MC test: Coloured complexes

Multiple choice test on 13.2 Coloured complexes

Use the following 'quiz' to test your knowledge and understanding of this sub-topic. You will need access to a periodic table (Section 6 of the IB data booklet).

If you get an answer wrong, read through the explanation carefully to learn from your mistakes.

Which factors affect the colour of a transition metal complex?

I. the oxidation state of the metal

II. the shape of the complex

III. the nature of the ligands

All three of them affect the colour as all three affect the amount the d orbitals are split.

 

Which factor explains why [Cu(NH3)4(H2O)2]2+ is a different colour to [Cu(H2O)6]2+?

The energy of light absorbed changes when water is substituted for ammonia as the splitting of the d orbitals is different.

 

Which factor determines the order of the spectrochemical series?

The spectrochemical series lists the ligands in order of their ability to split the d orbitals in octahedral complexes.

 

Which factor explains why the splitting in the d orbitals in [Fe(H2O)6)]2+ is different to [Fe(H2O)6)]3+

Because there is one less d electron in [Fe(H2O)6]3+ than in [Fe(H2O)6]2+ the attraction between the nucleus and the d sub level is different and this affects the splitting.

 

Which will form a colourless aqueous solution?

I. [Ti(H2O)6)]3+

II. [Sc(H2O)6)]3+

III. [Zn(H2O)6)]2+

[Ti(H2O)6)]3+ contains one d electron so a transition can occur when the d orbitals are split. In [Sc(H2O)6]3+ there are no d electrons and in [Zn(H2O)6]2+ the d sub-level is completely full so no transitions can occur.

 

Which describes the location of the d orbitals relative to the x, y and z axes in octahedral transition metal ion complexes?

The dz2 and dx2-y2 orbitals lie along the axes and move to higher energy when the ligands approach and the dxy, dyz and dzx orbitals lie between the axes and move to lower energy when the ligands approach.

 

Which accounts for the fact that solid anhydrous copper(II) sulfate is white?

The Cu2+ ion contains nine d electrons but as there are no ligands the d sub level is not split.

 

Which factor accounts for the pink colour of [Mn(H2O)6]2+?

It is true that the wavelength of the light emitted when an electron moves from a higher to a lower energy level in the complex corresponds to the colour pink but the pink colour is not caused by the emitted light. It is caused by the transmitted light which is the complementary colour to the light absorbed.

 

Cyanide ions are above water molecules in the spectrochemical series, i.e. H2O < CN−. What changes will occur when cyanide ions replace all six water molecules in Fe(H2O)6]2+?

The coordination number will remain at 6 as there are six monodentate ions surrounding the transition metal ion. The spectrochemical series places ligands in order of their ability to split the d orbitals so cyanide ions will increase the splitting which will decrease the wavelength of the light absorbed when a transition occurs from a lower to a higher level .

 

The diagram shows the visible spectrum of a transition metal ion, [M(H2O)6]3+ and the colour wheel from Section 17 of the IB data booklet.

Which colour best describes an aqueous solution of this complex ion?

The maximum absorption occurs between about 460 and 600 nm. This is a very broad region and yellow, orange and green are all absorbed as their wavelengths lie in this region. Both red and violet are colours that will be transmitted but the only complementary colour to this region in the four listed answers is violet. (The spectrum is in fact that of [Ti(H2O)6]3+ which is purple).

 

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