Coordination Compounds for NEET: Crystal Field Theory Explained

Introduction

Coordination Compounds form one of the most scoring yet conceptual parts of Inorganic Chemistry for NEET aspirants. Among the various theories that explain bonding in these compounds, Crystal Field Theory (CFT) stands out for its explanatory power. It plays a crucial role in helping students understand essential chemical phenomena like color, magnetism, bonding strength, and stability of coordination complexes—topics that frequently appear in NEET exams.

With the increasing emphasis on application-based questions in NEET, it’s more important than ever for students to go beyond surface-level memorization and truly understand the underlying principles. Crystal Field Theory, when taught using visuals, analogies, and conceptual tricks, can become one of the most engaging and intuitive parts of Inorganic Chemistry.

At Deeksha Vedantu, we equip students with clarity-driven learning tools—like orbital splitting diagrams, tables for quick comparison, and high-yield NEET MCQs—to build a strong foundation. This blog explores CFT in depth, connecting theory with practice to help you retain better and solve faster.

NEET Weightage of Coordination Compounds

Coordination Compounds contribute approximately 2–3 questions in NEET Chemistry every year. Despite being a sub-topic in the Class 12 syllabus, it has high conceptual value and is often the basis for indirect or tricky NEET questions.

Commonly tested subtopics include:

Geometry and hybridization of complexes
IUPAC naming conventions
Coordination number and oxidation states
Magnetic behavior prediction using CFT
Electronic configuration in ligand fields
Color of complexes and absorption of visible light

These questions are usually straightforward and scoring, provided the student has practiced enough diagram-based and logical application questions.

Basics of Coordination Compounds

Before exploring CFT, it’s essential to revisit the basic structure and terminology of coordination compounds:

Central metal ion/atom: Atom that accepts electron pairs from ligands (typically a transition metal)
Ligands: Ions/molecules that donate lone pairs to the metal (Lewis bases)
Coordination number: Number of ligand atoms directly bonded to the central atom
Coordination sphere: The metal-ligand entity enclosed in square brackets $\boldsymbol{[ ]}$

Classification of Ligands

Monodentate: Donate one lone pair (e.g., $\boldsymbol{H_2O}$ , $\boldsymbol{NH_3}$ , $\boldsymbol{Cl^-}$ )
Bidentate: Donate two lone pairs (e.g., $\boldsymbol{C_2O_4^{2-}}$ , ethylenediamine)
Polydentate: Donate more than two (e.g., EDTA has six donor sites)

Knowing how to count coordination numbers and identify ligand types helps simplify hybridization and geometry problems.

What is Crystal Field Theory (CFT)?

Crystal Field Theory is an electrostatic model developed to explain the electronic structure, color, and magnetic behavior of coordination compounds. Unlike the Valence Bond Theory (VBT), which offers a more generalized view, CFT emphasizes orbital-level interaction and explains the observed physical properties more accurately.

Why CFT is Preferred Over VBT

VBT cannot explain why certain complexes are colored while others are colorless
It doesn’t account for magnetic differences in similar-looking compounds
CFT provides quantitative data regarding orbital energy splitting

Key Assumptions of CFT

Ligands are treated as point charges (or dipoles for neutral ligands)
Metal-ligand bond is purely electrostatic, not covalent
Energy of d-orbitals is affected differently depending on ligand approach
Splitting pattern depends on complex geometry (octahedral, tetrahedral, square planar)

These assumptions allow CFT to predict spin states and other properties.

Crystal Field Splitting in Octahedral Complexes

In an octahedral field, six ligands approach the central metal ion along the x, y, and z axes. This symmetric arrangement leads to a splitting of the five d-orbitals into two distinct energy levels:

$\boldsymbol{t_{2g}}$ (lower energy): dxy, dxz, dyz
$\boldsymbol{e_g}$ (higher energy): $\boldsymbol{d_{x^2 - y^2}}$ , $\boldsymbol{d_{z^2}}$

The energy difference between these two sets is termed the Crystal Field Splitting Energy ( $\boldsymbol{\Delta_0}$ ).

Factors That Affect $\boldsymbol{\Delta_0}$ :

Nature of ligand (position in spectrochemical series)
Oxidation state of metal ion (higher charge = more splitting)
Size of metal ion (smaller ions have stronger interaction)
Period of metal (3d, 4d, 5d metals show different $\boldsymbol{\Delta_0}$ values)

Greater $\boldsymbol{\Delta_0}$ means a greater likelihood of electron pairing in the lower orbitals, impacting magnetic behavior and color.

Strong Field vs Weak Field Ligands

Spectrochemical Series (From Weak to Strong):

$\boldsymbol{I^- < Br^- < SCN^- < Cl^- < F^- < OH^- < H_2O < NH_3 < en < NO_2^- < CN^- < CO}$

Behavior:

Weak field ligands (e.g., $\boldsymbol{Cl^-}$ , $\boldsymbol{F^-}$ , $\boldsymbol{OH^-}$ ): Cause small $\boldsymbol{\Delta_0}$ → electrons prefer to remain unpaired → high spin complexes
Strong field ligands (e.g., $\boldsymbol{CN^-}$ , $\boldsymbol{CO}$ ): Cause large $\boldsymbol{\Delta_0}$ → pairing of electrons → low spin complexes

Pairing Energy ( $\boldsymbol{P}$ ):

If $\boldsymbol{\Delta_0 > P}$ → electrons pair up → low spin
If $\boldsymbol{\Delta_0 < P}$ → electrons remain unpaired → high spin

Tetrahedral and Square Planar Field Splitting

Tetrahedral Splitting:

Ligands approach between the axes → minimal overlap with orbitals on the axes
$\boldsymbol{\Delta_t}$ (splitting) is smaller: $\boldsymbol{\Delta_t \approx \frac{4}{9} \Delta_0}$
Usually results in high spin complexes due to low $\boldsymbol{\Delta_t}$

Square Planar Splitting:

Typically seen in $\boldsymbol{d^8}$ metal ions ( $\boldsymbol{Ni^{2+}}$ , $\boldsymbol{Pd^{2+}}$ )
Strong field ligands lead to larger splitting
Frequently results in low spin, diamagnetic behavior
Common in $\boldsymbol{[Ni(CN)_4]^{2-}}$ , $\boldsymbol{[PtCl_4]^{2-}}$

Applications of CFT

Crystal Field Theory helps predict the following:

1. Color of Complexes

Caused by d–d transitions, where electrons absorb light to move from t₂g to eₙ
Energy gap $\boldsymbol{(\Delta_0)}$ determines the wavelength absorbed → observed color is complementary

2. Magnetic Behavior

Based on unpaired electrons:
- Paramagnetic: Has unpaired electrons
- Diamagnetic: All electrons paired

3. Thermodynamic Stability

Strong field ligands stabilize complexes better
Less electronic repulsion, more efficient bonding

Examples:

$\boldsymbol{[\text{Fe}(\text{H}_2\text{O})_6]^{3+}}$ → weak field → $\boldsymbol{5}$ unpaired electrons → paramagnetic
$\boldsymbol{[\text{Fe}(\text{CN})_6]^{3-}}$ → strong field → $\boldsymbol{1}$ unpaired electron → less magnetic

Complexes vs Spin vs Magnetic Behavior

Complex	Ligand Type	Field Strength	Spin Type	Magnetic Nature
$\boldsymbol{[\text{Fe}(\text{H}_2\text{O})_6]^{2+}}$	$\boldsymbol{\text{H}_2\text{O}}$	Weak	High	Paramagnetic
$\boldsymbol{[\text{Fe}(\text{CN})_6]^{3-}}$	$\boldsymbol{\text{CN}^-}$	Strong	Low	Slightly Paramagnetic
$\boldsymbol{[\text{CoF}_6]^{3-}}$	$\boldsymbol{\text{F}^-}$	Weak	High	Paramagnetic
$\boldsymbol{[\text{Ni}(\text{CN})_4]^{2-}}$	$\boldsymbol{\text{CN}^-}$	Strong	Low	Diamagnetic
$\boldsymbol{[\text{Cu}(\text{NH}_3)_4]^{2+}}$	$\boldsymbol{\text{NH}_3}$	Intermediate	High	Paramagnetic
$\boldsymbol{[\text{PtCl}_4]^{2-}}$	$\boldsymbol{\text{Cl}^-}$	Weak	High	Paramagnetic

Practice Questions

Identify the hybridization and geometry of $\boldsymbol{[\text{Fe}(\text{CN})_6]^{3-}}$ .
Predict the number of unpaired electrons in $\boldsymbol{[\text{NiCl}_4]^{2-}}$ .
Determine whether $\boldsymbol{[\text{Co}(\text{NH}_3)_6]^{3+}}$ is paramagnetic or diamagnetic
Compare spin states and magnetic properties of $\boldsymbol{[\text{Fe}(\text{H}_2\text{O})_6]^{2+}}$ vs $\boldsymbol{[\text{Fe}(\text{CN})_6]^{3-}}$
Match the following ligands to their field strengths:
- A. $\boldsymbol{\text{CN}^-}$ → i. Strong field
- B. $\boldsymbol{\text{Cl}^-}$ → ii. Weak field
- C. $\boldsymbol{\text{NH}_3}$ → iii. Intermediate field

Common Mistakes to Avoid

Assuming all complexes are octahedral: While octahedral complexes are common, NEET questions may involve tetrahedral or square planar geometries, especially with $\boldsymbol{d^8}$ configurations like $\boldsymbol{[Ni(CN)_4]^{2-}}$ .
Confusing field strength with charge: A ligand’s field strength is not determined by its charge alone. For instance, both $\boldsymbol{Cl^-}$ and $\boldsymbol{CN^-}$ are negatively charged, but they lie at opposite ends of the spectrochemical series.
Ignoring the spectrochemical series: Many students fail to use the spectrochemical series when predicting spin states or magnetic properties. Always refer to the ligand’s position to assess whether the complex is high or low spin.
Forgetting to consider the oxidation state in $\boldsymbol{\Delta_0}$ prediction: Higher oxidation states increase the metal-ligand attraction, thus increasing $\boldsymbol{\Delta_0}$ . For example, $\boldsymbol{Fe^{3+}}$ leads to more splitting than $\boldsymbol{Fe^{2+}}$ .
Mixing up geometry in tetrahedral vs square planar: Tetrahedral geometry is common for $\boldsymbol{d^5}$ and $\boldsymbol{d^{10}}$ configurations, while square planar is favored by low-spin $\boldsymbol{d^8}$ complexes. Misidentifying these leads to errors in hybridization and spin state determination.

FAQs

Do I need to memorize the spectrochemical series?

Yes. Especially focus on common NEET ligands like $\boldsymbol{\text{CN}^-}$ , $\boldsymbol{\text{NH}_3}$ , $\boldsymbol{\text{H}_2\text{O}}$ , $\boldsymbol{\text{Cl}^-}$ .

How do I decide between high spin and low spin?

Compare $\boldsymbol{\Delta_0}$ with pairing energy ( $\boldsymbol{P}$ ). Use the spectrochemical series as a guide.

Are the colors of complexes frequently asked in NEET?

Yes. Questions on color arise due to d–d transitions and $\boldsymbol{\Delta_0}$ variation.

Can CFT be used to predict reactivity?

Yes, indirectly. Complex stability based on ligand strength can influence reactivity.

Does CFT apply to s- and p-block elements?

No. CFT is only applicable to transition metals with partially filled d-orbitals.

What’s the easiest way to draw orbital diagrams?

Start with $\boldsymbol{t_{2g}}$ and $\boldsymbol{e_g}$ levels, assign electrons according to $\boldsymbol{\text{Hund's rule}}$ , then calculate spin.

Conclusion

Crystal Field Theory provides a structured and logical approach to understanding the bonding and properties of coordination compounds. From predicting magnetic behavior to understanding the origin of color, it offers invaluable insights.

At Deeksha Vedantu, we ensure that CFT and other Inorganic Chemistry topics are taught using visual methods, concept maps, and question-based learning. With consistent practice, you’ll be able to recognize patterns, eliminate wrong options quickly, and master NEET-level questions on coordination compounds.