Ligands and 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
• 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
• 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 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
• • 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 (C2O42- ) which is sometimes written as ‘ox’
• • 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
• • 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
• • Octahedral complexes are formed when a central metal atom or ion forms six coordinate bonds
  • 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
• • 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 109.5o
• • 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 90o