Crystal Field Theory (CFT) & Colour of Complexes - Free MCQ Practice Test

In [Fe(H₂O)₆]³⁺, iron has 3d⁵ configuration. Since, H₂O is weak ligand the distribution is .
Hence, CFSE = - 3 x 0.4 + 2 x 0.6 = 0.0

Among all the coordination aforesaid in complex [Co(NH₃)₅Cl]²⁺ unpaired electrons are present with the central element cobalt so it required less energy for excitation. Thus, the wavlength of light absorbed will be the highest.

CFSE for [CoCI₄]^2- will be 
i.e.  = 8000cm^-1

Crystal field splitting energy depends on strength of ligand. Strong ligands have more value of CFSE (Δ_o).
Hence, Br^- < F^- < NCS < CN^-

The correct answer is Option A.
Electronic configuration of Cu²⁺  ion in [Cu(NH₃)₄]²⁺.
Cu²⁺ ion =[Ar]3d⁹4s⁰.
∴Cu²⁺ ion has one unpaired electron.
Magnetic moment of [Cu(NH₃)₄]²⁺ (μ) = BM
where, n = no. of unpaired electrons

Whereas Ni²⁺ in [Ni(CN)₄]²⁻ , Ti⁴⁺ in TiCl₄ and Co²⁺ ion [COCl₆]⁴⁻ has 2,0 and 3 unpaired electrons respectively.

Ligand field strength refers to the ability of a ligand to influence the energy levels of metal d-orbitals in coordination complexes. Ligands vary in their field strength, which affects the stability and properties of the complex.

In terms of ligand field strength, ligands can be ordered based on their ability to split the d-orbitals:

• Weak field ligands: These ligands cause a small splitting of the d-orbitals.
• Strong field ligands: These ligands create a larger splitting of the d-orbitals.

The general order of ligand field strength is as follows:

• Water (H₂O): A weak field ligand.
• Chloride (Cl^-): Slightly stronger than water.
• Ammonia (NH₃): A strong field ligand.
• Carbon monoxide (CO): The strongest field ligand among these.

This order indicates that as we move from water to carbon monoxide, the ligand field strength increases. Understanding this order helps in predicting the electronic structure and properties of coordination complexes.

For high spin d⁶, CFSE = - 4 x 0.4 + 2 x 0.6 = -0.4
For low spin d⁶, CFSE= - 6 x 0.4 = -2.4

Correct Answer :- d

Explanation : Bonding Energy :

[CoCI₄]²      -2084.67

[CoBr₄]²      -3139.39

[Co(NCS)₄]^2-  - 8559.48

Correct order is : III > II > I

Increasing order of ligand field strength
l^- < Br^- < SCN^- < CI^- < S^2- < F^- < OH^-< H₂O < NCS^- < edta^4- < NH₃ < en < CN^- < CO

Stronger ligand causes greater splitting between two energy levels.

The energy of light absorbed is more and the wavelength of light absorbed is less for the stronger ligand.

Hence order of wavelength absorption will be reverse of the order of the strength of ligands.

Hence A is correct.

Ti³⁺ has 3d¹ configuration and Cu²⁺ has 3d⁹ configuration.

If Δ_o < P high spin complexes are favoured.

Geometry of complex, charge on metal ion, nature of ligands and series of d orbital are the factors on which CFSE value depends.

Cr (III) ion have 3d³ configuration.
Cr (III) ion

Flence, it will form always d²sp³ hybridisation and show colour due to the presence of 3 unpaired electrons.

The total crystal field stabilisation energy is given by CFSE_(octahedral) = are the number of electron occupying the t_2g and e_g orbitatls respectively.
For d⁷ in strong field,

CFSE = -0.4 x 6 + 0.6 x 1 = -2.4 + 0.6 = -1.8Δ_o 

In [Mn (H₂O)₆]²⁺, manganese have d⁵ configuration. H₂O is a weak field ligand. Hence, it will form high spin complex.


In [Mn(CN)₆]^4-, manganese have d⁵ configuration. CN^- ion is a strong field ligand. It will form low spin complex by pairing of electron in t_2g orbitals.

(i) → (r), (ii) → (s), (iii) → (q), (iv) → (p)

Matching Column I with Column II:

1. [Ti(H₂O)₆]³⁺ (i)

   • Weak field (Δ₀<P), d1 → t_2g¹e_g⁰ → q (−0.4Δ₀​)

2. [Fe(F)₆]³⁻ (ii)

   • Weak field (Δ₀<P), d5 → t_2g³e_g²​ → p (given config.), s (−0.0Δ₀​)

3. [Fe(CN)₆]⁴⁻ (iii)

   • Strong field (Δ₀>P), d₆ → t_2g⁶e_g⁰ → r (given config.), t (Δ₀>P)

4. [Zn(H₂O)₆]²⁺ (iv)

   • d₁₀ → No CFSE → s (−0.0Δ_0​)

Correct Option:

a) i → q, ii → p & s, iii → r & t, iv → s

In d⁶ low spin octahedral complex.

Hence, zero number of unpaired electrons.

NCS^- edta^4- , and PPh3 are strong field ligand than H₂O.

d⁷ (weak octahedral field)

Hence, 3 unpaired electrons.

The pairing starts with six electrons in the tetrahedral complexes which have weak field ligand.

In [Co(NH₃)₆]³⁺ Co, have d⁶ configuration and NH₃ is strong field ligand.

CFSE_(OCt) = -0.4 x 6 + 0.6 x 0 = -2.4Δ_o 

C) The number of unpaired electrons determines the magnetic properties of a complex.

Solution:
Unpaired electrons generate magnetic moments, making the substance paramagnetic. The more unpaired electrons present, the stronger the paramagnetism. Paired electrons do not contribute to magnetism, and unpaired electrons can be in any orbital (s, p, d, or f)

B) The splitting of d-orbitals and absorption of certain wavelengths of light

Solution:
Crystal Field Theory explains that the colour arises because ligands cause the d-orbitals of the metal ion to split into different energy levels. When visible light hits the complex, certain wavelengths are absorbed to promote electrons between these split d-levels, and the remaining transmitted or reflected light gives the complex its characteristic colour.