Transition metals

• A transition metal is an element that forms at least one stable ion with an incomplete d-subshell.
• Transition metals are located in the d-block of the periodic table (Scandium to Zinc).
• Not all d-block elements are transition metals:

  • Scandium (Sc³⁺) and Zinc (Zn²⁺) are not transition metals because their stable ions have empty or full d-subshells.
• The key characteristics of transition metals include:

  • variable oxidation states
  • formation of coloured ions
  • formation of complex ions
  • catalytic activity
• Transition metals can lose different numbers of electrons, leading to variable oxidation states.

  • This is due to the similar energy levels of 4s and 3d electrons
• Transition metal ions often have partially filled d-orbitals, which allows them to absorb certain wavelengths of light.

  • The remaining light is reflected, allowing them to form coloured ions.
• Transition metals bond with ligands to form complex ions.

  • Complex ions is where a central transition metal ion is surrounded by ligands bonded by dative covalent bonds.
• A ligand is a molecule or ion that forms a coordinate bond with a transition metal by donating a pair of electrons.

  • They have at least one lone pair of electrons
  • They can be monodentate, bidentate or multidentate.
• Many transition metals act as catalysts due to their ability to change oxidation states and adsorb reactants onto their surface.
• Coordination number is the number of coordinate bonds to the central atom or ion.
• Monodentate ligands have one lone pair of electrons which they donate. For example:

  • H₂O (Water)
  • NH₃ (Ammonia)
  • Cl⁻ (Chloride)
• Bidentate ligands have two lone pairs of electrons which they donate. For example:

  • Ethane-1,2-diamine (en) → NH₂CH₂CH₂NH₂
  • Oxalate ion (C₂O₄²⁻)
• Multidentate ligands have three or more lone pairs which they donate. For example:

  • EDTA⁴⁻ – forms six coordinate bonds.
• Ligand substitution occurs when one ligand is replaced by another, changing the complex ion’s colour.
• The shape of complex ions are dependent on the size of the ligands and the coordination number.

  • Ligands such as H₂O and NH₃ are small, so 6 can fit around a central metal ion.
  • Cl- is a larger ligand, so only 4 can fit around the central metal ion
  • EDTA⁴⁻ and en are larger still, so only 3 can fit around a central metal ion
• Complexes with a coordination number of 6 form octahedral shapes. All bond angles are 90°.
• Complexes with a coordination number of 4 form tetrahedral and square planar shapes.

  • Bond angles in a tetrahedral complex are 109.5° - e.g. formed with Cu²⁺
  • Bond angles in a square planar complex are 90° - e.g. formed with Pt²⁺
• A specific example of a square planar complex is the anti-cancer drug cis-platin - Pt[(NH3)2(Cl)2].
• Complexes with a coordination number of 2 form linear shapes. A specific example are some silver complexes e.g. Tollen's reagent - [Ag(NH3)2]+(aq)

  • Bond angles in a linear complex are 180°
• Complexes have an overall charge which is the same as it's total oxidation state.

  • Total oxidation state of metal = total oxidation state - total oxidation state of ligands
• Haem is an iron(II) complex with a multidentate ligand that is found in the molecule haemoglobin.

  • Oxygen forms a coordinate bond to Fe(II) in haemoglobin, enabling oxygen to be transported in the blood
  • Carbon monoxide is toxic because it replaces oxygen coordinately bonded to Fe(II) in haemoglobin
  • CO bonds strongly, so it is not readily replaced by O2
• The Chelate effect is when bidentate and multidentate ligands replace monodentate ligands from complexes as they are more stable.

  • Reason: Substituting monodentate ligands with bidentate/multidentate ligands increases entropy, making the reaction more thermodynamically favourable.
• Cis-trans isomerism occurs in octahedral and square planar complexes where identical monodentate ligands are arranged differently.
• Optical Isomerism (Enantiomers) occurs in octahedral complexes when bidentate ligands form non-superimposable mirror images.
• Transition metal ions are coloured because:

  1. They have partially filled d-orbitals.
  2. When ligands bond to the metal ion, the d-orbitals split into two energy levels.
  3. Electrons absorb light energy to move from a lower energy d-orbital (ground state) to a higher energy d-orbital (excited state).
  4. The remaining wavelengths of light are transmitted or reflected, giving the complex its observed colour.
• The energy gap (ΔE) between the ground state and the excited state of the d electrons is dependent on:

  • The central metal ion and its oxidation state
  • The type of ligand
  • The coordination number
• The energy difference (ΔE) between the ground state and the excited state of the d electrons is given by: ΔE = hν = hc/λ

  • h = Planck’s constant
  • ν = Frequency of light (Hz)
  • c = Speed of light
  • λ = Wavelength of light absorbed (m)
• The colour of a complex ion is the complementary colour to the one absorbed.
• Colour of complex ions depends on the size of energy gap (ΔE) which is affected by change of oxidation state, coordination number and change of ligand.
• Calorimetry is used to measure the concentration of transition metal ions in solution.

  • The intensity of the colour of a transition metal solution is directly proportional to the concentration of the ion.
• Colorimetry:

  1. A solution of unknown concentration is placed in a colorimeter.
  2. A filter is used to select the wavelength of light absorbed by the metal ion.
  3. The absorbance is measured.
  4. A calibration curve is used to determine the unknown concentration.