Coordination Compounds — Part 2: Bonding, colour, carbonyls and uses: visual lesson, CBSE Class 12 Chemistry

Lesson

Bonding in coordination compounds

Part 2 — why some complexes are magnetic and others not, why they are coloured, and how carbon monoxide bonds to metals.

In this lesson you will

Use valence bond theory to predict hybridisation, shape and magnetic behaviour of complexes
Explain crystal-field splitting in octahedral and tetrahedral fields, high-spin and low-spin complexes, and the spectrochemical series
Explain the colour of complexes and the limitations of the bonding theories
Describe synergic bonding in metal carbonyls and state uses of coordination compounds

Concept

Valence bond theory: hybridisation and shape

The metal provides empty hybrid orbitals that accept electron pairs from the ligands. The hybridisation fixes the shape. Strong-field ligands such as $\ce{CN-}$ , CO and $\ce{NH3}$ (for Co³⁺) can force d electrons to pair, freeing inner d orbitals.

Table of hybridisations for coordination numbers four and six with the shapes and example complexes
Coordination number	Hybridisation	Shape	Example
4	$sp^3$	Tetrahedral	$\ce{[NiCl4]^2-}$ , $\ce{[Ni(CO)4]}$
4	$dsp^2$	Square planar	$\ce{[Ni(CN)4]^2-}$
6	$d^2sp^3$ (inner orbital)	Octahedral	$\ce{[Co(NH3)6]^3+}$ , $\ce{[Fe(CN)6]^3-}$
6	$sp^3d^2$ (outer orbital)	Octahedral	$\ce{[CoF6]^3-}$ , $\ce{[FeF6]^3-}$

Worked example

Worked example: two cobalt(III) complexes

Problem. Explain why $\ce{[Co(NH3)6]^3+}$ is diamagnetic but $\ce{[CoF6]^3-}$ is paramagnetic.

Step 1. $\ce{Co^3+}$ is $3d^6$ . Ammonia acts as a strong-field ligand here: the six d electrons pair into three orbitals, leaving two 3d orbitals empty → $d^2sp^3$ , inner-orbital, no unpaired electrons: diamagnetic.

Step 2. Fluoride is weak-field: no pairing; four unpaired electrons remain; 4s, 4p and two 4d orbitals are used → $sp^3d^2$ , outer-orbital, 4 unpaired electrons: $\mu = \sqrt{24} \approx 4.90$ BM.

Table comparing the two cobalt three plus complexes: inner-orbital diamagnetic with ammonia, outer-orbital paramagnetic with fluoride
Complex	Hybridisation	Unpaired electrons	Magnetic behaviour
$\ce{[Co(NH3)6]^3+}$	$d^2sp^3$	0	Diamagnetic
$\ce{[CoF6]^3-}$	$sp^3d^2$	4	Paramagnetic (4.90 BM)

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