Valence-shell electron-pair repulsion model lone pairs

In the valence-shell electron-pair repulsion model, or VSEPR model, we focus attention on the central atom of a molecule, such as the B atom in BF3 or the C atom in C02. We then imagine that all the electrons involved in bonds to the central atom and the electrons of lone pairs belonging to that atom lie on the surface of an invisible sphere that surrounds it (Fig. 3.3). These bonding electrons and lone pairs are regions of high electron concentration, and they repel one another. To minimize their repulsions, these regions move as far apart as possible on the surface of the sphere. Once we have identified the most distant ... 

The observed variations in the N—C bond distances and < C3—N—C angles are not in agreement with the valence-shell electron-pair repulsion model. This predicts that partial removal of the lone-pair electrons on the nitrogen atom would lead to a shortening of the N—C bonds. It would further predict that < C3—N—C should be greater than tetrahedral in the free donor and decrease on complex formation. 

VSEPR The valence shell electron pair repulsion model, originally introduced by Nyholm and Gillespie (with antecedents from Sidgwick and Powell), which assumes that molecular geometry associated with a central atom is determined by the number of groups (single bonds, double bonds, triple bonds, or lone pairs) surrounding that atom. 

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 ... 

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. 

While valence shell electron pair repulsion and steric repulsion between ligands may act in a synergetic manner on the acceptor molecule, they are expected to be opposed on the donor molecule the VSEPR model predicts that partial removal of a lone pair on the donor atom should lead to a reduction of D-C bond distances and an increase of the CDC valence angles. Steric repulsion between the methyl groups bonded to D and the entire acceptor... 

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... 

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. 

Molecular Shapes The shapes of molecules can be predicted by combining Lewis theory with valence shell electron pair repulsion (VSEPR) theory. In tiiis model, electron groups— lone pairs, single bonds, double bonds, and triple bonds—aroxmd the central atom repel one another and determine the geometry of the molecule. 

According to the valence-shell electron-pair repulsion (VSEPR) model, electron pairs in the valence shell of an atom repel one another. An electron domain is a lone pair or a bond. Any bond (single, double, or triple) constitutes one electron domain. 

Once computed on a 3D grid from a given ab initio wave function, the ELF function can be partitioned into an intuitive chemical scheme [30], Indeed, core regions, denoted C(X), can be determined for any atom, as well as valence regions associated to lone pairs, denoted V(X), and to chemical bonds (V(X,Y)). These ELF regions, the so-called basins (denoted 2), match closely the domains of Gillespie s VSEPR (Valence Shell Electron Pair Repulsion) model. Details about the ELF function and its applications can be found in a recent review paper [31],... 

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... 

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