3.5 Formation Of Coloured Complexes

Question 1

Why may a complex be coloured?

Answer 1

D orbitals are split into two groups at different energies
A partially filled d sub shell is required
certain frequencies of visible light can be absorbed to promote an electron from the lower set of d orbitals to the higher set.
The light that is transmitted is the complementary colour to the light absorbed.

Question 2

Why are complexes containing Sc3+ and Ti4+ ion colourless?

Answer 2

There are no electrons in the 3d sub shell.

Question 3

Why are complexes containing Cu+ and Zn2+ ion colourless?

Answer 3

They have a full 3d sub shell.

Question 4

What can the colours of transition metal complex ions be related to?

Answer 4

The amount of splitting of the d orbitals the bigger the difference in energy between the lower and higher set of d orbitals the higher the frequency (short wavelength) of light absorbed.

Question 5

What are factors that affect the colour?

Answer 5

The identity of the metal since different metals have different electron configurations.
The oxidation state of the metal the higher charge on the ion causes greater splitting of d orbitals.
The nature of the ligand.

Question 6

How can ligands be arranged into a spectrochemical series?

Answer 6

According to how much they split d orbitals.

Question 7

What is the spectrochemical series of ligands?

Answer 7

I- < Br- < Cl- < F- < OH- < H2O < NH3 < CO = CN-
CN- causes a greater splitting than Cl- therefore a higher frequent of light will be absorbed by complexes containing CN- ions.

Question 8

How can the position of some ligands in the spectrochemical series be explained?

Answer 8

In terms of charge density the F- ion is the smallest and therefore has the highest charge density meaning ti causes the greater splitting of the d orbitals than I-