Octahedral Complexes (Edexcel A Level Chemistry): Revision Note

Octahedral 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

  • As there are six ligands, these complexes are sometimes described as having six-fold coordination

Table showing Examples of Common Monodentate Ligands

Common monodentate ligands:

Example of an octahedral complex with monodentate ligands

• 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

Example of an octahedral complex with bidentate ligands

• It could be six-coordinate bonds with one multidentate ligand

  • The multidentate ligand, for example, EDTA^4-_, forms all six-coordinate bonds

Example of an octahedral complex with a polydentate ligand 

• The bond angles in an octahedral complex are 90^o

• The coordination number of a complex is the number of dative bonds formed between the central metal ion and the ligands

  • Since there are 6 dative bonds, the coordination number for the complex is 6