JEE Main 2025 — Coordination Compounds Question with Solution

Given low-spin complexes:

$$K_3\left[Co{(N{O}_2)}_6\right]-$$ Yellow colour

$$K_4\left[Fe{(CN)}_6\right]-$$ light yellow colour

$$K_3\left[Fe{(CN)}_6\right]-$$ bright red colour not yellow.

$$C{u}_2\left[Fe{(CN)}_6\right]-$$ reddish brown color not yellow

$$Z{n}_2\left[Fe{(N{O}_2)}_6\right]-$$ bluish-white colour not yellow

$$K_3\left[Co{(N{O}_2)}_6\right]$$

Oxidation state of $$Co:+3$$

Configuration:

$$Co{d}^7{s}^2$$

$$C{o}^{3+}{d}^6$$

$$3(+1)+x+6(-1)=0$$

$$3+x-6=0$$

$$x=6-3=+3$$

For low-spin d${}^6$ configuration, no unpaired electrons are present :

spin-only magnetic moment, $$\mu =\sqrt{n(n+2)}$$

$$n=0$$, $$\mu =0$$

This complex shows yellow colour (bright yellow)

$$K_4\left[Fe{(CN)}_6\right]$$

Oxidation state of $$Fe:+2$$

$$4(+1)+x+6(-1)=0$$

$$4+x-6=0$$

$$x=6-4=+2$$

Configuration:

$$Fe{d}^6{s}^2$$

$$F{e}^{+2}{d}^6$$

$$\mu =0$$

For low-spin d${}^6$ configuration, no unpaired electrons are present :

This complex shows yellow colour (light yellow)

$$K_3\left[Fe{(CN)}_6\right]$$

Oxidation state of $$Fe:+3$$

$$3(+1)+x+6(-1)=0$$

$$3+x-6=0$$

$$x=6-3=+3$$

Configuration:

$$Fe{d}^6{s}^2$$

$$F{e}^{+2}{d}^5$$

low spin d${}^5$

One unpaired electron is present.

Spin-only magnetic moment, $$\mu =\sqrt{n(n+2)}$$

n (number of unpaired electrons) = 1

So, $$\mu =\sqrt{n(n+2)}$$

$$=\sqrt{1\times 3}$$

$$=\sqrt{3}$$

= 1.732 BM

Colour of the complex is not yellow.

$$C{u}_2\left[Fe{(CN)}_6\right],C{u}^{2+}$$ and $${\left[Fe{(CN)}_6\right]}^{4-}$$

$$x+6(-1)=-4$$

$$x=-4+6$$

$$=+2$$

Oxidation state of $$Fe:+2$$

Configuration:

$$Fe{d}^6{s}^2$$

$$F{e}^{2+}{d}^6$$

For low-spin d${}^6$ configuration, no unpaired electrons are present

$$n=0,\mu =\sqrt{n(n+2)}=0$$

Colour of the complex is not yellow.

$$Z{n}_2\left[Fe{(CN)}_6\right]$$

$${\left[Fe{(CN)}_6\right]}^{4-}$$ and $$Z{n}^{2+}$$

$$x+6(-1)=-4$$

$$x=-4+6=+2$$

Oxidation state of $$Fe:+2$$

Configuration:

$$Fe{d}^6{s}^2$$

$$F{e}^{2+}{d}^6$$

For low-spin d${}^6$ configuration, no unpaired electrons are present.

$$n=0,\mu =\sqrt{n(n+2)}=0$$

Colour of the complex is not yellow

The complexes having yellow colour are $$K_3\left[Co{(N{O}_2)}_6\right]$$ and $$K_4\left[Fe{(CN)}_6\right]$$

The sum of the spin-only magnetic moment values of complexes having yellow colour is 0 (zero).