Why do transition metals form colored compounds?

 

Most of the compounds of transition metals are colored in the solid or in the solution states. In the case of free metal ion, all the five d-orbitals have the same energy. That is, they are degenerate, meaning energetically alike.

But when a uniformly spread negative charge or a ligand approaches towards the d-orbitals of the metal ions, then the electrostatic repulsion between d-electrons and this negative charge would result in uniform increase in the energies of d-orbitals. 

Though, under this condition the five d-orbitals of metal ions would still remain degenerate. Now consider that the negative charge or ligand surrounding d-orbitals of the metal ions gets concentrated in a number of point charges in certain specific positions.

 

The energy of the orbitals lying in the directions of the point charges would be raised to a larger extent than that of the orbitals lying in between the point charges.

 Because of the electrons in the former type of d-orbitals would experience greater repulsion from point charges than the electrons in the later type of d-orbitals.

Consequently some of the d-orbitals would get associated with a higher energy than the other d-orbitals of the metal ions.

That is, the five degenerate d-orbitals of the metal ions split up into two sets of orbitals. They are d-orbitals of lower energy sets and d-orbitals of higher energy sets.

The splitting of the five degenerate d-orbitals under the influence of negative charge or ligand is called crystal field splitting.

 

Now the color of transition metal ions arises due to excitation of electrons from the d-orbitals of lower energy to the d-orbitals of higher energy which is called d-d transition or d-d excitation.

Now light radiations corresponding to such small amounts of energy which are required for the above mentioned d-d electron transitions, are available in the visible range.

This means that the transition metals ions or transition metal compounds have the property to absorb certain radiations from the visible region and exhibit the complementary colors to that of the absorbed light.

For example, hydrated Cu (II) ion absorbs radiations corresponding to the red light and transmits radiation of wave lengths corresponding to the blue color which is the complementary color of red color.

Again, anhydrous Co (II) compounds also absorbs red light and therefore, appear blue. Similarly, the hydrated Co (II) compounds absorb radiations of wave lengths corresponding to blue color and hence appear red.

The transition metals ions which have completely filled d-orbitals or have completely empty d-orbitals are colorless.

Why Zn²⁺, Cd²⁺ and Hg²⁺ are colorless?

Zn²⁺, Cd²⁺ and Hg²⁺ are colorless. This fact can be explained on the basis of their outer electronic configuration. The outer electronic configuration of Zn²⁺, Cd²⁺ and Hg²⁺ are (n-1) d¹⁰.

That is, they have completely filled d-orbitals. Hence, in the case of Zn²⁺, Cd²⁺ and Hg²⁺, d-d transition does not occur. Therefore, no energy absorbed from visible range of light. So no color appeared.

 

Why Sc³⁺ and Ti⁴⁺ are colorless?

 

Sc³⁺ and Ti⁴⁺ are colorless. This fact can be explained on the basis of their outer electronic configuration. The outer electronic configuration of Sc³⁺ and Ti⁴⁺ are (n-1) d⁰.

That is, they have completely vacant d-orbitals. Hence, in the case of Sc³⁺ and Ti⁴⁺, d-d transition does not occur. Therefore, no energy absorbed from visible range of light. So no color appeared.

 

What is d-d transition in co-ordination chemistry?

 

In the atom or ion of a transition element, the transfer of electrons takes place from the lower energy d-orbital to the higher energy d-orbital belonging to the same sub-shell. This transfer requires a small amount of energy.

 

This transfer of electrons is usually accomplished by absorbing light rays of a specified frequency from visible light. This transfer of electrons is called d-d transition.

 

Note that the d-d transition process cannot take place if the d-sub-shell has no electrons or if the d-sub-shell is completely filled with electrons. As a result, the corresponding ions as well as compounds become colorless.

 

 

• Why do transition metals form colored compounds?
• Why Zn²⁺, Cd²⁺ and Hg²⁺ are colorless?
• Why Sc³⁺ and Ti⁴⁺ are colorless?
• What is d-d transition in co-ordination chemistry?

 

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