Question

Amongst $\left[ Ni ( CO )_{4}\right],\left[ Ni ( CN )_{4}\right]^{2-}$ and $\left[ NiCl _{4}\right]^{2-}$

• A. $\left[ Ni ( CO )_{4}\right]$ and $\left[ NiCl _{4}\right]^{2-}$ are diamagnetic and $\left[ Ni ( CN )_{4}\right]^{2-}$ is paramagnetic
• B. $\left[ NiCl _{4}\right]^{2-}$ and $\left[ Ni ( CN )_{4}\right]^{2-}$ are diamagnetic and $\left[ Ni ( CO )_{4}\right]$ is paramagnetic
• C. $\left[ Ni ( CO )_{4}\right]$ and $\left[ Ni ( CN )_{4}\right]^{2-}$ are diamagnetic and $\left[ NiCl _{4}\right]^{2-}$ is paramagnetic
• D. $\left[ Ni ( CO )_{4}\right]$ is diamagnetic and $\left[ NiCl _{4}\right]^{2-}$ and $\left[ Ni ( CN )_{2}\right]^{2-}$ are paramagnetic.

Answer 1

Answer: (C)

$CO$ and $CN^-$ are strong field ligands and force electrons to pair up so that a diamagnetic low spin complex is resulted. However $Cl ^{-}$ is a weak field ligand and hence does not force electrons to pair up so that a paramagnetic high spin complex results.