An aqueous solution of $\mathrm{NiCl}_{2}$ was heated with excess sodium cyanide in presence of strong oxidizing agent to form $\left[\mathrm{Ni}(\mathrm{CN})_{6}\right]^{2-}$. The total change in number of unpaired electrons on metal centre is \_\_\_\_ .

JEE Main · 2021 · 20 Jul 2021 · Shift 2 · Coordination Compounds

An aqueous solution of

NiCl_{2}

was heated with excess sodium cyanide in presence of strong oxidizing agent to form

[Ni (CN)_{6}]^{2 -}

. The total change in number of unpaired electrons on metal centre is ____ .

Answer: 2

Solution

In aqueous nickel(II) chloride the metal is

N i^{2 +}

, a

3 d^{8}

ion, and water is a weak field ligand, so the octahedral aqua ion keeps two unpaired electrons in its

e_{g}

set. In

[Ni (CN)_{6}]^{2 -}

the six cyanide ions carry

- 6

in all against an overall charge of

- 2

, so nickel is in the

+ 4

state and is

3 d^{6}

. Cyanide is a strong field ligand, so those six electrons pair within the

t_{2 g}

set and the complex has no unpaired electron. The number of unpaired electrons on the metal falls from two to zero, a change of 2.

JEE Main · 2021 · 20 Jul 2021 · Shift 2 · Integer answer

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