Bonding in Coordination Compounds — Part 2: Crystal field theory, stability and importance: visual lesson, ISC Class 12 Chemistry

Lesson

Splitting the d orbitals

Part 2 — crystal field theory, colour, stability constants and why complexes matter.

In this lesson you will

Draw d-orbital splitting diagrams for octahedral and tetrahedral complexes
Explain colour and magnetism of complexes using crystal field splitting and ligand strength
Compare the stabilities of complexes from their stability constants
Give examples of complexes in qualitative analysis, metal extraction and living systems

Diagram

Octahedral and tetrahedral splitting

Schematic energy levels; delta-t is about four-ninths of delta-o.

Ligands are point charges (anions) or point dipoles (neutral molecules); bonding is treated as purely electrostatic. In an octahedral field, $d_{x^2-y^2}$ and $d_{z^2}$ point at the ligands and are raised ( $e_g$ ); $d_{xy}$ , $d_{yz}$ , $d_{xz}$ lie between the axes and are lowered ( $t_{2g}$ ). The gap is $\Delta_o$ : $t_{2g}$ lies $0.4\Delta_o$ below and $e_g$ $0.6\Delta_o$ above the average. In a tetrahedral field the order reverses ( $e$ below $t_2$ ) and $\Delta_t \approx \tfrac{4}{9}\Delta_o$ .

Concept

Spectrochemical series; high and low spin

Ligands ranked by the splitting they produce (weak → strong):

\ce{I-} < \ce{Br-} < \ce{SCN-} < \ce{Cl-} < \ce{F-} < \ce{OH-} < \ce{C2O4^2-} < \ce{H2O} < \ce{NCS-} < \ce{NH3} < \text{en} < \ce{CN-} < \ce{CO}

Table of high-spin and low-spin filling
Case	Filling for d⁴–d⁷	Result
Δo < P (weak field)	e_g filled before pairing in t₂g	High spin
Δo > P (strong field)	t₂g filled and paired first	Low spin
d⁵ weak field	t₂g³e_g²	5 unpaired
d⁵ strong field	t₂g⁵e_g⁰	1 unpaired

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