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28. Chemistry of transition elements

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  1. 28.1 Physical and chemical properties of first-row transition elements

    1. • Define a transition element as a d-block element which forms one or more stable ions with incomplete d orbitals

    2. • Sketch the shape of a 3dxy orbital and 3dz² orbital

    3. • Understand: transition elements have the following properties:: (a) they have variable oxidation states; (b) they behave as catalysts; (c) they form complex ions; (d) they form coloured compounds

    4. • Explain why transition elements have variable oxidation states in terms of the similarity in energy of the 3d and the 4s sub-shells

    5. • Explain why transition elements behave as catalysts in terms of having more than one stable oxidation state, and vacant d orbitals that are energetically accessible and can form dative bonds with ligands

    6. • Explain why transition elements form complex ions in terms of vacant d orbitals that are energetically accessible

  2. 28.2 Chemical properties of first-row transition elements

    1. 28.2.1Ligand reactions forming complexes

      • Describe/explain: the reactions of transition elements with ligands to form complexes, incl. complexes of copper(II) and cobalt(II) ions with water and ammonia molecules and hydroxide and chloride ions

    2. 28.2.2Terms: ligand as a species that contains

      • Define: ligand as a species that contains a lone pair of electrons that forms a dative covalent bond to a central metal atom / ion

    3. 28.2.3Terms

      • Use terms:: (a) monodentate ligand e.g. H2O, NH3, Cl – and CN–; (b) bidentate ligand e.g. 1,2-diaminoethane, en, H2NCH2CH2NH2, and the ethanedioate ion, C2O4; 2–; (c) polydentate ligand including as an example EDTA4–

    4. 28.2.4Complex ions

      • Define: complex as a molecule or ion formed by a central metal atom / ion surrounded by one or more ligands

    5. 28.2.5Shapes of transition metal complexes

      • Describe the geometry (shape and bond angles) of transition element complexes which are linear, square planar, tetrahedral or octahedral

    6. 28.2.6Coordination number and complex formula

      • The ligand and its coordination number or geometry: (a) state what is meant by coordination number; (b) predict the formula and charge of a complex ion, given the metal ion, its charge or oxidation state,

    7. 28.2.7Ligand exchange reactions

      • Explain qualitatively that ligand exchange can occur, incl. complexes of copper( II) ions and cobalt(II) ions with water and ammonia molecules and hydroxide and chloride ions

    8. 28.2.8Predict, using E

      • Predict, using E ⦵ values, the feasibility of redox reactions involving transition elements and their ions

    9. 28.2.9Transition metal redox calculations

      • Describe the reactions of, and perform calculations involving:: (a) MnO 4 – / C2O4; 2– in acid solution given suitable data; (b) MnO 4 – / Fe2+ in acid solution given suitable data; (c) Cu 2+ / I– given suitable data

    10. 28.2.10Calculations: other redox systems

      • Calculate: other redox systems given suitable data

  3. 28.3 Colour of complexes

    1. • Define/use: degenerate and non-degenerate d orbitals

    2. • Describe the splitting of degenerate d orbitals into two non-degenerate sets of d orbitals of higher energy, and use of Δ E in:: (a) octahedral complexes, two higher and three lower d orbitals; (b) tetrahedral complexes, three higher and two lower d orbitals

    3. • Explain why transition elements form coloured compounds in terms of the frequency of light absorbed as an electron is promoted between two non-degenerate d orbitals

    4. • Describe, in qualitative terms, the effects of different ligands on Δ E, frequency of light absorbed, and hence the complementary colour that is observed

    5. • Use the complexes of copper( II) ions and cobalt(II) ions with water and ammonia molecules and hydroxide and chloride ions as examples of ligand exchange affecting the colour observed

  4. 28.4 Stereoisomerism in transition element complexes

    1. • Describe the types of stereoisomerism shown by complexes, including those associated with bidentate ligands: - (a) geometrical (cis/trans) isomerism, e.g. square planar such as [Pt(NH₃)₂Cl ₂] and octahedral such as [Co(NH3)4(H2O)2]2+ and [Ni(H2NCH2CH2NH2)2(H2O)2]2+ - (b) optical isomerism, e.g. [Ni(H2NCH2CH2NH2)3]2+ and [Ni(H2NCH2CH2NH2)2(H2O)2]2+

    2. • Deduce the overall polarity of complexes such as those described in 28.4.1(a) and 28.4.1(b)

  5. 28.5 Stability constants, K stab

    1. • Define the stability constant, Kstab, of a complex as the equilibrium constant for the formation of the complex ion in a solvent (from its constituent ions or molecules)

    2. • Write an expression for a Kstab of a complex ([H₂O] should not be included)

    3. • Use Kstab expressions to perform calculations

    4. • Describe/explain: ligand exchanges in terms of K stab values and understand that a large Kstab is due to the formation of a stable complex ion