Lone pair repulsion, VSEPR

When lone pairs are present, several different repulsive forces exist between bonding pairs, between bonding pair and a lone pair, and between lone pairs. The VSEPR model is based on different repulsive forces for the three situations. These are shown here ... 

Like so many other molecular properties, shape is determined by the electronic structure of the bonded atoms. The approximate shape of a molecule can often be predicted by using what is called the valence-shell electron-pair repulsion (VSEPR) model. Electrons in bonds and in lone pairs can be thought of as "charge clouds" that repel one another and stay as far apart as possible, thus causing molecules to assume specific shapes. There are only two steps to remember in applying the VSEPR method ... 

An important group of cations that shows electronically distorted environments are those of the main group elements in lower oxidation states. These contain nonbonding electron pairs in their valence shells, the so-called lone pairs . Such atoms are usually found displaced from the center of their coordination sphere so as to form between 3 and 5 strong bonds and a number of weaker ones. The effect can be described using the Valence Shell Electron Pair Repulsion (VSEPR) Model [43] in which it is assumed that the cation is surrounded uniformly by between 4 and 7 electron pairs occupying valence shell orbitals. One or more of these is a lone pair... 

Xep4 (Sec. 1.11) is an unusual example of a neutral molecule that takes a square-planar structure. This structure is predicted by the valence shell electron pair repulsion (VSEPR) theory [1089] which states that two lone-pairs in the valence shell occupy the axial positions because they exert greater mutual repulsion than... 

Organic chemists find that the information obtained from MO theory, where valence electrons occupy bonding and antibonding molecular orbitals, does not always yield the needed information about the bonds in a molecule. The valence-shell electron-pair repulsion (VSEPR) model combines the Lewis concept of shared electron pairs and lone-pair electrons with the concept of atomic orbitals and adds a third principle the minimization of electron repulsion. In this model, atoms share electrons by overlapping... 

Knowledge Required (1) Valence-shell-electron-pair-repulsion (VSEPR) model for predicting molecular shape. (2) Effect of lone pairs on predicted bond angles. 

Use valence shell electron pair repulsion (VSEPR) to predict geometry about any carbon or nitrogen atom. Count the number of other atoms attached to it and add to that the number of lone pair(s) it may contain. Two = linear and sp hybridized 3 = trigonal planar and sp2 hybridized 4 = tetrahedral and sp3. Not complicated. 

THE VSEPR MODEL (SECTION 9.2) The valence-shell electron-pair repulsion (VSEPR) model rationalizes molecular geometries based on the repulsions between electron domains, which are regions about a central atom in which electrons are likely to be found. Bonding pairs of electrons, which are those involved in making bonds, and nonbonding pairs of electrons, also called lone pairs, both create electron domains aroimd an atom. According to the VSEPR model, electron domains orient themselves to minimize electrostatic repulsions that is, they remain as far apart as possible. 

The coordination number of a compound is defined as the number of attachment sites of the various ligands to the metal center. The valence-shell electron-pair repulsion (VSEPR) model does not work well for transition compounds having partially filled d-subshells. The Kepert model is sometimes used instead. As with the VSEPR model, the metal ion is assumed to be spherical with the ligands lying along the surface of the sphere. The ligands will repel one another for either electronic or steric reasons and will tend to distribute themselves around the sphere so as to avoid each other. In the Kepert model, the lone pair electrons (which are the low-lying d-electrons in the... 

Fig. 8.33. The most stable structures according to VSEPR. The structures are assigned by assuming that the lone pair-lone pair repulsion is larger than the lone pair-bonding pair repulsion, the latter still larger than the bonding pair-bonding pair repulsion.

The concept of valence-shell electron-pair repulsion (VSEPR) is presented in introductory organic chemistry as a way to predict molecular geometries. The idea behind VSEPR is that areas of electron density repel each other so that the geometry of bonds and/or lone pairs of electrons around any one atom places these areas as far apart as possible. Por four areas of electron density a tetrahedral geometry is predicted. Eor three areas of electron density a trigonal planar geometry is predicted. Two areas of electron density lead to a linear geometry. 

Molecular Geometry Molecular geometry refers to the three-dimensional arrangement of atoms in a molecule. For relatively small molecules, in which the central atom contains two to six bonds, geometries can be rehably predicted by the valence-shell electron-pair repulsion (VSEPR) model. This model is based on the assumption that chemical bonds and lone pairs tend to remain as far apart as possible to minimize repulsion. 

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