Transition Metals

• What is a transition metal?

  The transition metals are elements in the d-block of the periodic table with a partially filled d-orbital, they lose electrons to form positive ions with the s-orbital electrons being removed first
• Chemical Properties:

  They form complexes

  They form coloured ions

  They have variable oxidation states

  They are good catalysts
• Ligand Substitution Reactions

  Copper complex with NH3:

  [Co(H2O)6]2+ + 6NH3 ---> [Co(NH3)6]2+ +6H2O

  When NH3 excess:

  [Cu(H2O)6]2+ + 4NH3 ---> [Cu(H2O)2(NH3)4]2+ + 4H2O - Forms deep blue solution

  With Cl-: [Cu(H2O)6]2+ + 4Cl- ---> [CuCl4]2- + 6H2O - Tetrahedral
• What is a bidentate ligand?

  These ligands are able to form two coordinate bonds to the central metal ion as they have two lone electron pairs

  E.g. Ethandioate ions or ethane-1,2-diamine
• Discuss the role of iron as a heterogenous catalyst in the Haber process:

  (6)

  Heterogenous means in a different phase from reactants

  Catalyst speeds up a reaction and is left unchanged

  Hydrogen and nitrogen adsorb onto the surface (active sites of iron)

  Bonds weaken, products leave from the surface of the iron

  Large surface area, catalyst poisoned makes blocked active sites
• Explain why complexes formed from transition metal ions are coloured (3)


  They absorb wavelengths to excite electrons in the d-orbital and the complementary colour shows
• Describe how a calibration graph is produced and used to find the concentration of the iron (III) complex (3)

  Measure absorbance for a range of known concentrations

  Plot graph of absorbance v concentration

  Read value of concentration for the measured absorbance from this graph
• Vanadium Oxide is used as a catalyst in the manufacture of sulfur trioxide (The Contact Process)

  Give an equation for this reaction

  V2O5 + SO2 ---> V2O4 + SO3

  V2O4 + 1/2O2 ---> V2O5
• Explain why colorimetry cannot be used to determine the concentration of solutions containing [CuCl2-]

  Has a full d-orbital and is colourless so cannot absorb frequencies of light
• Explain the meanings of the term 'multidentate' and 'ligand'

  Multidentate - can form six dative bonds with central cation

  Ligand - Lone pair can form dative bond with copper (II) ions
• Outline the practical steps that you would follow, using colorimetry, to determine the concentration of this complex in a sample of water:

  (About EDTA)

  Calibrate a colorimeter by testing the colorimeter with solutions of copper-EDTA complex of known concentration, add excess EDTA salt to the sample
• Explain how this co-ordinate bond is formed

  An electron pair on the ligand is donated from the ligand to the central metal ion
• Describe what you would observe when dilute aqueous ammonia is added dropwise, to excess, to an aqueous solution containing copper(II) ions

  Blue ppt, dissolves to give a dark blue solution

  [Cu(H2O)6]2+ + 2NH3 ---> Cu(H2O)4(OH)2 + 2NH4+

  Cu(H2O)4(OH)2 + 4NH3 ---> [Cu(NH3)4(H2O)2]2+ + 2OH- + 2H2O
• Characteristics of Transition Metals

  Complex formation, high density, high m.p, formation of coloured ions, variable oxidation states and catalytic activity
• What is a ligand?


  A molecule or ion that forms a co-ordinate bond with a transition metal by donating a pair of electrons
• What is a complex?

  A central metal atom or ion surrounded by ligands
• What is meant by 'co-ordination number'?


  Number of co-ordinate bonds to the central atom or ion
• Describe what is meant by 'the chelate effect'?


  When bidentate and multidentate substitute monodentate ligands, a more stable complex is formed...
• Cl- Ligand Substitution Reaction


  The Cl- ligand is much larger than NH3 or H2O ligands, so the ligand results in a change in coordination number for that complex

  Equation example: [Cu(H2O)6]2+ + 4Cl- ---> [CuCl4]2- + 6H2O

  Shape of complex produced is tetrahedral (coordination number is 4)
• Multidentate Ligand Reaction


  These ligands can form up to 6 coordinate bonds to the central metal ion as they have multiple lone electron pairs - most common ligand is EDTA

  Equation example: [Fe(H2O)6]3+ + EDTA4- ---> [FeEDTA]-1 + 6H2O

  • Haem is also common component of a common multidentate ligand - consists of 6 coordinate bonds around a central FE2+ ion - its shape/structure allows for transport of oxygen in the body
  • Carbon monoxide is toxic as it replaces the oxygen in the haem complex - preventing it from being transported
• Octahedral Complexes

  Transition metal complexes with H2O and NH3 ligands commonly form octahedral complexes with a bond angle of 90 degrees

  A) Yellow-brown in sol

  
• Cis-trans Isomerism

  Shown by octahedral complexes with different types of unidentate ligands - ligands of the same type can be either next to or opposite each other

  The trans isomer has two of the same ligands opposite each other and cis isomer has two of the same ligands next to each other

  
• Optical Isomerism 


  This is shown by octahedral complexes with bidentate ligands, the two isomers are mirror images of each other

  
• Tetrahedral Complexes


  When complexes form with larger ligands such as Cl-, they form tetrahedral complexes with a bond angle of 109.5 degrees

  Tetrahedral complexes can show optical isomerism

  Examples: [CuCl4]2- , [CoCl4]2-
• Square Planar Complexes


  Platinum and nickel complexes form in a square planar shape - this consists of 4 coordinate bonds with a bond angle of 90 degrees

  Common example is cisplatin (used as an anti-cancer drug) - [Pt(NH3)2Cl2]

  • It is neutral because 2+ charge of the original platinum ion is exactly cancelled by two negative charges supplied by the chloride ions
  • Can show geometric isomerism - cis and trans
• Linear Complexes

  Silver complexes always have a linear shape with 2 coordinate bonds around the central metal ion - the bond angle is 180 degrees

  This is the type of complex present in Tollen's reagent [Ag(NH3)2]+
• Coloured Ions

  Transition metal ions can be identified by colour

  Colour arises because substances absorb & reflect light, when white light shines on a substance, some wavelengths of light are absorbed but remaining wavelengths are reflected/transmitted - these reflected wavelengths correspond to a specific colour - electrons in the d-orbital exist in a series of energy states, when they are given energy, they move from ground state to 'excited state'

  Change in energy between states corresponds to wavelength & frequency of light which can be calculated
• Vanadium's Variable Oxidation States

  Vanadium has 4 possible states from 5+ to 2+ of which produces a different coloured compound

  These different species can be produced from the oxidation of vanadium by zinc in acidic solution

  • Colours can be remembered by the phrase: You Better Get Vanadium (v for violet) - Order of ions: VO2+ (2 down), VO2+ (2 up), V3+, V2+
• Vanadium Equations


  The pH of the reaction conditions determines whether a transition metal is oxidised or reduced

  For ions to be reduced (oxidation state more negative) acidic conditions are required - Equation: 2VO2+ (down) + 2e- + 4H+ ---> 2VO2+ (top) + 2H2O

  For ions to be oxidised (oxidation state more positive), they react with water to produce OH- ions and therefore an alkaline solution - Equation: VO2+(top) + H2O ---> VO2+(bottom) + 2H+ + e-

  Tollen's reagent [
• Tollens Reagent


  The silver complex [Ag(NH3)2]+ is reduced by aldehydes to form silver atoms, seen as silver mirror - used as a test for aldehydes

  Equation: RCHO + 2[Ag(NH3)2]+ + 3OH- ---> RCOO- + 2Ag + 4NH3 + 2H2O
• Redox Reactions


  MnO4- + 8H+ + 5e- ---> Mn2+ + 4H2O

  Fe2+ ---> Fe3+ + e-

  C2O42- ---> 2CO2 + 2e-

  Reacting Molar Ratios

  MnO4-: C2O42- is 2:5

  MnO4-: Fe2+ is 1:5

  C2O42-: Fe2+ is 1:2
• Catalysts


  A catalyst is a substance that speeds up the rate of reaction without being used up in the reaction. It provides an alternative reaction path with a lower activation energy

  Catalysts do not affect the position of equilibrium but allow it to be reached faster
• Heterogenous Catalyst 


  These are catalysts that are in a different state of phase from reactants

  Example is the Haber Process, where a solid iron catalyst is used to speed up the reaction between hydrogen and nitrogen gases

  Transition metals make good catalysts due to their variable oxidation states, electrons are transferred to produce a reactive intermediate

  Example is the Contact Process, which uses a vanadium oxide catalyst to speed up the conversion of sulfur dioxide to sulfur trioxide
• Contact Process


  Reactions:

  V2O5 + SO2 ---> V2O4 + SO3

  V2O4 + 1/2O2 ---> V2O5

  Overall:

  2SO2 + O2 --V2O5-> 2SO3

  Vanadium is reduced from 5+ to 4+ and is then reformed in its original oxidation state, showing it has acted as a catalyst
• Adsorption

  A solid catalyst works absorbed molecules onto an active site on the surface of the catalyst

  These active sites increase the proximity of molecules and weaken the covalent bonds in the molecules so that reactions occur more easily and rate is increased

  The strength of the adsorption depends on the type of catalyst and can affect how much it affects the rate of reaction

  Iron, cobalt and nickel make good catalysts - they are cheap and increase the rate the most out of all period 4 transition metals
• Catalyst Poisoning


  Heterogenous catalysts can be poisoned by impurities which block the active sites and prevent adsorption

  Reaction impurities can lead to an increase in chemical production costs as the catalyst has to be cleaned or replaced regularly
• Homogenous Catalysts

  E.g. Reaction between S2O82- ions and I- ions where Fe2+ has to be used as a catalyst (without catalyst, these negative ions would naturally repel each other/never react)

  Reactions: S2O82- + 2Fe2+ ---> 2Fe3+ + 2SO42-

  2Fe3+ + 2I- ---> 2Fe2+ + I2

  Overall: S2O82- + 2I- -Fe2+--> I2 + 2SO42-

  The catalyst works by combining with the reactants to produce a reactive intermediate, this changes the reaction path as the enthalpy for formation of the intermediate is much lower than the original reaction - the activation energy is lower so the reaction is more feasible
• Autocatalysis


  In some reactions, one of the products can act as a catalyst - over time as the amount of product increases, the rate of reaction also increases as it becomes catalysed

  E.g: 4Mn2+ + MnO4- + 8H+ ---> 5Mn3+ + 4H2O

  2Mn3+ + C2O42- ---> 2CO2 + 2Mn2+

  A small amount of initial heating is required to produce some Mn2+ ions so the reaction can be autocatalysed, then rate of reaction increases without further heating
• Ethanedioate ions react with aqueous iron(III) ions in a ligand substitution reaction

  Write an equation for this reaction:

  Suggest why the value of enthalpy change for this reaction is close to zero


  [Fe(H2O)6]3+ + 3C2O42- ---> [Fe(C2O4)3]3- + 6H2O

  Same number and type of bonds are made/broken
• Suggest one reason why people who eat foods with ethanedioate ions do not suffer from poisoning

  Only present in very small amounts