Electron-pair repulsion theory

Valence shell electron pair repulsion theory, 1, 564 Valence tautomerism photochromic processes and, 1, 387 y-Valerolactone, o -allyl-a -2-(pyrido[2,3-6]-imidazolyl)-synthesis, 5, 637 Validamycin A as fungicide, 1, 194 Valinomycin... 

Valence shell electron pair repulsion theory, 1,32-39 effective bond length ratios, 1.34 halogenium species, 3, 312 noble gas compounds, 3,312 repulsion energy coefficient, 1, 33 Valency... 

Now that we know how to determine hybridization states, we need to know the geometry of each of the three hybridization states. One simple theory explains it all. This theory is called the valence shell electron pair repulsion theory (VSEPR). Stated simply, all orbitals containing electrons in the outermost shell (the valence shell) want to get as far apart from each other as possible. This one simple idea is all you need to predict the geometry around an atom. First, let s apply the theory to the three types of hybridized orbitals. 

Frequently, directionality is a property attributed to the covalent bond which supposedly is taken to be the cause of the resulting structures. However, as the success of the valence electron pair repulsion theory shows, there exists no need to assume any orbitals directed a priori. The concept of directed orbitals is based on calculations in which hybridization is used as a mathematical aid. The popular use of hybridization models occasionally has created the false impression that hybridization is some kind of process occurring prior to bond formation and committing stereochemistry. 

In one respect the valence shell electron-pair repulsion theory is no better (and no worse) than other theories of molecular structure. Predictions can only be made when the constitution is known, i.e. when it is already known which and how many atoms are joined... 

Very low energy differences also result for different polyhedra with higher coordination numbers, including coordination number 7. In these cases the electron pair repulsion theory no longer allows reliable predictions. 

To derive the values of the coefficients at, Ph y, and 8i so that the bond energy is maximized and the correct molecular structure results, the mutual interactions between the electrons have to be considered. This requires a great deal of computational expenditure. However, in a qualitative manner the interactions can be estimated rather well that is exactly what the valence shell electron-pair repulsion theory accomplishes. 

Redress can be obtained by the electron localization function (ELF). It decomposes the electron density spatially into regions that correspond to the notion of electron pairs, and its results are compatible with the valence shell electron-pair repulsion theory. An electron has a certain electron density p, (x, y, z) at a site x, y, z this can be calculated with quantum mechanics. Take a small, spherical volume element AV around this site. The product nY(x, y, z) = p, (x, y, z)AV corresponds to the number of electrons in this volume element. For a given number of electrons the size of the sphere AV adapts itself to the electron density. For this given number of electrons one can calculate the probability w(x, y, z) of finding a second electron with the same spin within this very volume element. According to the Pauli principle this electron must belong to another electron pair. The electron localization function is defined with the aid of this probability ... 

There are some unique structural aspects of some of the sulfur fluorides that will need to be discussed in order to understand the 19F NMR spectra. The geometry of tetracoordinate group VI compounds is predicted on the basis of Gillespie s electron-pair repulsion theory to be trigonal bipyramid, with an electron pair occupying one of the equatorial sites.2 Thus, the SF3 substituent as well as the molecule SF4 have structures as depicted in Scheme 7.12, with nonequivalent (axial and equatorial) fluorines, and thus their 19F NMR spectra consist of two 19F signals, with the fluorines being coupled if the system is scrupulously dry. 

Due to the simplicity and the ability to explain the spectroscopic and excited state properties, the MO theory in addition to easy adaptability for modern computers has gained tremendous popularity among chemists. The concept of directed valence, based on the principle of maximum overlap and valence shell electron pair repulsion theory (VSEPR), has successfully explained the molecular geometries and bonding in polyatomic molecules. 

VSEPR theory The VSEPR (valence shell electron-pair repulsion) theory says that the electron pairs around a central atom will try to get as far as possible from each other in order to minimize the repulsive forces. This theory is used to predict molecular geometry. 

Gibson model, 38 174-176 Gillespie-Nyholm valence shell electron-pair repulsion theory, 18 325 Glass-formers , 4 294 Glauber s salt, 4 17... 

The applications of valence bond theory, the valence shell electron pair repulsion theory, to the bonding and bond angles of triatomic molecules... 

Valence Shell Electron Pair Repulsion Theory... 

Valence shell electron pair repulsion theory was applied to all the examples used in the chapter. 

The molecular structures adopted by simple carbonyl complexes are generally compatible with predictions based on valence shell electron pair repulsion theory. Three representative examples from the first transition series are shown in Fig. 15.2. 

VALENCE SHELL ELECTRON PAIR REPULSION THEORY 32... 

The most useful approach in predicting the stereochemistry of coordination compounds is the Valence Shell Electron Pair Repulsion theory (VSEPR theory), which is based on the idea that the electron pairs around an atom will be arranged so as to minimize the repulsion between them. The qualitative foundations of this field were laid by Sidgwick and Powell1 in 1940 and by Gillespie... 

A recent electron diffraction study has shown that the structure is consistent with those predicted by the Valence Shell Electron Pair Repulsion Theory (137). 

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