Ligand Substitution and Isomerism in Complexes

Ligand Substitution and Isomerism in Complexes

Recap: Complex Ions

A complex ion consists of a central transition metal ion surrounded by ligands — molecules or ions that donate a lone pair of electrons to the metal, forming a dative (coordinate) bond.

The coordination number is the total number of dative bonds to the central metal ion.

Common Ligands

Ligand	Formula	Type	Denticity
Water	H₂O	Neutral	Monodentate
Ammonia	NH₃	Neutral	Monodentate
Chloride	Cl⁻	Anionic	Monodentate
Cyanide	CN⁻	Anionic	Monodentate
Hydroxide	OH⁻	Anionic	Monodentate
Ethane-1,2-diamine (en)	H₂NCH₂CH₂NH₂	Neutral	Bidentate
Ethanedioate (oxalate)	C₂O₄²⁻	Anionic	Bidentate
EDTA⁴⁻	(OOCCH₂)₂NCH₂CH₂N(CH₂COO)₂⁴⁻	Anionic	Hexadentate

Monodentate ligands donate through one lone pair.

Bidentate ligands donate through two lone pairs (they have two donor atoms).

Hexadentate EDTA has six donor atoms (four O and two N), so one EDTA molecule can occupy all six coordination sites.

Ligand Substitution Reactions

In a ligand substitution (or ligand exchange), one type of ligand replaces another in a complex. The incoming ligand displaces the outgoing one from the coordination sphere.

Example 1: Water replaced by ammonia (no change in coordination number)

[Cu(H₂O)₆]²⁺ + 4NH₃ → [Cu(NH₃)₄(H₂O)₂]²⁺ + 4H₂O

• Pale blue → deep blue (royal blue)
• Coordination number remains 6 (octahedral, but only 4 of 6 waters are replaced because the reaction does not go to completion with excess NH₃ alone)
• H₂O and NH₃ are similar in size, so the shape is retained

Example 2: Water replaced by chloride (change in coordination number)

[Cu(H₂O)₆]²⁺ + 4Cl⁻ → [CuCl₄]²⁻ + 6H₂O

• Pale blue → yellow/green
• Coordination number changes from 6 to 4 because Cl⁻ is larger than H₂O — only four Cl⁻ ligands fit around the Cu²⁺
• Shape changes from octahedral to tetrahedral

Example 3: Cobalt complexes

[Co(H₂O)₆]²⁺ + 4Cl⁻ → [CoCl₄]²⁻ + 6H₂O

• Pink → blue
• Octahedral (coordination number 6) → tetrahedral (coordination number 4)

Example 4: EDTA replacing water

[Cu(H₂O)₆]²⁺ + EDTA⁴⁻ → [Cu(EDTA)]²⁻ + 6H₂O

One EDTA molecule replaces all six water ligands. This substitution is strongly favoured thermodynamically — see below.

The Chelate Effect

Substitution by polydentate ligands is strongly favoured because it increases the entropy of the system.

When one EDTA⁴⁻ (hexadentate) replaces six H₂O ligands:

• Reactants: 2 species (1 complex + 1 EDTA)
• Products: 7 species (1 complex + 6 H₂O)

The number of particles increases from 2 to 7, so ΔS is positive. Since ΔH is approximately zero (similar bond strengths), the positive TΔS term makes ΔG negative, driving the reaction forward.

This thermodynamic stability of chelate complexes is called the chelate effect. EDTA is used in water treatment (binds Ca²⁺ and Mg²⁺) and medicine (treatment of heavy metal poisoning).

Isomerism in Complexes

Cis-trans isomerism (a form of geometric isomerism) occurs in square planar and octahedral complexes:

Square planar example: [Pt(NH₃)₂Cl₂]

• Cis isomer: both Cl ligands on the same side — this is cisplatin, a vital anti-cancer drug
• Trans isomer: Cl ligands on opposite sides — this is transplatin, biologically inactive

Octahedral example: [Co(NH₃)₄Cl₂]⁺

• Cis: the two Cl ligands are adjacent (90° apart)
• Trans: the two Cl ligands are opposite (180° apart)

Optical isomerism occurs in octahedral complexes with bidentate or polydentate ligands:

[Cr(en)₃]³⁺ has three ethane-1,2-diamine ligands arranged around the Cr³⁺ ion. The two mirror image forms are non-superimposable — they are enantiomers that rotate plane-polarised light in opposite directions.

[Cr(C₂O₄)₃]³⁻ (three oxalate ligands) is similarly optically active.

Requirements for optical isomerism in complexes:

• Octahedral complex with three identical bidentate ligands, OR
• Octahedral complex with two bidentate ligands and two other ligands in cis positions

Stability Constants (Kstab)

The stability constant measures the equilibrium position for the formation of a complex from the aqua ion:

[Cu(H₂O)₆]²⁺ + 4NH₃ ⇌ [Cu(NH₃)₄(H₂O)₂]²⁺ + 4H₂O

Kstab = [Cu(NH₃)₄(H₂O)₂]²⁺ / ([Cu(H₂O)₆]²⁺ × [NH₃]⁴)

A larger Kstab means the complex is more thermodynamically stable.

EDTA complexes have very large stability constants due to the chelate effect.

Exam Tips

• Always state the colour change when describing ligand substitution
• Explain the chelate effect in terms of entropy (more particles on the product side), not just by saying "more bonds"
• When drawing cis/trans isomers, clearly show the spatial arrangement and label each
• Cisplatin is cis-[Pt(NH₃)₂Cl₂] — know this as a real-world application of geometric isomerism