3.2.5 Transition metals

• Properties of transition metals:

  > Catalytic activity

  > variable oxidation states

  > ability to form complexes

  > The ability to form coloured ions
• A transition metal is an element whose atom or ion has an incomplete d sub-level
• Ethanedioate ions have the formula (C2O4)2- When MnO4-(aq) is titrated with Ethanedioate ions 2 observations are seen

  • The purple colour decolourises slowly at first
  • As more MnO4- is added the purple colour decolourises inmediately.
• An autocatalyst is where the catalyst for the reaction is a reaction product
• A ligand is an ion or molecule that can donate a lone pair of electrons to the central metal ion by a co-ordinate bond
• A complex is a central metal ion surrounded by ligands
• Types of mono dentate ligands

  Negative ions

  Halides, :OH- , :CN-

  Neutral molecules

  :NH3 , H2O:
• A ligand that can donate one lone pair of electrons is a unidentate / mono dentate ligand.
• A ligand that can donate two lone pair of electrons is a bidentate ligand.
• A ligand that can donate more than 2 lone pair of electrons is a multidentate ligand.
• Co-ordination number is the number of pairs of electrons donated to the central metal ion

  The numbers are 2,4,6
• The central metal ion has a positive charge

  The overall charge on the complex depends on the charge of the metal ion and the charge on the ligands
• Shapes of complexes

  For mono dentate ligands:

  > 2 ligands give a linear shape (180)

  > 4 ligands give a tetrahedral shape (109.5) or square planar (90)

  > 6 ligands give an octahedral shape (90)
• Square planar complexes

  Only Ni and Pt can form a square planar complexes

  Show lone pairs on correct atom on the ligand

  Type of bonding - co-ordinate bonding
• Linear complexes (monodentate)

  Silver forms complexes with a co-ordinate number of 2

  > The shape of the complex is linear

  e.g. Diamminesilver (I) ion , [Ag(NH3)2] used in Tollens reagent

  [H3N: ->Ag+<-:NH3]

  The silver ions is reduced to silver

  Aldehyde is oxidised to carboxylic acid
• Tetrahedral complexes

  > Large charged ligands, such as the chloride ion (Cl-) and (CN-) form tetrahedral complexes

  > The ligands are large in size and charged so four can fit around the central metal ion

  > This also minimises repulsion between the ligands

  e.g. Tetrachlorocuprate(II)ion [CuCl4]2-

  
• Octahedral complexes

  > Small uncharged ligands such as water (H2O) and ammonia (NH3) form octahedral coplexes

  > Ligands are small in size, so six can fit can fit around the central metal ion

  e.g. Hexaaquacopper(II)ion, [Cu(H2O)6]2+

  
• Multidentate complexes

  The two multidentate ligands are

  > EDTA4- ( can donate 6 lone pairs of electrons )

  > Haem ( can donate 4 lone pairs of electrons )

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

  Oxygen forms a co-ordinate bond to Fe(II) in haemoglobin, enabling oxygen to be transported in the blood.

  Carbon monoxide is toxic because it replaces oxygen co- ordinately bonded to Fe(II) in haemoglobin.
• Stereoisomerism and Complexes

  Complexes can show two types of stereoisomerism:

  > cis-trans isomerism (same as E-Z isomerism)

  > optical isomerism

  Cis-trans isomerism is when two ligands are on the opposite or the same side as each other

  Optical isomerism

  Complexes with 3 bidentate ligands can form two optical isomers ( non-superimposable mirror images)
• Cisplatin

  formula : [PtCl2(NH3)2]

  Name: Diamminechloroplatinum

  shape: square planar

  The Pt(Il) complex cisplatin is used as an anticancer drug

  The cisplatin version only works as two chloride ions are displaced and the molecule joins on to the DNA. In doing this it stops the replication in cancer cells by a ligand substitution reaction with DNA in which a coordinate bond is formed between platinum and a nitrogen atom on guanine

  Side effect

  > Kills healthy cells by attaching to the DNA

  Limited dosage and target specific site will limit the effects of the side effects
• Mechanism of action (cisplatin)

  > Cisplatin is coverted into a reactive form inside cells

  > One of the chloride ligands is substituted by a water ligand

  [PtCl2(NH3)2] + H2O -> Cl- + [PtCl(H2O)(NH3)2]+

  > The water ligand is then subsituted by a guanine base on DNA

  > The complex is attached to the DNA 

  > The other Chloride ligand is then substituted by another guanine base on the DNA 

  > The complex binds to the DNA and stops the cell dividing causing it to die.
• Electromagnetic spectrum

  > A substance appears coloured if it absorbs, some of the wavelengths (colours) of visible light. The colour we observe is due to the wavelengths (colours) of visible light which are transmitted
• Electronic transitions

  Colour arises from electronic transitions from the ground state to excited states: between different d orbitals. A portion of visible light is absorbed to promote d electrons to higher energy levels. The light that is not absorbed is transmitted to give the substance colour.
• A solution will appear blue if it absorbs orange light. The energy split in the d orbitals ΔE will be equal to the frequency of orange light(5 x1014 s-1) x Planck’s constant ΔE in a blue solution = hv

  =6.63×10–34 x5x1014 = 3.32 × 10–19 J
• Energy difference between ground state and excited state

  ΔE=hv orΔE=hc/λ

  v = frequency of light absorbed (unit s-1 or Hz)

  ΔE = energy difference between split orbitals (J)

  h = Planck’s constant 6.63 × 10–34 (J s)

  c = speed of light 3.00 x 108 (m s–1)

  λ = wavelength of light absorbed (m)
• d-d transitions

  d-d transitions can only occur if there is a d electron to promote

  and an empty space in the higher energy d sub-level

  > Therefore a transition metal ion must have an incomplete d sub-level to be coloured
• The presence of ligands cause the d sub-levels to split into two different energy levels
• Factors affecting colour

  > Oxidation state of the metal

  > Type of ligand

  > Co-ordination number
• Explain fully how a colorimeter can be used to determine the concentration of an unknown sample

  Add an appropriate ligand to intensify the colour (if the colour of the complex is pale)

  > Make up solutions of known concentration

  > Measure absorption

  > Plot a calibration curve

  > Measure absorption of unknown on the calibration curve and read the concentration
• Choosing the correct acid for manganate titrations

  Using a weak acid like ethanoic acid would have the same effect as it cannot supply the large amount of hydrogen ions needed (8H+).

  It cannot be conc HCl as the Cl- ions would be oxidised to Cl2 by MnO4- as the E MnO4-/Mn2+ > E Cl2/Cl- This would lead to a greater volume of manganate being used and poisonous Cl2 being produced.

  It cannot be nitric acid as it is an oxidising agent. It oxidises Fe2+ to Fe3+ as Eo NO3-/HNO2> Eo Fe3+/Fe2+ This would lead to a smaller volume of manganate being used.
• Chelation

  The substitution of monodentate ligand with a bidentate or a multidentate ligand leads to a more stable complex. This is called the chelate effect.

  This chelate effect can be explained in terms of a positive entropy change in these reactions as there are more molecules of products than reactants.
• Chelation

  [Cu(H2O)6]2+ (aq) + EDTA4- (aq) -> [Cu (EDTA)]2- (aq) + 6H2O (l)

  The copper complex ion has changed from having unidentate ligands to a multidentate ligand.

  In this reaction there is an increase in the entropy because there are more moles of products than reactants (from 2 to 7), creating more disorder.

  The enthalpy change is small as there are similar numbers of bonds in both complexes. Free energy ΔG will be negative as ΔS is positive and ΔH is small.
• Chelation

  Co(NH3)6]2+ + 3NH2CH2CH2NH2 -> [Co(NH2CH2CH2NH2)3]2+ + 6NH3

  This reaction has an increase in entropy because of the increase in moles from 4 to 7 in the reaction.  ΔS is positive.

  Its enthalpy change ΔH is close to zero as the number of dative covalent and type (N to metal coordinate bond) are the same so the energy required to break and make bonds will be the same. Therefore Free energy ΔG will be negative and the complex formed is stable.
• Bidentate ligands

  The two bidentate ligands are

  > 1,2-diaminoethane

  > Ethanedioate ion

  Three bidentates ligands can fit around the central metal ion

  > This gives an octahedral complex with co-ordination number of 6

  A) 1,2-diaminoethane

  B) ethanedioate ion