Electron pair distribution geometry

VSEPR Model valence shell electron pair repulsion model, model used to predict the geometry of molecule based on distribution of shared and unshared electron pairs distributed around central atom of a molecule... 

Fig. 1 Comparison of the experimentally determined geometries of the hydrogen-bonded complex H3N-- -HC1 and its halogen-bonded analogue H3N- C1F (both drawn to scale) with a non-bonding electron-pair (n-pair) model of NH3. Here, and in other figures, the n-pair electron distribution is drawn in the exaggerated style favoured by chemists. The key to the colour coding of atoms used in this and similar figures is also displayed...

Before discussing the AIM theory, we describe in Chapters 4 and 5 two simple models, the valence shell electron pair (VSEPR) model and the ligand close-packing (LCP) model of molecular geometry. These models are based on a simple qualitative picture of the electron distribution in a molecule, particularly as it influenced by the Pauli principle. 

If the electronegativity of the ligands X is much less than the electronegativity of the central atom A, the electrons in the valence shell of A are not well localized into pairs and therefore have a small or zero effect on the geometry. In such molecules the bonds are very ionic in the sense A X+, and the central atom A is essentially an anion with a spherical electron density distribution. In this case the VSEPR model is not valid, and the geometry of the molecule is determined by ligand-ligand repulsions. 

Figure 6.7 shows the calculated electron density distributions for the H2 and N2 molecules in their equilibrium geometry together with the standard deformation densities. There is clearly a buildup of electron density in the bonding region in both molecules. In the N2 molecule there is also an increase in the electron density in the lone pair region and a de-... 

We can interpret the trans geometry of OsO + [87], which has the d2 configuration, in a similar way. Here the first oxo-ligand acts, in the first order of perturbation theory, to induce a quadrupolar charge distribution in the valence shell, as the d-electron pair occupies either the xy or x2—y2 components in response to a ligand on the z axis the second ligand must then follow in the trans position. 

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