Why are Transition Metal Complexes Coloured?

In this video we want to explain why transition metal complexes and compounds are coloured.

Before that let us consider the criteria for transition metal complexes to be coloured in the first place.

colour of transition metal complexes criteria for colour

1. d-d orbital splitting

When a metal ion forms a complex with ligands, the surrounding ligands interact with the d-orbitals within the d-subshell to different extent.

This results in a d-d orbital splitting where some of the d-orbitals have higher energy level while others have lower energy level.

The energy difference between the 2 energy levels happen to correspond to the energy level of a particular colour in the visible light region.

2. partially filled d-subshell

When the d-subshell is partially filled (d1 to d9), the transition or promotion of an electron from a lower energy state to a higher energy state is possible.

When there are no electrons (d0), no d-d transition is possible since there are no electrons.

When the d-subshell is fully filled (d10), there is no available space in the higher energy level for d-d transition to take place.

Let's have an example using Mn+

colour of transition metal complexes d d transition diagram

When Mn+ is in the gaseous state, the d-subshell is partially filled but there is no d-d orbital splitting.

Therefore no d-d transition is possible so Mn+(g) has no colour.

When Mn+ dissolves in water to form Mn+(aq) or a metal complex with water ligands, a d-d orbital splitting occurs with 2 orbitals at a higher energy level and 3 orbitals at the lower energy level.

Let's say for example the energy gap corresponds to the energy level of orange light.

So when white light is shone at the complex, electrons can absorb orange light and get promoted from the lower d-level to the higher d-level.

Since orange light is absorbed by the complex, we cannot detect this orange light and will see the complementary colour as the colour of the complex.

The explanation for the colour of transition metal complexes is summarised below:

colour of transition metal complexes explanation

We can also use the colour wheel to determine the colour of the complex if we know what colour corresponds to the energy gap.

colour of transition metal complexes colour wheel

In our example the energy gap corresponds to orange colour.

The complementary colour is simply the colour in the opposite sector of the colour wheel, blue in this case.

This means if the complex absorbs orange light, we will see this complex as blue.

If the complex absorbs red light, we will see it as green.

If the complex absorbs violet light, we will see it as yellow.

For the detailed step-by-step discussion on how to explain the colour of transition metal complexes, check out this video!

Topic: Transition Elements, Inorganic Chemistry, A Level Chemistry, Singapore

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