Ligands & Complex Ions (OCR A Level Chemistry)

• • Transition element ions can form complexes which consist of a central metal ion and ligands
• A ligand is a molecule or ion that forms a co-ordinate bond with a transition metal by donating a pair of electrons to the bond
  • This is the definition of a Lewis base - electron pair donor

• This means ligands have a negative charge or a lone pair of electrons capable of being donated
  • This definition may seem familiar: a ligand is the same as a nucleophile

• Different ligands can form different numbers of dative bonds to the central metal ion in a complex
  • Some ligands can form one dative bond to the central metal ion
  • Other ligands can form two dative bonds, and some can form multiple dative bonds

• Co-ordination number is number of co-ordinate bonds to the central metal atom or ion

Examples of ligands Table

Ligand name	Ligand formula
Water	H2O
Ammonia	NH3
Chloride	Cl–
Cyanide	CN–
Hydroxide	OH–
Ethanedioate (ox)	–COO–COO– C2O42–
1,2-diaminoethane (en)	H2NCH2CH2NH2

 

Monodentate Ligands

• Monodentate ligands can form only one dative bond to the central metal ion
• Examples of monodentate ligands are:
  • Water (H₂O) molecules
  • Ammonia (NH₃) molecules
  • Chloride (Cl^–) ions
  • Cyanide (CN^–) ions

Examples of complexes with monodentate ligands

Bidentate Ligands

• Bidentate ligands can each form two dative bonds to the central metal ion
• This is because each ligand contains two atoms with lone pairs of electrons
• Examples of bidentate ligands are:
  • 1,2-diaminoethane (H₂NCH₂CH₂NH₂) which is also written as ‘en’
  • Ethanedioate ion (C₂O₄^2- ) which is sometimes written as ‘ox’

Examples of complexes with bidentate ligands

• Depending on the size of the ligands and the number of dative bonds to the central metal ion, transition element complexes have different geometries
  • Dative bonds can also be referred to as coordinate bonds, especially when discussing the geometry of a complex

Six-coordinate complexes

• Octahedral complexes are formed when a central metal atom or ion forms six coordinate bonds
• This could be six coordinate bonds with six small, monodentate ligands
  • Examples of such ligands are water and ammonia molecules and hydroxide and thiocyanate ions

• It could be six coordinate bonds with three bidentate ligands
  • Each bidentate ligand will form two coordinate bonds, meaning six coordinate bonds in total
  • Examples of these ligands are 1,2-diaminoethane and the ethanedioate ion
• The bond angles in an octahedral complex are 90^o

Examples of octahedral complexes

Four-coordinate complexes

Tetrahedral

• When there are four coordinate bonds the complexes often have a tetrahedral shape
  • Complexes with four chloride ions most commonly adopt this geometry
  • Chloride ligands are large, so only four will fit around the central metal ion

• The bond angles in tetrahedral complexes are 109.5^o

Example of a tetrahedral complex

Square planar

• Sometimes, complexes with four coordinate bonds may adopt a square planar geometry instead of a tetrahedral one
  • Cyanide ions (CN^-) are the most common ligands to adopt this geometry
  • An example of a square planar complex is cisplatin

• The bond angles in a square planar complex are 90^o

Cisplatin is an example of a square planar complex