Transition Metals

Cards (77)

  • a transition metal is an element with an incomplete d sub-shell in its atoms or in one of its common ions
  • properties of transition metals:
    • variable oxidation states
    • catalytic action
    • coloured compounds
    • form complexes
  • a ligand is a particle with a lone pair of electrons that bonds to metals through a co-ordinate bond
  • complex - metal ion with co-ordinately bonded ligands
  • co-ordination number - the number of co-ordinate bonds from ligands to metal ion
  • a lewis base is a lone pair donor
  • a lewis acid is a lone pair acceptor
  • complexes are formed when ligands form co-ordinate bonds to a metal ion
    • ligands act as Lewis bases when they bond to transition metals as they donate a lone pair of electrons to form a co-ordinate bond
    • the metal ion acts as a Lewis acid as it accepts lone pairs
  • shapes of complexes:
    • linear
    • square planar
    • tetrahedral
    • octahedral
  • linear complexes:
    • co-ordination number is 2
    • bond angle is 180
    • Ag+ typically forms this complex shape
  • square planar complexes:
    • co-ordination number is 4
    • bond angle is 90
    • occurs in Pt2+ and Ni2+ complexes
  • tetrahedral complexes:
    • co-ordination number is 4
    • bond angle is 109.5
    • occurs with larger ligands such as Cl-, when ligands are too big for 6 to fit
  • octahedral complexes:
    • co-ordination number is 6
    • bond angle is 90
    • occurs in most complexes with small ligands e.g. H2O, NH3
  • all ligands must have at least one lone pair of electrons to form a co-ordinate bond to a metal ion
  • unidentate ligands:
    • ligands that form one co-ordinate bond to a metal ion
    • e.g. Cl-, OH-, CN-, H2O, NH3
  • bidentate ligands:
    • ligands that form two co-ordinate bonds to a metal ion
    • 1,2-diaminoethane (en), ethanedioate ion (C2O4 2-)
  • multidentate ligands:
    • ligands that form more than two co-ordinate bonds to a metal ion
    • e.g. EDTA 4-, porphyrin
    • EDTA forms 6 bonds
    • porphyrin forms 4 bonds
  • haemoglobin:
    • globular protein
    • contains four Fe2+ centres, each with a porphyrin ligand taking up four of the six co-ordination sites
    • one of the remaining two sites is bonded to the rest of the haemoglobin structure and the last site is left for oxygen to bond to as a ligand
    • oxygen isn't a very good ligand so drops off easily
    • CN- and CO are better ligands than oxygen and can bind to haemoglobin more easily - CO and cyanide compounds are toxic as they prevent transfer of oxygen in the body
  • stereoisomerism:
    • cis-trans isomerism
    • optical isomerism
  • cis-trans isomerism:
    • special case of E-Z isomerism
    • occurs in octahedral and square planar complexes where there are two ligands that are the same and are different to the other ligands
    • cis - next to each other
    • trans - opposite each other
    • e.g. cisplatin and transplatin
  • optical isomerism:
    • occurs in an octahedral complex with three bidentate ligands
  • a ligand substitution reaction is a reaction where one ligand is replaced by another ligand
  • ligand substitution:
    • if the ligands are similar in size then there is no change in co-ordination number after a ligand substitution reaction
    • if the ligands are a different size then the co-ordination number may change
    • e.g. Cl- ligands are much bigger than the O in H2O so only four Cl- ligands can bind to most transition metal ions as opposed to 6H2O ligands
  • chelate effect:
    • ligands can be replaced by ligands that form more co-ordinate bonds, e.g. six monodentate ligands can be replaced by three bidentate ligands
    • in many ligand substitution reactions, ΔH is negligible as the same number of the same type of bonds are being broken and formed
    • when ligands are replaced by ligands that form more co-ordinate bonds there is a significant increase in entropy
    • if ΔH is negligible and ΔS is very positive then ΔG is very negative and the reaction is feasible
  • if ligands are replaced by ligands that form fewer co-ordinate bonds then the reaction would not be feasible because of the decrease is entropy
    • ligands that form more than one co-ordinate bond are called chelating agents
    • they are very good at bonding to a metal ion and are very difficult to remove
    • this makes the metal harmless as it cannot bond to anything else - cure for metal poisoning
    • EDTA4- is a strong chelating agent as it forms 6 co-ordinate bonds
  • vanadium oxidation states and colours:
    • 5+ is yellow
    • 4+ is blue
    • 3+ is green
    • 2+ is violet
  • transition metals have variable oxidation states because the electrons sit in 4s and 3d orbitals which are very close together
  • the shape of the complex is dependent on the size of the ligands and the co-ordination number
    • total oxidation state of the metal = total complex oxidation state - total oxidation state of ligands
    • the total complex oxidation state is the charge outside the bracket
  • d-orbital splitting:
    • the d subshell is split into 2 when ligands bond with the central metal ion
    • in the cobalt ion example, when ligands bond to the ion some orbitals gain energy which creates an energy gap - ΔE
    • when electrons absorb light energy some move from the lowest energy level (ground state) to higher energy level orbitals (excited state)
    • for this to happen, the energy from the light must equal ΔE
  • the size of ΔE is dependent on:
    • the central metal ion and it's oxidation state
    • the type of ligand
    • the shape of the complex
    • co-ordination number
  • the factors that affect ΔE:
    • affect the size of the energy gap between d orbitals so the wavelength of visible light absorbed is also affected
  • coloured complexes:
    • some frequencies of visible light are absorbed by transition metal complexes
    • the frequencies absorbed depends on the size of ΔE
    • the larger the energy gap the higher the frequency of light absorbed
    • any frequencies that aren't absorbed are reflected or transmitted
  • complexes that have a full or empty 3d subshell are observed as colourless or white because there are no electrons that can migrate to the higher energy level
  • colourimetry:
    • transition metal complexes can be analysed using colorimetry
    • the colours we see are complementary to the colours that the solution absorbs
    • any frequencies that are not absorbed are reflected or transmitted
    • white light is made up of the colours in the rainbow, when white light hits a transition metal only one frequency is absorbed
  • absorption colour wheel:
    A) colour of light absorbed
    B) directly
    C) opposite
    D) absorbed
  • colourimetry is used to measure the concentration of transition metal ions in a solution
    make samples by diluting different concentrations of the metal ion
    use the same metal ion and solvent as the one being tested
    use a pipette to ensure accurate volumes are used
  • colorimetry process:
    • colourimeter is set to zero/calibrated by measuring the absorbance of a blank sample e.g. distilled water
    • white light is filtered into a narrow range of frequencies using a light filter to produce a monochromatic light (single colour)
    • the monochromatic light passes through the sample which is held in a cuvette and some of the light is absorbed
    • light that isn't absorbed travels to the detector - this measures the level of absorbance by comparing the absorbance to the blank sample
    • colorimeter must be calibrated after every sample