Valence shell electron pair repulsion lone pairs effect

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

The lone-pair effect is an orbital overlap phenomenon due to electronic s-p rearrangements and is not caused by interpair repulsion within the valence shell, in disagreement with the VSEPR model. The lone pair is usually not localized at the central atom, but occupies an antibonding MO with pronounced ligand contributions. 

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... 

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