, optical data, photodynamic therapy of tnmois, liquid crystals, pigments and dyes. Porphyrazines have been of considerable interest to spectroscopist and theoreticians for their high symmetrical, planar stractnre and electron delocalisation [2]. 

Benzene is unusually stable and it is the delocalised electrons that account for this stability. The presence of the delocalised electrons also explains why benzene does not undergo addition reactions. Addition reactions would disrupt the electron delocalisation and so reduce the stability of the ring. Substitution reactions, on the other hand, can occur without any such disruption and the stability of the benzene ring is maintained. The delocalised electrons in the % molecular orbital make benzene susceptible to attack by electrophiles (electron pair acceptors). As a result, benzene undergoes electrophilic substitution reactions and some of these are outlined at the top of the next page. Note that the electrophiles are shown in red, the reagents in blue and the reaction names in green. 

There are many other aromatic hydrocarbons, i.e. compounds like benzene, which contain rings of six carbon atoms stabilised by electron delocalisation. For example, if one of the hydrogen atoms in benzene is replaced by a methyl group, then a hydrocarbon called methylbenzene (or toluene) is formed. It has the structural formulae shown. Methylbenzene can be regarded as a substituted alkane. One of the hydrogen atoms in methane has been substituted by a or —group, which is known as a phenyl group. So an alternative name for methylbenzene is phenylmethane. Other examples of aromatic hydrocarbons include naphthalene and anthracene. 

Computational studies of the bonding situation have so far focussed on two issues, namely the r-electron delocalisation and its perturbation by phos-... 

The force constant of carbon monoxide can be associated with a bond order of three that of an organic carbonyl (such as formaldehyde) can be associated with a bond order of two, and that of CO in metal carbonyl complexes can be found by intrapolation. Thus (to the extent that the uncertainties of Sections II.l—II.8 can be ignored) we have a facile probe for d-electron delocalisation within a molecule. 

Partial covalency in essentially ionic bonds changes somewhat the distribution of electrons, detectable as electron delocalisation by the modem methods of nuclear magnetic and electron spin resonance (NMR and ESR). Although the interpretations of these measurements widely differ (see 292, 293, 320) they doubtless prove the existence of partial covalency (in the order of magnitude of 10%) even in the most ionic fluorides AMeFg. Little work seems to have been done one fluorides of the heavier transition elements (96), but there is an abundant literature on first transition series fluorides, of which an arbitrary selection is given below for further information. ... 

In the spinel structure three coordination octahedra meet in one corner, the corresponding Cr—O—Cr bond-angles being about 95°, however, which is unfavourable for electron-delocalisation efiects, i. e. superexchange interactions. 

The formation of anion radicals by chemical or electrochemical reduction may provide information about electron delocalisation in an inorganic ring system through the determination of the EPR spectrum in conjunction with molecular orbital calculations. In practice, however, the LUMO of the homo- or heterocycle is often an antibonding orbital. Consequently, occupation of the LUMO... 

The most intriguing difference between the chemical properties of cyclopolysilanes and those of cycloalkanes is the ability of the former to form either anion or cation radicals upon one-electron reduction or oxidation, respectively. For example, the cyclic pentamer (Mc2Si)5 is reduced to the corresponding radical anion by sodium-potassium alloy in diethyl ether [see eqn (4.1) in Section 4.1.3], whereas the hexamer (Me2Si)6 is oxidised by aluminium trichloride in dichlor-omethane to the corresponding cation radical. In both cases the EPR spectra of the radical ions can be interpreted in terms of a-electron delocalisation over the entire polysilane ring (see Section 10.1.4.1). In this respect, the cyclosilanes resemble aromatic hydrocarbons rather than their aliphatic analogues. 

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