Which of these statements about ${\left[\mathrm{C}\mathrm{o}{\left(\mathrm{C}\mathrm{N}\right)}_6\right]}^{3-}$ is true?

• Correct answer
  A

  ${\left[\mathrm{C}\mathrm{o}{\left(\mathrm{C}\mathrm{N}\right)}_6\right]}^{3-}$ has no unpaired electrons and will be in a low-spin configuration.
• B

  ${\left[\mathrm{C}\mathrm{o}{\left(\mathrm{C}\mathrm{N}\right)}_6\right]}^{3-}$ has four unpaired electrons and will be in a low-spin configuration.
• C

  ${\left[\mathrm{C}\mathrm{o}{\left(\mathrm{C}\mathrm{N}\right)}_6\right]}^{3-}$ has four unpaired electrons and will be in a high-spin configuration.
• D

  ${\left[\mathrm{C}\mathrm{o}{\left(\mathrm{C}\mathrm{N}\right)}_6\right]}^{3-}$ has no unpaired electrons and will be in a high-spin configuration.

Solution

Due to strong Ligand field of $\mathrm{C}{\mathrm{N}}^{-}$ ions pairing of electrons takes place in inner 3d orbital it is a low spin complex with no unpaired electrons.

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