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Advanced level inorganic chemistry - 3d block-transition metals - shapes and isomerism of complexes

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Appendix 3 Complexes of the 3d block transition metals: more on shapes and types of isomerism

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The previous page described transition metal complexes in general, but here we focus on isomerism in transition metal complex ions. Examples of positional isomerism, R/S isomerism (optical isomers) and E/Z isomerism (trans/cis isomers) of transition metal complexes are described with the aid of diagrams.

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Appendix 3. Complexes: more on shapes and isomerism

Read Appendix 2. Introduction to complexes & ligands first

  • Valence shell electron pair repulsion theory (VSEPR) can quite often be used to predict the shape of a complex, which is usually one of four shapes (at least at pre–university level):

  • Principal shapes:

    • Examples of complexes have been described in Appendix 2

    • For isomerism analysis it is crucial to the think of the orientation of the co-ordinate bonds and not the number of ligands.

    • Isomerism can occur because of the directional nature of the dative covalent bonds emanating from the central metal ion.

    • Based on the complex ion shape - usually determined by the ligands themselves (as opposed to lone pair involvement).

    • Octahedral (co–ordination number 6): very common, positional geometrical (E/Z) and optical (R/S) isomerism possible.

    • Tetrahedral (co–ordination number 4): with more bulky ligands like chloride ion, optical isomerism possible but very rare.

    • Square planar (co–ordination number 4): E/Z isomerism (geometrical/geometric cis/trans isomerism) possible.

    • Linear (co–ordination number 2): relatively rare and can't form isomers.

    • Three are illustrated in the diagram above, and the isomerism in three cases illustrated below.

  • Principal forms of isomerism:

    • E/Z isomerism (was called cis/trans geometric isomerism, but Z = cis and E = trans, so take care):

      • Examples of (1) above with monodentate ligands in a square planar complex:

        • Z and E isomers (cis and trans) of [Pt(NH3)2Cl2]  shown below

        • cisplatin

        • You could not get isomers if it had a tetrahedral shape, you couldn't form two non-superimposable structures.

      • Examples of (2) above with unidentate ligands in an octahedral complex:

        • Z and E isomers (cis and trans) of [Cr(NH3)4Cl2]+  or  [Cr(H2O)4Cl2]+  (shown below, same with NH3)

        • and

    • Optical isomerism: Examples of (3) above with a bidentate ligand.

      • and 

      • (iv) the optical (R/S) isomers (mirror images) of [Co(H2NCH2CH2NH2)3]2+ are represented above.

      • H2NCH2CH2NH2, ethane–1,2–diamine (ethylenediamine), is often represented in shorthand by en,

        • and the complex simply written as [Co(en)3]2+.

        • This complex has mirror image forms i.e. enantiomers of optical isomers.

          • This optical isomerism can be illustrated thus

          •  where L–L represents the electrically neutral ligand H2NCH2CH2NH2

          • The ligand bonds via the lone pairs of electrons on the nitrogen which are donated to form the metal–ligand dative covalent bonds.

          • The chromium(III) ion forms a similar complex exhibiting R/S isomerism.

            • [Cr(en)3]3+. The diagrams would be the same as above.

INORGANIC Part 10 3d block TRANSITION METALS sub–index:

10.1–10.2 Introduction to 3d–block Transition Metal chemistry

10.3 Scandium * 10.4 Titanium * 10.5 Vanadium  *  10.6 Chromium * 10.7 Manganese  *  10.8 Iron

10.9  Cobalt * 10.10 Nickel * 10.11 Copper * 10.12 Zinc  *  10.13 Other Transition Metals e.g. Ag and Pt

Appendix 1. Hydrated salts, acidity of hexa–aqua ions * Appendix 2. Complexes & ligands

Appendix 3. Complexes and isomerism * Appendix 4. Electron configuration & colour theory

Appendix 5. Redox equations, feasibility, Eø * Appendix 6. Catalysis * Appendix 7. Redox equations

Appendix 8. Stability Constants of complexes and entropy changes

Appendix 9. Colorimetric analysis and complex ion formula * Appendix 10 3d block – extended data table

Appendix 11 3d–block compounds - transition metal complexes, oxidation states & electrode potentials

Appendix 12 Hydroxide complex precipitate 'pictures', formulae and equations

Advanced Level Inorganic Chemistry Periodic Table Index: Part 1 Periodic Table history * Part 2 Electron configurations, spectroscopy, hydrogen spectrum, ionisation energies * Part 3 Period 1 survey H to He * Part 4 Period 2 survey Li to Ne * Part 5 Period 3 survey Na to Ar * Part 6 Period 4 survey K to Kr and important trends down a group * Part 7 s–block Groups 1/2 Alkali Metals/Alkaline Earth Metals * Part 8  p–block Groups 3/13 to 0/18 * Part 9 Group 7/17 The Halogens * Part 10 3d block elements & Transition Metal Series * Part 11 Group & Series data & periodicity plots * All 11 Parts have their own sub–indexes near the top of the pages

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