5.6.2 Ligands & Complex Ions

• 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

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