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JEE Main levelSingle correct optionDifficulty 2 of 3df129

Which of the following nickel(II) or cobalt(II) complexes is diamagnetic?

A $[Ni (H X_{2} O) X_{6}] X^{2 +}$
B $[NiCl X_{4}] X^{2 -}$
C $[Ni (CN) X_{4}] X^{2 -}$
D $[CoCl X_{4}] X^{2 -}$

Answer: C

In

[Ni (CN) X_{4}] X^{2 -}

, strong-field

CN X^{-}

pairs the

d^{8}

electrons into a square-planar low-spin

d s p^{2}

complex with

n = 0

.

[NiCl X_{4}] X^{2 -}

is tetrahedral with 2 unpaired electrons,

[CoCl X_{4}] X^{2 -}

has 3, and octahedral

[Ni (H X_{2} O) X_{6}] X^{2 +}

has 2.

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