Non-bonding valence shell electrons

There is an alternative way of calculating the bond flux using the Kirchhoff equations ((2.7) and (2.11)) in place of the network equations ((3.3) and (3.4)), the problem in this case being to determine the appropriate bond capacitances which are not now all equal. Where the multipole produces a shorter bond, a larger capacitance is needed, and conversely where the multipole produces a longer bond, a smaller capacitance is needed. Transferable bond capacitances have been successfully used to model the asymmetries in d° transition metal environments as discussed in Section 8.3.2 below. 

The remaining sections of this chapter examine particular classes of cations that show electronically induced distortions. Section 8.2 explores the distortions caused by lone pairs and Section 8.3 explores the distortions found in transition metals. 

One of the best known examples of electronically induced distortion is the steric effect of the non-bonding valence-shell electrons found in main group cations in low oxidation states, cations such as S , and Sn, in which one or more pairs of valence electrons are not involved in chemical bonding. Such nonbonding electrons are popularly known as lone pairs because they occur as localized spin-paired electrons. 

The distortion produced by the lone pairs is traditionally described using the Valence Shell Electron Pair Repulsion Model (VSEPR model) (Gillespie and Hargittai 1991), which assumes that each pair of electrons in the valence shell is 

Displacement of the cation towards a face gives three primary and three secondary bonds and is favoured by low-valence cations such as Tl . Cations with intermediate valence, e.g. Sn +, tend to move towards an edge giving four primary and two secondary bonds while high-valence cations such as Xe favour displacement towards a corner to give five primary bonds (one strong 

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The fundamental principle of the Valence-Shell Electron-Pair Repnlsion theory is that the bonding pairs and lone, non-bonding pairs of electrons in the valence level of an atom repel one another. As you know, electron pairs of atoms are localized in orbitals, which are shapes that describe the space in which electrons are most likely to be found around a nucleus. 

Q The valence shell of the central atom of a molecule possesses two bonding and three non-bonding pairs of electrons. State, with reasoning, the shapes of (i) the electron pair distribution and (ii) the molecule. 

It is a small step from van der Waals, electron-domain models of the C—H bonds of, e.g., biphenyl, cyclohexane, or methane (Figs. 1—3), to molecular models in which to a first, and useful, approximation each valence-shell electron-pair is represented by a spherical, van der Waals-like domain 7h (Non-spherical domains may be useful for describing, e.g., lone pairs about atoms with large atomic cores, -electrons, and the electron-pairs of multiple bonds vide infra.)... 

The valence shell electron pair repulsion (VSEPR) model is based on the observation that the geometrical arrangement of bonds around an atom is influenced by non-bonding electrons present. 

The Laplacian thus displays where the electronic charge is locally concentrated or depleted [25, 26]. The topology of the valence shell charge concentration (VSCC), the region of the outer shell of an atom over which V p < 0, is in accordance with the Lewis and valence shell electron pair repulsion model. To each local maximum in the VSCC, a pair of bonded or non-bonded electrons can be assigned (Fig. 2). 

Valence-shell electron-pair repulsion (VSEPR) theory and the concept of hybridization suggest that the water molecule has two O—H bonds and two non-bonded pairs arranged tetrahedrally. More accurate calculations show that this does not provide a true picture of the total electron density in H20. 

The reason for this unusual geometry was the availability of only the stereochemically active electrons for bonding. Thus, a SO modification of the valence shell electron pair repulsion (VSERP) theory was suggested in [130]. Han et al. [129], however, claim that this modification should be applied with caution, since no non-linear II8F2 structure has been detected as a minimum at the HF level of theory. An important observation was made that the fluorides of element 118 will most probably be ionic rather than covalent, as in the case of Xe. This prediction might be useful for future gas-phase chromatography experiments. 

Atoms as well as molecules have electronic transitions that are not of the Rydberg type. For atoms the famous D-lines of sodium (3s,3p) are an example. For molecules all the familiar (vr, n ) and (n, rr ) transitions of olefins and aromatic molecules are examples of non-Rydberg, valence-shell (or intravalency) type transitions. For typical valence-shell transitions the orbital of the excited electron is not much larger than the molecular core. Bands due to such transitions cannot be ordered into series. The orbital of the excited electron is usually antibonding in one or more bonds wliile Rydberg orbitals because of their large size are, in most cases, essentially non-bonding. 

In everyday practice, chemists often use a minimal model of molecules that enables them to compare the geometry and vibrational frequencies with experiment to the accuracy of about 0.01 A for bond lengths and about 1 for bond angles. This model assumes that the speed of light is infinite (non-relativistic effects only), the Born-Oppenheimer approximation is valid (i.e., the molecule has a 3-D structure), the nuclei are boimd by chemical bonds and vibrate in a harmonic way, the molecule moves (translation) and rotates as a whole in space. In many cases, we can successfully predict the 3-D structure of a molecule by using a very simple took the Valence Shell Electron Pair Repulsion (VSEPR) algorithm. 

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