Valence Shell Electron Pair Repulsion model Group 18 elements

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... 

In all of the above cases, the feometry is in afreement with the expectations of the valence shell electron pair repulsion (VSEPR) model of Gillespie and Nyholm [77]. The VSEPR model accounts for the molecular geometry in nearly all cases for main group elements in free molecules. Deviations are observed in solids because of the contribution from the lattice energy. For example, in SnO and red PbO, the coordination MO4E is square pyramidal, instead of seesaw expected by the VSEPR model [78,79]. 

---