Crystal Field Theory, d-Orbital Splitting and Colour Class 12 Chemistry | Coordination Compounds

Understand it

In an octahedral field, ligands approach along the axes, so the dz2 and dx2-y2 orbitals experience greater repulsion and form the higher-energy eg set, while dxy, dxz, and dyz form the lower-energy t2g set.
The energy gap is called crystal field splitting energy, written as Delta_o.
In a tetrahedral field, the splitting pattern is reversed and the gap Delta_t is smaller than Delta_o.
Electron arrangement depends on splitting energy and pairing energy, producing high-spin or low-spin complexes.
Colour often arises when electrons absorb visible light and undergo d-d transitions between split d levels.

Example

[Ti(H2O)6]3+ is coloured because Ti³⁺ has a d1 configuration. The d electron can absorb light and move from lower t2g to higher eg level in the octahedral field, so the transmitted or reflected light appears coloured.

Rule to remember

Octahedral splitting: t2g lower and eg higher, separated by Delta_o.

Tetrahedral splitting: e lower and t2 higher, separated by Delta_t.

Approximate relation: Delta_t is about 4/9 of Delta_o for similar metal and ligands.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

Explain whether [Ti(H2O)6]3+ can show colour.

1H₂O is neutral, so Ti is +3.
2Titanium has atomic number 22, and Ti³⁺ is d1.
3In an octahedral field, the one d electron occupies t2g and can absorb energy to move to eg.
4This d-d transition makes the complex coloured.

Method you can reuse

1Calculate oxidation state.
2Write the d configuration of the metal ion.
3Identify the geometry as octahedral or tetrahedral.
4Draw the correct splitting order.
5Fill electrons according to ligand field strength and pairing energy.
6Use the arrangement to explain colour or magnetism.

Visual tip

A d-orbital splitting diagram is strongly recommended, showing octahedral t2g and eg levels, tetrahedral e and t2 levels, and electron promotion for colour.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

In an octahedral crystal field, which set of d orbitals has higher energy?

Think first, then show answer

The eg set, containing dz2 and dx2-y2 orbitals, has higher energy in an octahedral field.

Common wrong answer

A common wrong answer is reversing the octahedral levels by placing eg lower than t2g, which leads to incorrect electron filling and wrong magnetic predictions.

Don't mix this up

Students often write that all coordination compounds are coloured. Complexes with d0 or d10 metal ions generally do not show colour due to d-d transitions because there is no suitable d-electron transition.

Exam tip

For colour and magnetism questions, find the metal oxidation state and d configuration first. Then connect d-electron availability, ligand field strength, and splitting pattern to the property asked.

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

How CBSE asks it

Questions ask students to draw splitting diagrams, compare octahedral and tetrahedral splitting, explain colour, predict high-spin or low-spin behaviour, or relate unpaired electrons to magnetic properties.

1
Enough for 1 mark
Crystal Field Theory explains coordination compounds by considering electrostatic interaction between metal d orbitals and ligands, causing the five d orbitals to split into sets of different energy in a ligand field.
2
Add for 2 marks
Example: [Ti(H2O)6]3+ is coloured because Ti³⁺ has a d1 configuration. The d electron can absorb light and move from lower t2g to higher eg level in the octahedral field, so the transmitted or reflected light appears coloured.
3
Add for 3 marks
In an octahedral field, ligands approach along the axes, so the dz2 and dx2-y2 orbitals experience greater repulsion and form the higher-energy eg set, while dxy, dxz, and dyz form the lower-energy t2g set.

Use the concept graph around this idea instead of revising it in isolation.

Werner's Theory, Coordination Sphere and Basic Terms

Werner's theory states that a metal in a coordination compound shows primary valence, usually ionisable and related to oxidation state, and secondary valence, non-ionisable and equal to the coordination number. The metal and directly attached ligands form the coordination sphere written inside square brackets.

IUPAC Naming of Coordination Compounds

IUPAC nomenclature of coordination compounds follows a fixed order: name the cation before the anion, name ligands alphabetically before the metal within the coordination entity, and write the oxidation state of the metal in Roman numerals.

Structural and Stereoisomerism in Coordination Compounds

Isomerism in coordination compounds occurs when compounds have the same overall composition but differ in the arrangement of ligands, ions, donor atoms, or spatial positions around the metal.

Valence Bond Theory, Hybridisation and Geometry

Valence Bond Theory explains bonding in coordination compounds by assuming that the central metal ion uses suitable hybrid orbitals to accept electron pairs donated by ligands, producing definite geometries such as octahedral, square planar, or tetrahedral.

Previous: Valence Bond Theory, Hybridisation and Geometry Back to full mind map

Crystal Field Theory, d-Orbital Splitting and Colour

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Crystal Field Theory, d-Orbital Splitting and Colour

Understand it

See it worked out

Check yourself

Write it in the exam

Related concepts

Practice this concept with focused MCQs

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