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Q.
Which of the following complex ions is not expected to absorb visible light?
AIPMTAIPMT 2010Coordination Compounds
Solution:
For the absorption of visible light, presence of unpaired d-electrons is the necessity.
(a) In $\left[ Ni ( CN )_{4}\right]^{2-}, Ni$ is present as $Ni ^{2+}$
$Ni ^{2+}=[ Ar ] 3 d^{8}\, 4 s^{0}$
Since, in $\left[ Ni ( CN )_{4}\right]^{2-}$, no unpaired electron is present in $d$-orbitals, no unpaired electron is presen $t$ in $d$-orbitals it does not absorb visible light.
(b) In $\left[ Cr \left( NH _{3}\right)_{6}\right]^{3+}, Cr$ is present as $Cr ^{3+}$
$Cr ^{3+}=[ Ar ] 3 d^{3} 4 s^{0}$ (Three unpaired electrons)
(c) In $\left[ Fe \left( H _{2} O \right)_{6}\right]^{3+}, Fe$ is present as $Fe ^{2+}$.
[AI] $3 d^{6} 4 s^{0}$ (Four unpaired electrons)
(d) In $\left[ Ni \left( H _{2} O \right)_{6}\right]^{2+}, Ni$ is present as $Ni ^{2+}$.
$Ni ^{2+}=[ Ar ] 3 d^{8} 4 s^{0}$ (Two paired elections)
The complex given in option (b), (c), (d) have unpaired electrons, thus absorb visible light.
