Ionic-covalent transitions

Figure 2.5 Charge transfer (CT percentage of e charge) from F to Li+ during ionic-bond formation. For reference, a dotted vertical line marks the equilibrium bond length. Note the steep rise corresponding to the onset of the ionic-covalent transition near R = 1 A.

Of much greater chemical significance is the short-range ionic-covalent transition, which marks the inevitable collapse of a purely ionic description at shorter distances. In LiF this transition is centered around RjJ) — 0.73 A, all but hidden under the... 

Figure 2.9 Variations of bond order (upper panel) and charge (lower panel) near the short-range ionic-covalent transition.

In practice, the NBO program labels an electron pair as a lone pair (LP) on center B whenever cb 2 > 0.95, i.e., when more than 95% of the electron density is concentrated on B, with only a weak (<5%) delocalization tail on A. Although this numerical threshold produces an apparent discontinuity in program output for the best single NBO Lewis structure, the multi-resonance NRT description depicts smooth variations of bond order from uF(lon) = 1 (pure ionic one-center) to bu 10n) = 0 (covalent two-center). This properly reflects the fact that the ionic-covalent transition is physically a smooth, continuous variation of electron-density distribution, rather than abrupt hopping from one distinct bond type to another. 

Figure 2.19 -dependent variation of metal charge (gsc) in Sc—F, showing Req (vertical dashed line) and the nearby short-range ionic-covalent transition. 

As discussed in Section 2.5, donor-acceptor interactions generally lead to progressive charge delocalization and ionic-covalent transition from one-center to... 

In a similar fashion the bonding in H2 might be formally regarded as a complementary pair of one-electron donor-acceptor interactions, one in the ot (spin up ) and the other in the 3 (spin down ) spin set.8 In the long-range diradical or spin-polarized portion of the potential-energy curve, the electrons of ot and (3 spin are localized on opposite atoms (say, at on HA and 3 on HB), in accordance with the asymptotic dissociation into neutral atoms. However as R diminishes, the ot electron begins to delocalize into the vacant lsB(a) spin-orbital on HB, while (3 simultaneously delocalizes into Isa on HA, until the ot and (3 occupancies on each atom become equalized near R = 1.4 A, as shown in Fig. 3.3. These one-electron delocalizations are formally very similar to the two-electron ( dative ) delocalizations discussed in Chapter 2, and they culminate as before (cf. Fig. 2.9) in an ionic-covalent transition to a completely delocalized two-center spin distribution at... 

Sousa, C. and Illas, F. (1994) Ionic-Covalent transition in titanium oxides Phys. Rev. B, 50, 13974-13980. 

The adsorption of AM s on Si(l 11)2x1, that is obtained by cleaving in ultra high vacuum, presents some interesting features. Details are given in Table 4. In the case of Na/Si(lll)2xl [94Man], two adsorption sites were found together with clear evidence of an ionic-covalent transition. 

Fig. 2 Expectation values of primary (black), secondary (red) and external functions (olive) as function of LiF bond length. The PCI energy curve (blue dashed, right ordinate) and ionic-covalent transition point are included for reference...

The main oxides of the elements of the second period are shown in Table 7.4. There is the expected pattern, but there are exceptions which are dealt with below. Across the period it would be expected that the compounds would be less ionic/more covalent as the differences in values of electronegativity coefficients between the elements and oxygen decrease. Lithium oxide is ionic and dissolves in water to give a solution of lithium hydroxide. It is a basic oxide. The ionic/covalent transition occurs very early, and BeO is a very high-melting (2530 °C) covalent oxide. The doubly charged Be-" ion would be very small, and the application of Fajans rules would imply that its compounds should be covalent. The oxide does not dissolve in water, but reacts with concentrated sulfuric... 

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