Question

Which of the following compounds are having white ppt?
(A) ${{K}_{2}}Fe\left[ Fe{{\left( CN \right)}_{6}} \right]$
(B) ${{\left[ Fe{{\left( {{H}_{2}}O \right)}_{5}}\left( SCN \right) \right]}^{2+}}$
(C) ZnS
(D) $Zn{{\left( OH \right)}_{2}}$

Answer 1

Hint: Think about d-d transitions. d-block elements are very famous for their variety of colour properties. Think about complexes which do not give any colour and the reason behind it.

Complete step by step answer:
-In the question, we can see that all the options are consisting of d-block elements as their central atom. From the question, we can directly relate the colour property to d-d transitions shown by these elements.
-d-d transitions are transitions which take place within the d-orbitals of the elements. If unpaired electrons are present in the element then they can absorb energy from the suitable wavelength of white light from the surroundings and get excited to higher orbitals, ${{t}_{2g}}\to {{e}_{g}}$ and the terminated energy will be the complementary colour of which wavelength was absorbed initially.
-Let’s look at the questions to determine which complexes have unpaired electrons.

(A) ${{K}_{2}}Fe\left[ Fe{{\left( CN \right)}_{6}} \right]$ - Here, the complexed metal Fe is in +2 oxidation state. Cyanide being a strong ligand will induce pairing of all the unpaired d (6) electrons. So, there won’t be any d-d- transitions in this complex resulting in white colour of the complex.

(B) ${{\left[ Fe{{\left( {{H}_{2}}O \right)}_{5}}\left( SCN \right) \right]}^{2+}}$ - Here, iron is in +3 oxidation state and has five aqua ligands bonded to it which are weak field ligands. So, there are 5 unpaired electrons present in the complex which means there will be a d-d transition taking place. This complex is generally blood red in colour.

(C) ZnS – Zinc in its +2-oxidation state has zero unpaired electrons so no d-d transitions occur in this compound. So, it will be white.

(D) $Zn{{\left( OH \right)}_{2}}$ - Similarly, here also zinc exists in +2 oxidation state. So, no d-d transitions occur in this compound. So, it will be white.
Therefore, compounds having white ppt are (A) ${{K}_{2}}Fe\left[ Fe{{\left( CN \right)}_{6}} \right]$, (C) ZnS and (D) $Zn{{\left( OH \right)}_{2}}$
So, the correct answer is “OptionA,C and D”.

Note: Remember that colour of a complex arises due to electronic transitions such as d-d transitions. Also, most of the zinc complexes will be white due to lack of unpaired electrons.