Question

Which of the following complex is/are low spin complex(es)?

• A. $\left[Co\right(N{H}_3{)}_6{]}^{3+}$
• B. $[Co{F}_6{]}^{3-}$
• C. $\left[Fe\right(H_{2}O{)}_6{]}^{2+}$
• D. $\left[Fe\right(CN{)}_6{]}^{4-}$

Answer 1

Answer: (A), (D)

$\left[Fe\right(CN{)}_6{]}^{4-}$

Presence of strong field ligands pair up the valence electrons of the metals. Since $C{N}^{-}$ and $N{H}_3$ are strong field ligands (from spectrochemical series) so $\left[Fe\right(CN{)}_6{]}^{4-}$ and $\left[Co\right(N{H}_3{)}_6{]}^{3+}$ are low spin complexes. Conversely, $F^{-}$ and $H_{2}O$ are weak field ligands and thus $[Co{F}_6{]}^{3-}$ and $\left[Fe\right(H_{2}O{)}_6{]}^{2+}$ are high spin complexes.