Which of the following complex has zero magnetic moment (spin only)?

• A

  $\left[\mathrm{N}\mathrm{i}{\left({\mathrm{N}\mathrm{H}}_3\right)}_6\right]{\mathrm{C}\mathrm{l}}_2$
• B

  ${\mathrm{N}\mathrm{a}}_3\left[{\mathrm{F}\mathrm{e}\mathrm{F}}_6\right]$
• C

  $\left[\mathrm{C}\mathrm{r}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6\right]{\mathrm{S}\mathrm{O}}_4$
• Correct answer
  D

  ${\mathrm{K}}_4\left[\mathrm{F}\mathrm{e}{\left(\mathrm{C}\mathrm{N}\right)}_6\right]$

Solution

Ferrocyanide ion ${\left[\mathrm{F}\mathrm{e}{\left(\mathrm{C}\mathrm{N}\right)}_6\right]}^{4-}$ is diamagnetic in nature hence ${\mathrm{K}}_4\left[\mathrm{F}\mathrm{e}{\left(\mathrm{C}\mathrm{N}\right)}_6\right]$ complex has zero magnetic moment.
$\text{Fe}$ atom ${\text{3d}}^{\text{6}}{\text{4s}}^{\text{2}}{\text{4p}}^{\text{0}}$
$\text{Fe}\text{(II)}$
${\text{d}}^{\text{2}}{\text{sp}}^{\text{3}}$ hybridisation

Hence, the correct answer is option: 'D'.