The colour of [Ni(NO_2)_6]^4- complex is ______.

The correct answer is Option (1) → Brown-yellow 

According to the principles of Crystal Field Theory (CFT) in Coordination Chemistry, the color of a complex is determined by the absorption of light during $d-d$ electronic transitions.

1. Oxidation State and Configuration

• In the complex $[Ni(NO_{2})_{6}]^{4-}$, the central metal ion is Nickel ($Ni$).

• The oxidation state of Nickel is $+2$, which gives it a $d^{8}$ electronic configuration.

2. Role of the Ligand ($NO_{2}^{-}$)

• The nitrite ion ($NO_{2}^{-}$) is a strong field ligand in the spectrochemical series.

• When these ligands approach the $Ni^{2+}$ ion, they cause a large splitting of the $d$-orbitals ($\Delta_{o}$ is high).

3. Light Absorption and Complementary Colors

The color we see is always the complementary color of the light absorbed by the complex:

• Because of the large energy gap caused by the $NO_{2}^{-}$ ligands, the complex absorbs high-energy light in the Blue and Violet regions of the visible spectrum.

• Based on the color wheel, the complement of blue/violet is Yellow/Brown.

• Therefore, the observed color of $[Ni(NO_{2})_{6}]^{4-}$ is Brown-yellow.

$[Ni(H_{2}O)_{6}]^{2+}$Weak  Field   Green

$[Ni(NH_{3})_{6}]^{2+}$Mid-Strong Field   Blue/Purple

$[Ni(NO_{2})_{6}]^{4-}$Strong Field   Brown-yellow