Transition metals

• The transition metals are elements in the d block of the periodic table with a partially filled d orbital.
• Transition metals can form complexes, which consist of a central metal atom or ion surrounded by ligands.
• Ligands are molecules or ions with a lone electron pair that is able to form a coordinate bond to a central metal atom or ion.
• The coordination number of a complex is equal to the number of coordinate bonds formed around the central metal.
• Complexes with just Cl - ligands always have a coordination number of four, producing a tetrahedral shape with a bond angle of 109.5 degrees.
• With copper and water complexes and ammonia in excess, the ligand substitution reaction is incomplete and the complex formed has a combination of water and ammonia ligands.
• Bidentate ligands can form two coordinate bonds to the central metal.
• The ox ligand is a bidentate ligand with the structure O = C (- O) - C (- O) = O.
• The en ligand is a bidentate ligand with the structure NH2 - CH2 - CH2 - NH2.
• Multidentate ligands can form up to six coordinate bonds to the central metal.
• EDTA is a multidentate ligand that forms six coordinate bonds with the central metal. (Structure doesn't need to be known).
• The chelate effect is the substitution of unidentate ligands for multidentate ligands as they give a greater positive entropy change and are therefore more stable.
• Enthalpy change for ligand substitution reactions is near zero as the bonds formed are very similar to the bonds broken.
• Complexes with H2O or NH3 ligands usually form octahedral complexes with a bond angle of 90 degrees.
• Cis - trans isomerism is shown in octahedral complexes with different types of unidentate ligands. The cis isomer has the same ligands next to each other and the trans isomer has the same ligands opposite to each other.
• Optical isomerism is shown in octahedral complexes with bidentate ligands.
• Platinum and nickel complexes form in a square planar shape consisting of four coordinate bonds with a bond angle of 90 degrees.
• Silver complexes always have a linear shape with two coordinate bonds and bond angle 180 degrees.
• Cisplatin is the cis isomer of a square planar complex of platinum and is used as a cancer therapy drug.
• Cells in the natural world are chiral so only one isomer of the drug will be in the correct orientation to work.
• Transition metal colour depends on the coordination number, type of ligand and the oxidation state.
• When electrons in the d orbital are excited, the change in energy corresponds to a frequency of light that is in the visible spectrum.
• Colorimetry is an analytic technique that uses the absorbance of visible light to determine the concentration of coloured ions by measuring absorbance.
• Vanadium has four possible oxidation states: + 5 (VO2 +), + 4 (VO 2+), + 3 (V 3+) and + 2 (V 2+).
• VO2 + is yellow, VO 2+ is blue, V 3+ is green and V 2+ is violet.
• The pH of the reaction conditions determines whether a transition metal is oxidised or reduced.
• In acidic conditions, transition metals are reduced, eg for VO2 +, VO2 + + e - + 2 H + -> VO 2+ + H2O.
• In alkali conditions, transition metals are oxidised, eg for VO 2+, VO 2+ + H2O -> VO2 + + 2 H + + e -.
• MnO4 - reacts with C2O4 2- with a 2:5 ratio.
• MnO4 - reacts with Fe 2+ with a 1:5 ratio.
• C2O4 2- reacts with Fe 2+ with a 1:2 ratio.
• Heterogeneous catalysts act in a different phase or state to the species in the reaction, for example the haber process uses a solid iron catalyst with hydrogen and nitrogen gases.
• Transition metals make good catalysts due to their variable oxidation states.
• An example of a heterogeneous transition metal catalyst is in the contact process, which uses a vanadium oxide catalyst to increase the rate of reaction of sulfur dioxide to sulfur trioxide.
• A solid catalyst works by adsorbing (sticking) molecules onto its active site, which increases the proximity and weakens the covalent bonds of the molecules.
• Heterogenous catalysts can be poisoned by impurities which block the active site and prevent adsorption.
• Homogeneous catalysts act in the same phase to the species in the reaction, for example Fe 2+ catalyst with S2O8 2- and I - ions as without a catalyst the ions would repel and not react.
• Autocatalysis is where one of the products of the reaction can act as a catalyst for the reaction, and thus rate increases over time as more catalyst is produced, for example Mn 2+ from the reaction between MnO4 - and C2O4 2-.
• 3+ aqua ions are more acidic than 2+ aqua ions as the 3+ ions dissociate more and have a greater attractive power to OH -, basic ions.
• The reaction of NaOH or NH3 with 2+ aqua ions causes two OH - to be substituted from H2O. Fe 2+ forms a green precipitate and Cu 2+ forms a blue precipitate.