Question

What colours does an aqueous solution does $ F{e^{3 + }} $ ions gives with $ SC{N^ - } $ and $ {\left[ {Fe{{\left( {CN} \right)}_6}} \right]^{4 - }} $ ions respectively
a) blue and red
b) red and blue
c) blue and green
d) green and blue

Answer 1

Hint: Transition metals are said to form a large number of complex compounds in which the metal atom gets bound to a number of anions or neutral molecules. In modern terminologies, such compounds are known as coordination compounds.

Complete answer:
One of the first chemists to work and research on coordination compounds was the swiss chemist Werner. He put forth his research in the form of postulates. Which are as follows
1. In coordination compounds metals are said to show two types of linkages (valences)-primary and secondary.
2. The primary valences are normally ionisable and are satisfied by negative ions.
3. The secondary valences are non-ionisable. These are satisfied by neutral molecules or negative ions. The secondary valence is equal to the coordination number and is fixed for a metal.
 4. The ions/groups bound by the secondary linkages to the metal have characteristic spatial arrangements corresponding to different coordination numbers.
One of the most distinguishing features of coordination compounds is their wide spectrum of colours. This can be explained by the hypothesis that some of the visible spectra are being removed from white light when it passes through the coordination compound sample, hence the light that emerges from it is no longer white. The colour of the complex is complementary to that which is absorbed. The complementary colour is the colour generated from the wavelength leftover after absorption. The crystal field theory attributes the colour of the coordination compounds to the d-d transition of the electron.
When $ F{e^{3 + }} $ ions react with $ SC{N^ - } $ we get $ Fe{(SCN)_3} $ .It is experimentally found that this compound appears as red in colour.
When $ F{e^{3 + }} $ ions react with $ {\left[ {Fe{{\left( {CN} \right)}_6}} \right]^{4 - }} $ we get $ F{e_4}{\left[ {Fe{{\left( {CN} \right)}_6}} \right]_3} $ .It is experimentally found that this compound appears as blue in colour.
Hence we conclude that that the correct option is (b)red and blue.

Note:
The coordination compounds are said to be of great importance. These compounds are widely present in the mineral plants and perform important functions in the area of analytical chemistry, metallurgy. The hardness of water can also be tested using coordination compounds.