Ligands

What is a ligand?

A species that donates a lone pair of electrons to a central metal ion to form a coordinate bond

What type of bond forms between ligand and metal?

Coordinate (dative covalent) bond

What is denticity?

The number of coordinate bonds a ligand can form with a metal ion

What is a monodentate ligand?

A ligand that forms one coordinate bond

What is a bidentate ligand?

A ligand that forms two coordinate bonds

What is a polydentate ligand?

A ligand that forms more than two coordinate bonds

What is a chelating ligand?

A polydentate ligand that forms a ring with the metal ion

Why are chelate complexes more stable?

Higher entropy as more particles are released when chelation occurs

H₂O ligand name

aqua

H₂O charge and denticity

Neutral, monodentate

H₂O donor atom

Oxygen

NH₃ ligand name

ammine

NH₃ charge and denticity

Neutral, monodentate

NH₃ donor atom

Nitrogen

Cl⁻ ligand name

chloro

Cl⁻ charge and denticity

Negative, monodentate

Cl⁻ donor atom

Chlorine

Br⁻ ligand name

bromo

Br⁻ charge and denticity

Negative, monodentate

Br⁻ donor atom

Bromine

I⁻ ligand name

iodo

I⁻ charge and denticity

Negative, monodentate

I⁻ donor atom

Iodine

F⁻ ligand name

fluoro

F⁻ charge and denticity

Negative, monodentate

F⁻ donor atom

Fluorine

OH⁻ ligand name

hydroxo

OH⁻ charge and denticity

Negative, monodentate

OH⁻ donor atom

Oxygen

CN⁻ ligand name

cyano

CN⁻ charge and denticity

Negative, monodentate

CN⁻ donor atom

Carbon

Why is CN⁻ a strong field ligand?

Strong overlap with metal d orbitals causing large splitting

CO ligand name

carbonyl

CO charge and denticity

Neutral, monodentate

CO donor atom

Carbon

Why is CO a strong field ligand?

π-backbonding strengthens metal–ligand interaction

NO₂⁻ possible ligand names

nitro or nitrito

What is special about NO₂⁻?

It shows linkage isomerism

Which atoms can NO₂⁻ donate from?

Nitrogen or oxygen

SCN⁻ ligand name

thiocyanate

What is special about SCN⁻?

It shows linkage isomerism

Which atoms can SCN⁻ donate from?

Sulfur or nitrogen

C₂O₄²⁻ ligand name

oxalate

Oxalate charge and denticity

Negative, bidentate

Why does oxalate form chelate rings?

It bonds through two oxygen atoms

en ligand name

ethane-1,2-diamine

en charge and denticity

Neutral, bidentate

Donor atoms in en

Two nitrogen atoms

EDTA⁴⁻ denticity

Hexadentate

Why is EDTA very stable?

Forms multiple coordinate bonds in one ligand

What is coordination number?

The number of coordinate bonds around the metal ion

Typical coordination number for octahedral complexes

6

Typical coordination number for tetrahedral complexes

4

Typical coordination number for square planar complexes

4

What causes ligand substitution reactions?

A ligand replaces another ligand in a complex

Common ligand substitution example

H₂O replaced by NH₃ or Cl⁻

What usually changes during ligand substitution?

Colour of the complex

What is geometric isomerism?

Different spatial arrangements of ligands

Which complexes show cis–trans isomerism?

Square planar or octahedral complexes

What causes optical isomerism?

Chiral complexes with no plane of symmetry

Which ligands often cause optical isomerism?

Bidentate ligands like en or oxalate

What is linkage isomerism?

Isomers formed when a ligand bonds through different atoms

Ligands that show linkage isomerism

NO₂⁻ and SCN⁻

What is a strong field ligand?

A ligand that causes large splitting of d orbitals

Examples of strong field ligands

CN⁻, CO

What is a weak field ligand?

A ligand that causes small splitting of d orbitals

Examples of weak field ligands

H₂O, Cl⁻, Br⁻