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Question: Which of the following is colourless? \(\begin{aligned} & a){{[Zn{{({{H}_{2}}O)}_{6}}]}^{2+}} ...

Which of the following is colourless?
a)[Zn(H2O)6]2+ b)[V(H2O)6]2+ c)[Mn(H2O)6]3+ d)[Fe(H2O)6]2+ \begin{aligned} & a){{[Zn{{({{H}_{2}}O)}_{6}}]}^{2+}} \\\ & b){{[V{{({{H}_{2}}O)}_{6}}]}^{2+}} \\\ & c){{[Mn{{({{H}_{2}}O)}_{6}}]}^{3+}} \\\ & d){{[Fe{{({{H}_{2}}O)}_{6}}]}^{2+}} \\\ \end{aligned}

Explanation

Solution

Hint: The reason why transition metal complexes in particular are colourful is because of the presence of unpaired electrons in their unfilled or either half-filled d orbitals. Therefore, your first course of action needs to be to try to find the only transition metal complex from the given options that bucks this trend.

Complete step-by-Step Solution:

Let us first analyse why transition metal complexes are usually more colourful than others.
According to the Crystal field theory which explains the splitting of the d orbital, the d orbital to a higher and lower orbital on the basis of energy. Now we know that the electrons of the transition metal can "jump". Note that light is absorbed for electrons to "jump", but these electrons will fall eventually back again to its ground state, releasing light of specific intensity and wavelength. We perceive this phenomenon as colours.
Since the electron can't transition if an orbital is already full, therefore no such “jump” occurs in the event of a fully filled d-orbital. Therefore, an element with a fully filled d-orbital will form colourless complexes.With this phenomenon and the criterion which explains it in mind, let us now check each of the given elements’ electronic configuration to check if any of them have a fully filled d-orbital.
Of all the given elements, since Zinc possesses a fully filled d-orbital with 10 electrons, therefore no splitting of light occurs making all of its complexes appear colourless.

Therefore, per our analysis, the answer to this question is (a)

Note: It is extremely important to know the electronic configuration of all transition state metals for you to be able to answer this question and it is imperative that you remember the electron configurations of all elements at least until atomic number 30. A profound knowledge of Crystal Field Theory also wouldn’t go amiss when it comes to this particular question’s solution.