The complex [Ni(CN)4]

The complex [Ni(CN)4]
The complex [Ni(CN)4]2- is diamagnetic but [NiCl4]2- is paramagnetic (both d8), with two unpaired electrons. Similarly, [Fe(CN)6]3- has only one unpaired electron, but [Fe(OH2)6]3+ has 5. Explain these experimental observations using:

a) the Valencia Bond Theory.

b) the Crystalline Field Theory.

The correct answer and explanation is :

Explanation of Magnetic Properties Using VBT and CFT

The complexes [Ni(CN)4]²⁻ and [NiCl4]²⁻ exhibit different magnetic properties despite both having d⁸ electronic configurations. Similarly, [Fe(CN)6]³⁻ and [Fe(OH2)6]³⁺ have different numbers of unpaired electrons despite both containing Fe³⁺ (d⁵).

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(a) Valence Bond Theory (VBT) Explanation

VBT explains the magnetic properties based on hybridization and electron pairing in the metal’s orbitals.

1. [Ni(CN)4]²⁻:

• Nickel is in the +2 oxidation state, meaning it has a d⁸ configuration.
• The strong field ligand CN⁻ forces the low-spin configuration.
• The Ni²⁺ ion undergoes dsp² hybridization, resulting in a square planar structure.
• All electrons pair up, making the complex diamagnetic.

1. [NiCl4]²⁻:

• Cl⁻ is a weak field ligand, leading to a high-spin configuration.
• The Ni²⁺ ion undergoes sp³ hybridization, resulting in a tetrahedral structure.
• Two unpaired electrons remain, making it paramagnetic.

1. [Fe(CN)6]³⁻:

• Fe³⁺ has a d⁵ configuration.
• CN⁻ is a strong field ligand, causing a low-spin state.
• Electrons pair up, leaving only one unpaired electron (t₂g⁵ in an octahedral field).

1. [Fe(OH2)6]³⁺:

• H₂O is a weak field ligand, leading to a high-spin configuration.
• The Fe³⁺ ion retains five unpaired electrons.

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(b) Crystal Field Theory (CFT) Explanation

CFT explains magnetism by considering the splitting of d-orbitals due to ligand interactions.

1. [Ni(CN)4]²⁻:

• Square planar structure: The strong field CN⁻ ligand causes a large d-orbital splitting.
• Electrons pair in the lower energy levels, leading to a low-spin, diamagnetic complex.

1. [NiCl4]²⁻:

• Tetrahedral structure: The weak field Cl⁻ ligand results in smaller d-orbital splitting.
• High-spin state occurs, leaving two unpaired electrons, making it paramagnetic.

1. [Fe(CN)6]³⁻:

• Octahedral field: Strong field CN⁻ ligand causes a large splitting.
• A low-spin configuration (t₂g⁵ eg⁰) forms, leaving only one unpaired electron.

1. [Fe(OH2)6]³⁺:

• Octahedral field: Weak field H₂O ligand results in a small splitting.
• A high-spin configuration (t₂g³ eg²) remains, keeping five unpaired electrons.

Conclusion

• Strong field ligands (CN⁻) induce low-spin states, reducing the number of unpaired electrons.
• Weak field ligands (Cl⁻, H₂O) lead to high-spin states, increasing unpaired electrons.
• Square planar (dsp²) hybridization results in diamagnetic behavior, while tetrahedral (sp³) hybridization allows unpaired electrons, leading to paramagnetic behavior.

Diagram for CFT Splitting in Different Complexes