Transition metals

• Define transition metal


  It is an element that forms at least one stable ion with a partly filled d-subshell of electrons
• Define transition metal, according to IUPAC.

  An element whose atom has an incomplete d sub-shell, or which can give rise to cations with an incomplete d sub-shell.

  
  

  A-Level focus on Ti to Cu
• What is the main property of transition metals and what is it useful for?

  Variable oxidation states, making them useful for catalysts
• Why can transition metals form complexes?

  The unfilled d-orbitals are able to accept (dative) coordinate bonds from other atoms/ions/ molecules
• Define complex

  A central metal atom or ion surrounded by ligands
• Define ligand

  An atom/ion/molecule that forms coordinate bonds to a transition metal by donating a pair of electrons
• Define coordination number

  The total number of coordinate bonds formed between a central metal ion and its ligands
• Examples of monodentate ligands

  :NH3, H2O:, [HO:]-, :Cl-, :CN-
• Examples of bidentate ligands

  H2NCH2CH2NH2, HOOC-COOH
• How are formulas of complexes written?

  Round brackets are used around ligands having more than one element like NH3, H2O, OH-

  
  

  Square rackets are used to enclose the complex, with the overall charge written outside.

  
  

  [Cu(H2O)6]2+ or [CuCl4]2-

  
• What information does the formula of a complex give you?

  - Number of ligands

  - Identity of each ligand

  - Identity of central metal ion and its oixdation state
• What are the shapes of complexes?

  - Linear - 2 coordinate bonds

  - Tetrahedral- 4 coordinate bonds

  - Octahedral - 8 bonds

  - Square planar - only d8 metals
• Difference between cis and trans isomers

  In cis, two of the same ligand are 90 degrees away from each other.

  
  

  Whereas, in trans, they are 180 degrees away from each other.

  
  

  Relevant for octahedral and square planar geometries.
• How do optical isomers arise in tetrahedral complexes?

  Four different ligands present
• How does optical isomers arise in octahedral complexes?

  Equal or more than 2 bidentate ligands present
• Give an example of a biological complex

  Haem is an iron (II) complex with a multidentate ligand.

  
  

  Oxygen forms a co-ordinate 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.
• What is ligand substitution?

  The replacement of one ligand for another
• Give an example of a ligand substitution

  [Co(H2O)6]3+ + 6NH3 ---> [Co(NH3)6]3+ + 6H2O
• Why is there a change in coordination number and geometry in [Co(H2O)6]3+ + 4Cl- ---> [CoCl4]- + 6H2O

  The larger chlorine ligand is not able to fit around in an octahedral shape, resulting in the tetrahedral complex product instead.
• Why is [CoCl4]- tetrahedral not a square planar?

  Cobalt is not a d8 metal, so it can't make a square planar complex.
• What is the Chelate effect?

  -Thermodynamic preference for ligands that form more than one coordinate bond to the metal.

  
  

  - Small enthalpy change, so it is driven by entropy

  
  

  -More molecule are made when a ligand with more coordinate bonds replaces a ligand with less
• What happen when white light is shown through a red coloured liquid?


  All the wavelength except the red wavelength is absorbed and the red wavelength is transmitted
• What happens to the absorbed wavelengths light?

  This excites electrons to a higher energy level.

  
  

  Electronic transition.

  
  

  Then, they eventually relax back down to the ground state through various mechanisms.
• What does the frequency of the energy absorbed depend on?

  The energy difference between the two states
• Equation to calculate energy change

  ∆E = Planck's constant multiplied by frequency
• Equation to find frequency

  Speed of light = frequency multiplied by wavelength
• Explain the splitting of d-subshell in transition metal complexes

  - Only alies to octahedral complexes

  - Before ligands attach the d orbitals are all at the same energy (degenerate)

  -The electric fields associated with the ligands cause repulsions in the d orbitals and that raises their energies. But it also affects the various d orbitals differently depending on how they are arranged in space.

  -Allof the d orbitals are now at a higher energy than in the uncombined ion due to the repulsions.

  -Split into two groups. The ligands are having more effect on the energies of two of the orbitals than of the other three.

  
• What must a transition metal have to be coloured?


  An incomplete d sublevel, zinc doesn't so it's not coloured
• What are the factors affecting colour?

  - Oxidation state of the metal

  
  

  - Type of ligand

  
  

  - Coordination number
• Oxidation State of metal

  Higher the oxidation state, greater difference in energy between the d sublevels

  
  

  This changes the wavelengths of light absorbed
• Example for Oxidation State of metal

  [Fe(H2O)6]2+ is a green solution

  
  

  Whereas [Fe(H2O)6]3+ is a purple solution
• Type of ligand

  Different ligands change the difference in energy between the d sublevels
• Example for type of ligand

  [Co(H2O)6]2+ is a pink solution

  
  

  [Co(NH3)6]2+ is a pale brown solution
• Coordination number

  - Difference in energy is greater in octahedral than in tetrahedral

  
  

  - So change in coordination number changes the colour

  
  

  - Change in coordination number involves change in ligand

  
  

  - Both of these factors cause colour change
• Example for coordination number

  [Cu(H2O)6]2+ - octahedral - blue solution

  
  

  [CuCl4] - tetrahedral- yellow solution
• What can a colorimeter be used to determine?

  The ratio of ligands to metal by determining the amount of light passing through the sample.
• How is the colorimeter used?

  A series of mixtures are made up of both ligand and metal and the maximum absorbance corresponds to the correct ligand to metal ratio.
• How can ammonium vanadate (V) be reduced?

  Using zinc and concentrated hydrochloric acid or concentrated sulfuric acid to get the VO3- ion
• What happens when NH4VO3 is dissolved in an acidic solution?

  A yellow solution of [VO2(H2O)4]+

  
  

  The vanadium will reduce from +5 to +4 to +3 to +2
• How is it reduced?

  In a small flask stoppered with cotton wool

  
  

  Allowing hydrogen from the zinc and acid reacting together, to escape.

  
  

  Prevents oxidation of the +2 species