|The University of Sheffield|
|Department of Chemistry
How to predict the geometry
A Lewis single bond is always regarded as a σ bond. Recall that Lewis dot structures allow double and triple bonds between some elements. For VSEPR calculation purposes, a double bond is always regarded as a σ+π bond interaction, while a triple bond is always treated as a σ+2π bond interaction. This simplification is permissible despite the MO treatments of Chapter 3 showing that the nature of some multiple bonds is somewhat different. However, double and triple bonds still only occupy one coordination vertex. The shape of a molecule is therefore dictated by the σ bond framework. Each vertex of the coordination polyhedron is necessarily occupied by a σ bond (possibly supported by π bonds) or a lone pair of σ symmetry, otherwise the vertex would not exist. The determination of a molecule's geometry therefore reduces to a calculation of the number of electrons contained in the σ orbitals. Electrons in π bonds must therefore be recognized and care taken not to confuse them with σ bond electrons. The following procedure, which is not unique, works well.
- Draw a Lewis structure based upon the atom of interest. In the following examples it will become clear that some simplifications are possible.
- Determine the number of valence electrons on the central atom.
- Write down a modified Lewis structure by assigning all atoms or groups bonded to the atom in question as singly, doubly, or triply bonded. It is not necessary to write in lone pairs, the calculation will determine these. Assign all singly bonded groups as shared electron pair bond types, with the exceptions of dative bound groups discussed below. Always regard groups such as =O and =S as double bonded to the central atom with the double bond consisting of a σ and a π bond, both of which are electron pair bonds. Regard groups such as ≡N and ≡P as always triple bonded, with the triple bond consisting of a σ and two π bonds, all of which are shared electron pair bonds.
- Groups for which the octet rule is satisfied, such as NH3, are regarded as lone pair electron donors to an atom such as B in H3N→BF3. That is, there is still a σ bond, but both electrons originate from the attached group (N). These are dative bonded groups. Conversely if N is under consideration as the central atom in H3N→BF3, then the BF3 group is bonded through a single bond to the central atom (N), but here both electrons of the bond originate from the central atom. In this case, there is no net contribution of electrons from the attached group to the central atom.
- The coordination geometry is dictated by the σ framework only. It is now necessary to discount those central atom electrons that are involved in π bonds. Since each π bond is a shared electron pair with one electron arising from each atom, subtract one electron for each π bond involving the central atom.
- Any overall charge on the molecule is always assigned to the central atom, even if later reflection requires that it may be best to assign it elsewhere. Thus, a negative charge constitutes an additional electron for the central atom, while a positive charge requires subtraction of one electron from the central atom electron count.
- Divide the total number of electrons associated with the σ framework by 2 to give the number of σ electron pairs. Assign a coordination geometry and perhaps distinguish between isomers.
Classification of formal bonding type to central atom
|F, Cl, Br, I, OH, SH, NH2, Me, Ph, H, SiMe3
||=O, =S, =NH, =PH, =CH2
||≡N, ≡P, ≡CH
A statement of rules always seems more imposing than their actual operation. It is therefore appropriate to examine some sample calculations.