Transition metals: Complex ions and colour

Edexcel A level Chemistry (2017)

Topic 15A: Principles of transition metal chemistry

Here are the learning objectives relating to the colour of complex ions:

15/7. To know that transition metals form coloured ions in solution.

15/8. To understand that the colour of aqueous ions, and other complex ions, results from the splitting of the energy levels of the d-orbitals by ligands.

15/9. To understand why there is a lack of colour in some aqueous ions and other complex ions.

15/10. To understand that colour changes in transition metal ions may arise as a result of changes in:

i) oxidation number,
ii) ligand,
iii) coordination number.

Transition metal complex ions and the origin of colour.

One of the distinctive features of transition metal complex ions is that they are usually coloured.

Here are some typical complex ions and their colours:

Name	Formula	Colour	Coordination number	Shape
Tetraammine copper (II)	[Cu(NH3)4]2+	deep blue	4	square planar
Tetrachloro cuprate(II)	[CuCl4]2—	yellow green	4	tetrahedral
Hexaaqua cobalt(II)	[Co(H2O)]2+	red	6	octahedral

Question: What process causes the colour of these ions?

The complex ions are coloured because they absorb light and we see the light that is not absorbed but transmitted. 

Here is a table that shows you the colour and wavelengths of light absorbed and transmitted by complex ions.

The wavelength (nm) of the transmitted color of the solution.	The transmitted color of the solution.	The complementary color of the solution.
400–435	violet	yellowish–green
435-480	blue	yellow
480–490	greenish–blue	orange
490–500	bluish–green	red
500–560	green	purple
560–580	yellowish–green	violet
580–595	yellow	blue
595–610	orange	greenish–blue
610–750	red	bluish–green

The process at the atomic level involves the excitation of electrons in the d orbitals of the complex ion.

It is possible to calculate the energy gap between these d orbitals if we know the wavelength (or frequency) of the light absorbed. 

The table below shows the details of the light absorbed from the visible region to give the colour of transition metal ions.

Colour Absorbed	Wavelength λ (nm)	Frequency ν (1014 Hz)	Energy (kJ.mol—1)	Energy ΔE (10—19J)
Infrared	1000	3.00	120	1.993
Red	700	4.28	171	2.839
Orange	620	4.84	193	3.205
Yellow	580	5.17	206	3.421
Green	530	5.66	226	3.753
Blue	470	6.38	254	4.218
Violet	420	7.14	285	4.732
Ultra violet	300	10.0	400	6.642

Remember the colour absorbed is complementary to the colour observed.  

So for example a purple coloured ion like manganate(VII) MnO₄^— is actually absorbing green light and ΔE is approx. 3.7  ×  10^—19 J.

If there are no d electrons then the ion is colourless and similarly if the d subshells are full there is no chance of a d—d transition and so these ions are also colourless.  

Examples would be the Scandium(III), Sc³⁺ ion, the Zinc(II), Zn²⁺ ion and the copper(I), Cu⁺ ion.

The absorption of light by a complex ion can also be used to calculate the concentration of this ion in solution.

My next post will discuss an experimental approach to this problem.