Chapter 20, Problem 73QRT

(a)

Interpretation Introduction

Interpretation:

Formula for the octahedral complex ion has to be written that has ${\text{Fe}}^{\text{3+}}$ with ${\text{NCS}}^{-}$ ligands and with ${\text{NO}}_{\text{2}}{}^{-}$ ligands.

Explanation of Solution

Octahedral complex contains six ligands inside the coordination sphere.  Central metal atom is given as ${\text{Fe}}^{\text{3+}}$.  The two ligands are ${\text{NCS}}^{-}$ and ${\text{NO}}_{\text{2}}{}^{-}$.  Octahedral complex ion that is formed from ${\text{Fe}}^{\text{3+}}$ and ${\text{NCS}}^{-}$ is given as ${[\text{Fe}{(\text{NCS})}_6]}^{3-}$.  Octahedral complex ion that is formed from ${\text{Fe}}^{\text{3+}}$ and ${\text{NO}}_{\text{2}}{}^{-}$ is given as ${[\text{Fe}{({\text{NO}}_{\text{2}})}_6]}^{3-}$.

(b)

Interpretation Introduction

Interpretation:

Complex that is formed from ${\text{Fe}}^{\text{3+}}$ ion, ${\text{NCS}}^{-}$ ligand and with ${\text{NO}}_{\text{2}}{}^{-}$ ligand are high-spin or low-spin has to be predicted.

Explanation of Solution

Octahedral complex ion that is formed from ${\text{Fe}}^{\text{3+}}$ and ${\text{NCS}}^{-}$ is given as ${[\text{Fe}{(\text{NCS})}_6]}^{3-}$.  ${\text{NCS}}^{-}$ ion is a weak field ligand.  Hence, the complex ion formed will be high spin.

Octahedral complex ion that is formed from ${\text{Fe}}^{\text{3+}}$ and ${\text{NO}}_{\text{2}}{}^{-}$ is given as ${[\text{Fe}{({\text{NO}}_{\text{2}})}_6]}^{3-}$.  ${\text{NO}}_{\text{2}}{}^{-}$ is a strong field ligand.  Hence, the complex ion formed will be low spin.

(c)

Interpretation Introduction

Interpretation:

Complex that has more paramagnetism has to be chosen.

Explanation of Solution

Octahedral complex ion that is formed from ${\text{Fe}}^{\text{3+}}$ and ${\text{NCS}}^{-}$ is given as ${[\text{Fe}{(\text{NCS})}_6]}^{3-}$.  ${\text{NCS}}^{-}$ ion is a weak field ligand.  Hence, the complex ion formed will be high spin.

Octahedral complex ion that is formed from ${\text{Fe}}^{\text{3+}}$ and ${\text{NO}}_{\text{2}}{}^{-}$ is given as ${[\text{Fe}{({\text{NO}}_{\text{2}})}_6]}^{3-}$.  ${\text{NO}}_{\text{2}}{}^{-}$ is a strong field ligand.  Hence, the complex ion formed will be low spin.

The crystal field model for high-spin and low-spin complex of ${\text{Fe}}^{\text{3+}}$ can be given as,

Number of unpaired electrons:

The ligand ${\text{NCS}}^{-}$ is a weak field ligand and therefore the complex ${[{\text{Fe(NCS)}}_{\text{6}}]}^{3-}$ forms high spin complex.  There are five unpaired electrons.  The ligand ${\text{NO}}_{\text{2}}{}^{-}$ is a strong field ligand and therefore the complex ${[{\text{Fe(NO}}_{\text{2}}{\text{)}}_{\text{6}}]}^{3-}$ forms low spin complex.  There is one unpaired electron.

Paramagnetism depends upon the number of unpaired electrons.  Therefore, the complex ion ${[{\text{Fe(NCS)}}_{\text{6}}]}^{3-}$ will be more paramagnetic.

(c)

Interpretation Introduction

Interpretation:

Crystal field diagram for the $d$ electrons in complex ${[\text{Fe}{(\text{NCS})}_6]}^{3-}$ and ${[\text{Fe}{({\text{NO}}_{\text{2}})}_6]}^{3-}$ has to be drawn.

Explanation of Solution

Octahedral complex ion that is formed from ${\text{Fe}}^{\text{3+}}$ and ${\text{NCS}}^{-}$ is given as ${[\text{Fe}{(\text{NCS})}_6]}^{3-}$.  ${\text{NCS}}^{-}$ ion is a weak field ligand.  Hence, the complex ion formed will be high spin.

Octahedral complex ion that is formed from ${\text{Fe}}^{\text{3+}}$ and ${\text{NO}}_{\text{2}}{}^{-}$ is given as ${[\text{Fe}{({\text{NO}}_{\text{2}})}_6]}^{3-}$.  ${\text{NO}}_{\text{2}}{}^{-}$ is a strong field ligand.  Hence, the complex ion formed will be low spin.

The central metal ion is ${\text{Fe}}^{\text{3+}}$.  ${\text{Fe}}^{\text{3+}}$ ion is said to contain five $3d$ electrons.  For high-spin complex, the number of unpaired electrons will be more and for low-spin complex, the number of unpaired electrons will be less.  The splitting of orbitals depends upon the ligand.  If the ligand is weak field ligand, then the crystal field splitting energy (${\Delta }_{\text{o}}$) will be smaller resulting in unpaired electrons.  If the ligand is a strong field ligand, then the crystal field splitting energy will be larger resulting in pairing of electrons.  The crystal field model for high-spin and low-spin complex of ${\text{Fe}}^{\text{3+}}$ can be given as,