Appendix 3. Complexes: more on shapes and isomerism

• Valence shell electron pair repulsion theory (VSEPR) can quite often be used to predict the shape of a complex, which is usually one of four shapes (at least at pre-university level):

• 

• Principal shapes:

  • Examples of complexes have been described in Appendix 2

  • For isomerism analysis it crucial to the think of the orientation of the bonds and not the number of ligands.

  • Isomerism can occur because of the directional nature of the dataive covalent bonds emanating from the central metal ion.

  • Octahedral (co-ordination number 6): very common, positional (E/Z) geometrical and optical (R/S) isomerism possible.

  • Tetrahedral (co-ordination number 4): with more bulky ligands like chloride ion, optical isomerism possible.

  • Square planar (co-ordination number 4): E/Z isomerism (geometrical/geometric isomerism) possible.

  • Linear (co-ordination number 2): relatively rare and can't form isomers

• 

• Principal forms of isomerism:

  • E/Z isomerism (was called geometrical isomerism):

    • Examples of (1) above:

      • Z and E isomers (cis and trans) of [Pt(NH₃)₂Cl₂]  shown below

      • 

    • Examples of (2) above:

      • Z and E isomers (cis and trans) of [Cr(NH₃)₄Cl₂]⁺  or  [Cr(H₂O)₄Cl₂]⁺  (shown below)

      • and

  • Optical isomerism: Examples of (3) above.

    • and 

    • (iv) the optical (R/S) isomers of [Co(H₂NCH₂CH₂NH₂)₃]²⁺ are represented above.

    • H₂NCH₂CH₂NH₂, ethane-1,2-diamine (ethylenediamine), is often represented in shorthand by en,

      • and the complex simply written as [Co(en)₃]²⁺.

      • This complex has mirror image forms i.e. enantiomers of optical isomers.

        • This optical isomerism can be illustrated thus

        •  where L-L represents H₂NCH₂CH₂NH₂

        • The ligand bonds via the lone pairs of electrons on the nitrogen which are donated to form the metal-ligand dative covalent bonds.

        • The chromium(III) ion forms a similar complex exhibiting R/S isomerism.

          • [Cr(en)₃]³⁺.