Octahedral & Tetrahedral Complexes (Edexcel International A Level Chemistry)

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Octahedral & Tetrahedral 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

Examples of monodentate ligands, downloadable IB Chemistry revision notes

octahedral-complex-1

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

Chemistry of Transition Elements - Bidentate Ligands, downloadable AS & A Level Chemistry revision notes

Example of an octahedral complex with bidentate ligands

  • It could be six-coordinate bonds with one multidentate ligand
    • The multidentate ligand, for example, EDTA4-, forms all six-coordinate bonds

Example of a multidentate ligand, downloadable IB Chemistry revision notes

Example of an octahedral complex with a polydentate ligand 

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

Examiner Tip

Electron pair repulsion theory can be extended to predict and explain the shape of transition metal complexes. The only difference is you should ignore the 3d elctrons in the transition metal ion and overall charge on the complex - just count the number of electron pairs donated by the ligands.

Changes to the central metal ion

  • The main example of the central metal ion changing oxidation state is when Fe2+ is oxidised to form Fe3+ 
    • Fe2+ → Fe3+ + e 
  • Hexaaquairon(II) is a pale green solution, which slowly oxidises on standing in air to give a yellow solution of hexaaquairon(III) ions. 
    • [Fe(H2O)6]2+ → [Fe(H2O)6]3+  
  • The same oxidation reaction can be achieved with different iron(II) complex ions
    • These are often performed by bubbling oxidising agents, such as chlorine gas, through a solution of the iron(II) complex ion
    • For example, bubbling chlorine gas through a yellow solution of potassium hexacyanoferrate(II), K4[Fe(CN)6], results in the formation of a red solution of potassium hexacyanoferrate(III), K3[Fe(CN)6]
    • 2K4[Fe(CN)6] + Cl2 → 2K3[Fe(CN)6] + 2K+ + 2Cl

Tetrahedral complexes

  • 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

Chemistry of Transition Elements - Tetrahedral Complexes, downloadable AS & A Level Chemistry revision notes

Example of a tetrahedral complex

Isomerism in transition element complexes

  • Transition element complexes can exhibit stereoisomerism
  • Even though transition element complexes do not have a double bond, they can still have geometrical isomers
  • Square planar and octahedral complexes with two pairs of different ligands exhibit cis-trans isomerism
  • Examples of octahedral complexes that exhibit geometrical isomerism are the [Co(NH3)4(H2O)2]2+ and [Ni(H2NCH2CH2NH2)2(H2O)2]2+ complexes
    • [Ni(H2NCH2CH2NH2)2(H2O)2]2+ can also be written as [Ni(en)2(H2O)2]2+

  • Like in the square planar complexes such as cis-platin and trans-platin, if the two ‘different’ ligands are next to each other then that is the ‘cis’ isomer, and if the two ‘different’ ligands are opposite each other then this is the ‘trans’ isomer
    • In [Co(NH3)4(H2O)2]2+, the two water ligands are next door to each other in the cis isomer and are opposite each other in the trans isomerChemistry of Transition Elements - Cis-trans in Octahedral (1), downloadable AS & A Level Chemistry revision notes

Octahedral transition metal complexes exhibiting geometrical isomerism

Optical isomerism

  • Octahedral complexes with bidentate ligands also have optical isomers
  • This means that the two forms are non-superimposable mirror images of each other
    • They have no plane of symmetry, and one image cannot be placed directly on top of the other

  • The optical isomers only differ in their ability to rotate the plane of polarised light in opposite directions
  • Examples of octahedral complexes that have optical isomers are the [Ni(H2NCH2CH2NH2)3]2+and [Ni(H2NCH2CH2NH2)2(H2O)2]2+ complexes
    • The ligand H2NCH2CH2NH2 can also be written as ‘en’ insteadChemistry of Transition Elements - Optical Isomerism in Octahedral, downloadable AS & A Level Chemistry revision notes

Octahedral transition metal complexes exhibiting optical isomerism

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Sonny

Author: Sonny

Expertise: Chemistry

Sonny graduated from Imperial College London with a first-class degree in Biomedical Engineering. Turning from engineering to education, he has now been a science tutor working in the UK for several years. Sonny enjoys sharing his passion for science and producing engaging educational materials that help students reach their goals.