Colour Formation in Compounds

Colour Formation in Compounds

Chemistry of Colour Formation in Compounds

Colour in compounds or ions is produced due to absorption and emission of radiation in the visible region of spectrum. Colour in general, is associated with the incomplete electron sub-shell and the ability of the compound or ion to promote an electron from one energy level to another by absorption of light of a particular wavelength.

If all the incident light (white light) falling on a substance is reflected then the substance appears white. If all the incident light is absorbed, the substance appears black. If, only a particular wavelength is absorbed from the incident light (and other are reflected) then the compound has a characteristic colour. The transmitted or reflected light has a colour complementary to the absorbed colour. e.g. when red light is eliminated from white light, the colour appears blue (i.e. blue is the combination of all the remaining colours except red). However, if radiations of all the wavelengths or colours except one are absorbed, then the colour of the substance will be the colour of the transmitted radiation .e.g. if a substance absorbs all colours except green, then it would appear green to the eyes.

The relationship between the colour of the absorbed radiation and that of the transmitted light i.e. the complementary colours is given below:

Reason of Colour of Transition Metal Compounds

Colour in transition metal compound is associated with partially filled (n – 1) d sub-shell i.e. colour is attributed by the unpaired d-electrons. In an isolated atom or ion of a transition metal, all the five d-orbitals are degenerate having same energy. In a complex ion, under the influence of combining ligands, the five d-orbitals split up into two levels of different energies namely lower energy trio called t_2g and higher energy pair called e_g and this splitting is termed as crystal field splitting (Crystal Field Theory). [The difference between the two energy levels depends upon the nature of the combining ligands, but corresponds to the energy associated the radiations in the visible region; wavelength = 380-760 nm]. In transition metal compounds, electrons are promoted to a higher energy level within the d-sub-shell by absorption of light of characteristic wavelength in the visible region and this electronic transition or shift is called d-d transition. The rest of the light transmitted is no longer white i.e. it appears coloured. The colour of the transition metal ions is due to the electronic transitions or shifts between the available d-orbitals. The colour of the ion is complementary to the colour absorbed. e.g.

1.  Cu²⁺ ions (3d⁹) or [Cu(H₂O)₆]²⁺ ions absorb red light from the visible range for promotion of 3d electrons reflecting excess of blue light and appear deep blue (which is complementary colour to red).

2. Hydrated Co²⁺ ions absorb radiation in the blue-green region, and therefore, appear red in sunlight.

The colour of an ion may be affected by altering the environments of the ions. For example; hydrated copper sulphate (CuSO₄.5H₂O) is blue while anhydrous form is white. The colour of co-ordination compounds or complexes also depends upon the nature of ligands as different ligands affect the energy levels of the d-orbitals.  For example:

	[Cu(H2O)4]2+	●●●●●	Blue
	[Cu(NH3)4]2+	●●●●●	Intense Deep Blue
	[Cu(Cl)4]2–	●●●●●	Green (Sea blue)
	[Cu(H2O)6]2+	●●●●●	Deep Blue
	[Cu(NH3)6]2+	●●●●●	Blue

A same transition metal ion has different colour in different oxidation states. This is because different oxidation states of the same transition metal ions possess different number of electrons and thus different amount of energy would be absorbed and emitted in electronic transitions with consequent manifestation of different colours of ions. Cr³⁺ ion is deep green while Cr²⁺ ion is blue.

Detailed Explanation of Colour in Terms of Crystal Field Theory

The cause of colour of transition metal compounds (complexes) can be explained on the basis of crystal field theory (CFT). According to this theory, the splitting of the five degenerate d-orbitals takes place under the influence of surrounding ligands. [In an isolated atom or ion of a transition metal, all the five d-orbitals are degenerate having same energy. In a complex ion, under the influence of the combining ligands, the five d-orbitals differ slightly in energy and become non-degenerate as a result of overlapping differently with the ligands]. The five degenerate d-orbitals are split up neighboring ligands into two levels of different energies namely lower energy trio t_2g called and higher energy pair called e_g and this splitting is termed as crystal field splitting. The energy gap between these levels is denoted as ∆○ and is comparatively small between the non-degenerate d-orbitals in a transition metal complex. [The difference between the two energy levels depends upon the nature of the combining ligands, but corresponds to the energy associated the radiations in the visible region having wavelength = 380-760 nm].

The transition metals in the elemental form or in the ionic form have one or more unpaired electrons. The excitation of an electron from a lower to a higher energy level within the d-sub-shell can be achieved by the absorption of visible light (white light) of a characteristic wavelength or colour. [This wavelength or colour of the absorbed light depends upon the energy difference between the two levels]. Rest of the light get transmitted i.e. in electronic transitions, some of the component wavelength of white light is removed, so the remaining components wavelength of the light reflected or transmitted appear coloured to the viewers and the substance looks coloured. The colour of the ion is complementary to the colour absorbed. 

Sc³⁺ ions (and Ti⁴⁺ ions) have no d-electrons and are colourless. (For this reason, it is thought to be non-transition metal).            

Zn²⁺, Cd²⁺, Hg²⁺ and Cu⁺ ions with a d¹⁰ configurations, no d-d electronic transitions is possible i.e. there is no possibility of promotion of electrons within d-sub-shell due to their completely filled sub-shell and hence these ions and their compounds are colourless (white).

Compounds of s and p-block elements are almost white. In s- and p-block elements, the d-orbitals are either missing or fully filled. There cannot be any d-d transitions (and also s-p or p-d transitions) and the energy to promote an s- or p-electron to a higher energy level is much greater and corresponds to ultraviolet region and hence the compounds of s- and p-block elements do not appear coloured in the visible region.