Concept explainers
(a)
Interpretation:
Formula for the octahedral complex ion has to be written that has Fe3+ with NCS− ligands and with NO2− ligands.
(a)

Explanation of Solution
Octahedral complex contains six ligands inside the coordination sphere. Central metal atom is given as Fe3+. The two ligands are NCS− and NO2−. Octahedral complex ion that is formed from Fe3+ and NCS− is given as [Fe(NCS)6]3−. Octahedral complex ion that is formed from Fe3+ and NO2− is given as [Fe(NO2)6]3−.
(b)
Interpretation:
Complex that is formed from Fe3+ ion, NCS− ligand and with NO2− ligand are high-spin or low-spin has to be predicted.
(b)

Explanation of Solution
Octahedral complex ion that is formed from Fe3+ and NCS− is given as [Fe(NCS)6]3−. NCS− ion is a weak field ligand. Hence, the complex ion formed will be high spin.
Octahedral complex ion that is formed from Fe3+ and NO2− is given as [Fe(NO2)6]3−. NO2− is a strong field ligand. Hence, the complex ion formed will be low spin.
(c)
Interpretation:
Complex that has more paramagnetism has to be chosen.
(c)

Explanation of Solution
Octahedral complex ion that is formed from Fe3+ and NCS− is given as [Fe(NCS)6]3−. NCS− ion is a weak field ligand. Hence, the complex ion formed will be high spin.
Octahedral complex ion that is formed from Fe3+ and NO2− is given as [Fe(NO2)6]3−. NO2− 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 Fe3+ can be given as,
Number of unpaired electrons:
The ligand NCS− is a weak field ligand and therefore the complex [Fe(NCS)6]3− forms high spin complex. There are five unpaired electrons. The ligand NO2− is a strong field ligand and therefore the complex [Fe(NO2)6]3− forms low spin complex. There is one unpaired electron.
Paramagnetism depends upon the number of unpaired electrons. Therefore, the complex ion [Fe(NCS)6]3− will be more paramagnetic.
(c)
Interpretation:
Crystal field diagram for the d electrons in complex [Fe(NCS)6]3− and [Fe(NO2)6]3− has to be drawn.
(c)

Explanation of Solution
Octahedral complex ion that is formed from Fe3+ and NCS− is given as [Fe(NCS)6]3−. NCS− ion is a weak field ligand. Hence, the complex ion formed will be high spin.
Octahedral complex ion that is formed from Fe3+ and NO2− is given as [Fe(NO2)6]3−. NO2− is a strong field ligand. Hence, the complex ion formed will be low spin.
The central metal ion is Fe3+. Fe3+ 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 (Δo) 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 Fe3+ can be given as,
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