Valence delocalisation

Theory and experiments on valence delocalisation in mixed-valence compounds. P. Day, Comments Inorg. Chem., 1981,1,155-167 (23). 

The uv/vis/nir spectra of mixed-valence species exhibit unique low-energy intervalence charge transfer (IVCT) transitions which are absent in the fully reduced and oxidised complexes. Qass (valence localised) mixed-valence systems have broad, weak, structureless IVCT bands which are sensitive to molecular environment (eg solvent ). Detailed study of these bands in trapped valence species allows os to calculate intramolecular electron transfer rates. Class III (valence delocalised) systems have more intense low-energy transitions which are often structured and are largely insensitive to solvent. We will consider the degree of metal-metal interactions within class III species by analysis of low-energy absorption transitions. 

The IR and resonance Raman spectra of [(NC)5Fe -CN-PF (NH3)4-NC-Fe (CN)5] (vCN) were used to monitor photochemical processes in this species (surface-attached to colloidal Ti02). Values of vCN for [(NC)5Fe(p-tz)Te(CN)5] " , where tz = 1,2,4,5-tetrazane, n = 4, 5 or 6, show that vCN increases with decreasing n - as expected for oxidation going through a valence-delocalised intermediate. " ... 

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

The valence-bond (resonance) description of the triphenylmethine dye Malachite Green (125) is illustrated in Figure 6.5. Comparison with Figure 6.4 reveals their structural similarity compared with cyanine dyes. Formally, the dye contains a carbonium ion centre, as a result of a contribution from resonance form II. The molecule is stabilised by resonance that involves delocalisation of the positive charge on to the p-amino... 

For the description of systems with conjugated double bonds force field calculations of the kind described here are not very useful since, in principle, they only allow the description of relatively localised valence effects and of pairwise nonbonded interactions. Effects of delocalisation as occurring in conjugated vr-systems represent a new element for whose description quantum-mechanical concepts are appropriate. 

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]. 

Radiation falling on a semiconductor will be absorbed by electrons in delocalised bands, particularly those near the top of the valence band, causing these electrons to be promoted to the conduction band. Because many closely packed levels are present in an energy band, the absorption spectrum is not a series of lines as in atomic spectra, but a broad peak with a sharp threshold close to the band gap energy. The absorption spectrum of GaAs, for example, is depicted in Figure 8.7. 

When discussing the electrical conductivity of metals, we described them in terms of ionic cores and delocalised valence electrons. The core electrons contribute a diamagnetic term to the magnetic susceptibility, but the valence electrons can give rise to paramagnetism or one of the cooperative effects we have described. 

ZnS is a semiconductor with a full valence band and empty conduction band. When the phosphor is illuminated, electrons are promoted to the conduction band. Because the orbitals in this band are delocalised, the energy can easily be transferred to other parts of the crystal, particularly to the dopant atoms. 

Figure 9.13—Effect of resonance on carbonyl-containing compounds. Representation of the delocalisation of valence electrons in mesomeric forms of organic compounds. In1 C NMR, the signal corresponding to a carbonyl in an ester is at 165 ppm, whereas it is at 205 ppm for a ketone.

The results of a valence bond treatment of the rotational barrier in ethane lie between the extremes of the NBO and EDA analyses and seem to reconcile this dispute by suggesting that both Pauli repulsion and hyperconjugation are important. This is probably closest to the truth (remember that Pauli repulsion dominates in the higher alkanes) but the VB approach is still imperfect and also is mostly a very powerful expert method [43]. VB methods construct the total wave function from linear combinations of covalent resonance and an array of ionic structures as the covalent structure is typically much lower in energy, the ionic contributions are included by using highly delocalised (and polarisable) so-called Coulson-Fischer orbitals. Needless to say, this is not error free and the brief description of this rather old but valuable approach indicates the expert nature of this type of analysis. 

Figure 8-16. A valence bond representation of a co-ordinated pyridine. The positive charge is delocalised and the 2- and the 4-positions of the ligand develop electrophilic character.

The molecular space is divided into core and valence basins, the latter being classified according to their connectivity to the core basins as monosynaptic basins, associated with electron pairs, or dissynaptic basins, associated with covalent bonds. Moreover, an integration of the electronic population over each basin gives the number of electrons in each of them and the fluctuation between basins is related to the electronic delocalisation [39]. 

The electronic structures of C2-symmetry N6 and quasi-linear (CNO)2 have attracted some attention recently [8,9]. The results of ab initio MO studies [8,9] indicate that these two species are respectively unstable and bound relative to their N2 and CNO dissociation products. The VB structures that we shall use to represent the primary features of the electronic structures of N6 and (CNO)2 are examples of increased-valence structures [2-5], Such structures may be generated from familiar (Kekul6-type) Lewis structures by delocalising non-bonding electrons into bonding localised MOs (LMOs). 

The increased-valence structures 39 and 45 for CH2N2 and CH3NO2 are generated from the Kekule-type Lewis structures 37 and 42 via the one-electron delocalisations that are indicated in structures 47 and 48. More... 

---