Appendix 3. Complexes: more on shapes and isomerism

Read Appendix 2. Introduction to complexes & ligands first

• 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 co-ordinate bonds and not the number of ligands.

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

  • Based on the complex ion shape - usually determined by the ligands themselves (as opposed to lone pair involvement).

  • Octahedral (co–ordination number 6): very common, positional geometrical (E/Z) 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 cis/trans isomerism) possible.

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

  • Three are illustrated in the diagram above, and the isomerism in three cases illustrated below.

• 

• Principal forms of isomerism:

  • E/Z isomerism (was called cis/trans geometric isomerism, but Z = cis and E = trans, so take care):

    • Examples of (1) above with monodentate ligands in a square planar complex:

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

      • 

      • You could not get isomers if it had a tetrahedral shape, you couldn't form two non-superimposable structures.

    • Examples of (2) above with unidentate ligands in an octahedral complex:

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

      • and

  • Optical isomerism: Examples of (3) above with a bidentate ligand.

    • and 

    • (iv) the optical (R/S) isomers (mirror images) 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 the electrically neutral ligand 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⁺. The diagrams would be the same as above.