Question

If color of ${[Ni{({H_2}O)_2}{(en)_2}]^{2 + }}$ is blue-purple then what would be the color of ${[Ni{(en)_3}]^{2 + }}$?
$(a)$ Green
$(b)$ Pale blue
$(c)$ Deep blue
$(d)$ Violet

Answer 1

Hint: The en in the compound ${[Ni{(en)_3}]^{2 + }}$ is ethylenediamine and the compound name is Nickel $(II)$ tris (ethylenediamine). The colors of the compound is due to the weak or strong ligands, ligands are classified as weak or strong according to the spectrochemical series:
(weak) $I < Br < Cl < SCN < F < OH < o{x^{2 - }} < ONO < {H_2}O < NCS < EDTA < N{H_3} < en < N{O_2} < CN$ (Strong)

Complete answer:
$Ni(II)$ has eight electrons in its d-orbitals. Both water and en (ethylene diamine) are strong ligands, meaning their presence will raise the energy of \[{t_{2g}}\] orbitals sufficiently above that of the \[{e_g}\] orbitals, fully leaving only two electrons to occupy the \[{t_{2g}}\] orbitals singly. En is somewhat stronger ligands than water, so the energy difference between lower and upper sets of orbitals will be greater for the nickel-en complex. The color arises from the change in energy between the \[{t_{2g}}\] and \[{e_g}\] orbitals. ‘en’ is a strong ligand, which pushes the orbitals further from one another as well as displaces the ${H_2}O$ ligands from the metal center.
When six ligands approach a metal center to form a octahedral complex, the five degenerate d-orbitals split into three lower-energy degenerate \[{t_{2g}}\] orbitals and two higher-energy degenerate \[{e_g}\] orbitals. The distance of the splitting between the and orbitals is dictated by the strength of the ligands according to the spectrochemical series.
The distance that electrons have to move from the lower \[{t_{2g}}\] state to the higher \[{e_g}\] state in the metal center dictates the energy of electromagnetic radiation that the complex absorbs. If that energy is in the visible region $(400 - 700nm{,_{}}1.77eV - 3.1eV)$, the complex absorbs high-energy light (i.e. blue or violet) and appears red-yellow in color. Complexes with ligands that are between strong and weak on the spectrochemical series, like ammonia, can adopt either a weak or strong field geometry.

Note:
\[{d_{xy}},{d_{xz}},{d_{yz}}\] are collectively called the \[{t_{2g}}\] orbitals, whereas \[d{z^2}\]and \[d{x^2} - d{y^2}\] are collectively called the \[{e_g}\] orbitals. The Jahn-teller effect causes the greater number of possible transitions and thus a greater number of peaks. The key is that the ${[Ni{({H_2}O)_6}]^{2 + }}$ complex transmit more visible light in the green part of the spectrum, whereas the ${[Ni{(en)_3}]^{2 + }}$ complex transmit more visible light in the blue part of the spectrum.