Revision:Transition Elements and Complexes

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Topic 19 - The Transition Elements

Before the examination you should be able to do the following:

• Demonstrate understanding of the terms: transition element, oxidation number, complex, ligand, stability constant and homogeneous catalysis.
• Recall the characteristic properties of transition elements vanadium to copper limited to simple examples of:
  1. typical physical properties
  2. variable oxidation number
  3. complex formation
  4. coloured compounds
  5. catalytic activity
• Construct electrode potential/ oxidation number charts (dumbbell) and use these to predict the feasibility of redox reactions
• Plan an investigation using an oxidising agent as the titrant, including calculation of the results and justification of the procedures involved.
• Identify for complex compounds:
  1. their relative stability by reference to stability constants (qualitative only)
  2. their ligands as monodentate, bidentate or hexadendate
  3. from diagrams, their shape as linear, tetrahedral, square planar or octahedral
  4. heir relative stability in terms of a predicted entropy change in ligand replacement reactions at the same central atom (qualitative only)
• Demonstrate understanding of methods of preparation of complex compounds.
• Interpret the catalytic behaviour of transition elements and their compounds in terms of alternative reaction pathways in homogenous catalysis
• Make links to previous work on metal compounds, oxidation number and catalysis.

Key points

1. This topic is often examined in conjunction with Topic 17 as it extends the ideas of simple electrochemistry.
2. Note that the characteristics of transition metals are:
   • They can have more than one oxidation state
   • They can form coloured compounds
   • They have high densities than group 1 and 2 metals
   • They have higher electrical conductivities than group 1 and 2 metals
   • They are relatively unreactive
   • They can form coloured complex ions
   • They can be magnetic or paramagnetic
3. These characteristics hinge on the fact that transition metals by definitions must form at least one ion with a partially filled d-subshell of electrons.
4. You should be able to recall the common colours of transition metal compounds.
5. You should be able to construct a redox potential chart and use it to predict the thermodynamic feasibility of a reaction, the possible products and observations. This is also useful for balancing redox equations.

   

   The chart shows that according to the anticlockwise rule SO₃^2- ions will reduce Cu²⁺. It shows the oxidation number increase and decrease to equal two units. Therefore equation is . Also it shows MnO_4- oxidises Cu in a ratio of 3:2, therefore equation is:

6. Complex ions are formed in solution when the transition metal is surrounded by ligands. Ligands are species that are capable of donating a lone pair of electrons to the transition metal atom or ion through a datively covalent bond. This will be able to accept them as, by definition, is has space in its d-subshell.
7. The ligands are responsible for the colour if the complex according to the ideas of the crystal field theory.
8. Remember that a hydrated cation or anion is a complex and the water ligand can be replaced by other ligands. This ligand replacement happens such that one ligand is replaced one at a time with each replacement having its own stability constant. These data are shown on page 128 of the Book of Data.
9. You should understand that ligand replacement depends upon the entropy change of the system, and that the replacement of several monodentate ligands by one polydentate ligand results in the entropy of the system increasing.
10. You should be familiar with the shape of simple complex ions limited to linear, tetrahedral, square planar and octahedral (2, 4, 4 and 6 coordination respectively).
11. You should be aware that one form of ligand replacement involves proton transfer to water molecules producing a hydroxide of the metal.
12. You should understand the concepts of charge density and the bearing this has on co-ordination number of a transition metal in a complex (as well as hydration energies).
13. You should be able to predict possible homogeneous catalysts from electrochemical data. For instance, a transition metal that has two different oxidation numbers that have an electrode potential between the two half reactions may act as a catalyst. Note that this is only a ‘may’ and the catalytic behaviour could only be determined by experimentation.
14. Transition metals are often used as heterogeneous catalysts in gaseous reactions e.g. Fe in the Haber process. They are able to adsorb gases onto the surface by Van der Waals forces (physical adsorbtion) or by allowing dative covalent bond formation (chemisorbtion).

Comments

• Suitable for: A Level Chemistry, Nuffield Board.
• Written by: lolific
• From here.