Geometry of Complexes (CIE A Level Chemistry)

• 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

Linear

• Central metal atoms or ions with two coordinate bonds form linear complexes
• The bond angles in these complexes are 180^o
• The most common examples are a copper (I) ion, (Cu⁺), or a silver (I) ion, (Ag⁺), as the central metal ion with two coordinate bonds formed to two ammonia ligands

Examples of a linear complex

A linear complex has a bond angle of 180^o

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

Tetrahedral complexes have a bond angle of 109.5^o

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

Example of a square planar complex

Cisplatin is an example of a square planar complex with a bond angle of 90^o

Octahedral

• 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
• It could be six coordinate bonds with one polydentate ligand
  • The polydentate ligand, for example EDTA^4-_, forms all six coordinate bonds
• The bond angles in an octahedral complex are 90^o 

Examples of octahedral complexes

Octahedral complexes have bond angles of 90^o

Types of ligands table

• The coordination number of a complex is the number of coordinate bonds that are formed between the ligand(s) and the central metal atom or ion
• Some ligands can form only one coordinate bond with the central metal ion (monodentate ligands), whereas others can form two (bidentate ligands ) or more (polydentate ligands)
• It is not the number of ligands which determines the coordination number, it is the number of coordinate (dative) bonds

Predicting complex ion formula & charge

• The formula and charge of a complex ion can be predicted if the following are known:
  • The central metal ion and its charge/oxidation state
  • The ligands
  • The coordination number/geometry