International A Level (IAL)ChemistryEdexcelRevision Notes5. Transition Metals & Organic Nitrogen ChemistryTransition Metals & ComplexesOctahedral & Tetrahedral Complexes

Octahedral & Tetrahedral Complexes (Edexcel International A Level (IAL) Chemistry): Revision Note

Exam code: YCH11

Stewart Hird

Written by: Stewart Hird

Reviewed by: Caroline Carroll

Updated on

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

Table of common monodentate ligands and their formulae: water H2O, ammonia NH3, chloride Cl-, cyanide CN- and hydroxide OH-
Octahedral complex [Fe(H2O)6]2+ with six water ligands, each donating a lone pair to the iron ion through a dative bond

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

Octahedral complexes with bidentate ligands: [Cu(en)3]2+ with three 1,2-diaminoethane ligands and [Co(C2O4)3]3- with three ethanedioate ligands

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

Skeletal formula of the EDTA4- ligand reacting with Cu2+ to form the octahedral copper(II)-EDTA complex [Cu(EDTA)]2- with six dative bonds

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 Tips and Tricks

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 electrons 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 an orange-yellow 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

Tetrahedral complex [CuCl4]2- with four chloride ligands and bond angles of 109.5 degrees

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 isomer

      Cis and trans isomers of an octahedral nickel complex with two bidentate 1,2-diaminoethane ligands and two chloride ligands: the chlorides are adjacent in the cis isomer and opposite in the trans isomer

cis and trans isomers of the octahedral complex [Ni(en)2Cl2]; [Co(NH3)4(H2O)2]2+ and [Ni(en)2(H2O)2]2+ show the same pattern

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 cis-[Ni(H2NCH2CH2NH2)2(H2O)2]2+ complexes (only the cis isomer is chiral)

    • The ligand H2NCH2CH2NH2 can also be written as ‘en’ instead

      Two non-superimposable mirror-image forms (enantiomers) of the octahedral complex [Ni(en)3]2+ drawn either side of a mirror line

Octahedral transition metal complexes exhibiting optical isomerism

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Stewart Hird

Author: Stewart Hird

Expertise: Chemistry Content Creator

Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Topic Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.

Caroline Carroll

Reviewer: Caroline Carroll

Expertise: Head of Content Delivery

Caroline graduated from the University of Nottingham with a degree in Chemistry and Molecular Physics. She spent several years working as an Industrial Chemist in the automotive industry before retraining to teach. Caroline has over 12 years of experience teaching GCSE and A-level chemistry and physics. She is passionate about delivering high-quality resources to help students achieve their full potential.