Q. The correct order for the wavelength of absorption in the visible region is
A) $\left\lbrack Ni\left( NO_{2} \right)_{6} \right\rbrack^{4 -} < \left\lbrack Ni\left( NH_{3} \right)_{6} \right\rbrack^{2 +} < \left\lbrack Ni\left( H_{2}O \right)_{6} \right\rbrack^{2 +}$
B) $\left\lbrack Ni\left( NH_{3} \right)_{6} \right\rbrack^{2 +} < \left\lbrack Ni\left( H_{2}O \right)_{6} \right\rbrack^{2 +} < \left\lbrack Ni\left( {NO}_{2} \right)_{6} \right\rbrack^{4 -}$
C) $\left\lbrack Ni\left( H_{2}O \right)_{6} \right\rbrack^{2 +} < \left\lbrack Ni\left( NH_{3} \right)_{6} \right\rbrack^{2 +} < \left\lbrack Ni\left( {NO}_{2} \right)_{6} \right\rbrack^{4 -}$
D) $\left\lbrack Ni\left( NO_{2} \right)_{6} \right\rbrack^{4 -} < \left\lbrack Ni\left( H_{2}O \right)_{6} \right\rbrack^{2 +} < \left\lbrack Ni\left( {NH}_{3} \right)_{6} \right\rbrack^{2 +}$
Explanation
(a)
The absorption of energy of the observation of colour in a complex transition compounds depends on the charge of the metal ion and the nature of the ligands attached. The same metal ion with different ligands shows different absorption depending upon the type of ligand. The presence of weak field ligands make the central metal ion to absorb low energies $ie$, of higher wavelength. The field strength of ligands can be obtained from spectrochemical series, $ie$,
(weak field)${\ I}^{-} < Br^{-} < S^{2 -} < {Cl}^{-} < NO_{3}^{-} < F^{-} < OH^{-}$
$< H_{2}O < NH_{3} < NO_{2} < CN^{-} < CO\ ($strong field)
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.