Chemguide: Support for CIE A level Chemistry

Learning outcomes 12.2(a) and 12.2(b)

These statements are an introduction to transition metal complexes.

Before you go on, you should find and read the statements in your copy of the syllabus.

I am taking these statements together because they are all part of the same topic. It is impossible to do 12.2(a) before you have done 12.2(b).


There is far more about complex ions on Chemguide than you will need for CIE purposes. So I am going to ask you to read some Chemguide material as background, and then I will pull all the specific information that you need together on this page.

First read the page introducing complex metal ions.

Don't try to remember all of this - there is more there than you really need. What you need to do is to understand what is going on.

For CIE purposes, the most important things to know in this introductory bit is:

  • A ligand is a molecule or ion with a lone pair of electrons, capable of forming a dative covalent (co-ordinate covalent) bond to a metal atom or ion.

  • A complex is a molecule or ion containing a central metal atom or ion surrounded by one or more ligands.

  • Ligands can be monodentate, bidentate or polydentate. "poly" in this context just means anything more than 2. The Chemguide page you have just looked at gives examples of quadridentate and hexadentate ligands - these both count as polydentate ligands. I would be inclined to concentrate on the monodentate and bidentate ligands.

  • The co-ordination number of a complex ion counts the number of co-ordinate bonds being formed by the metal ion at its centre.

    It does not count the number of ligands. If you had three bidentate ligands around a central ion, there would be a total of 6 co-ordinate bonds being formed with the three ligands.

Complexes don't have to involve a transition metal. You will find an example of an aluminium complex ion on that page. It is more common with transition metals, though.

Now read the page about the shapes of complex ions.

You don't need the later part about stereoisomerism at the moment. This is dealt with in Section 12.4.

The page gives you examples of octahedral, tetrahedral and square planar complexes, but not a linear one - which the syllabus also wants.

The simplest of these is the diamminesilver(I) ion, Ag(NH3)2+. The silver ion is in the middle with the two ammonia molecules joined on at 180°. They are joined using lone pairs on the two nitrogen atoms bonding to empty orbitals on the silver ion - dative covalent bonding again.

Then read the page about naming complex metal ions.

The names look scary, but are actually less complicated than the names of organic compounds. Again, make sure that you understand this page, so that if you come across a name, you aren't frightened by it.

The specific complexes mentioned by the syllabus

You will meet these again in statement 12.2(c), but I am just bringing them together here for tidiness.

ammonia solution (excess)[Cu(NH3)4(H2O)2]2+[Co(NH3)6]2+6octahedral
ammonia solution (few drops)Cu(H2O)4(OH)2Co(H2O)4(OH)26octahedral
hydroxide ionsCu(H2O)4(OH)2Co(H2O)4(OH)26octahedral
chloride ions[CuCl4]2-[CoCl4]2-4tetrahedral

We will look at these in some detail in the next statement. Notice for now how similar they are. The only essential difference is in the number of water molecules which are replaced by ammonia molecules if you use an excess of ammonia.

Predicting the formula and charge of a given complex

This is for statement 12.2(b)(iv). Two examples:

Example 1

Work out the formula and the charge of a 6-co-ordinated complex between Cr3+ ions and ammonia molecules.

Ammonia molecules are monodentate ligands - they only form one co-ordinate bond to the central ion. The complex will have 6 zero-charged ammonia molecules, attached to a 3+ chromium ion. The overall charge will be +3.


Example 2

Work out the formula and the charge of a 6-co-ordinated complex between Fe3+ ions and ethanedioate ions, (C2O4)2-. Ethanedioate ions are bidentate.

If the ion is 6-co-ordinated, and the ethanedioate ions each form two bonds to the central ion (they are bidentate), then you only need 3 ethanedioate ions.

The total charges will be 3+ for the ion and 6- for the three ethanedioate ion ligands - a net charge of 3-.


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© Jim Clark 2011 (last modified May 2014)