2.5 Transition Metals

• what is a transition metal?

  metals that have an incomplete d sub-shell
• what are chemical properties of transition metals?

  -form coloured compounds

  -can form complexes

  -variable oxidation states

  -can act as catalysts
• what is a ligand?

  a molecule, atom or ion that has one or more lone pair of electrons that it can donate to a central metal ion
• what is a complex?


  a central metal ion surrounded by ligands that form co-ordinate bonds
• what is a co-ordination number?

  the number of co-ordinate bonds formed by ligands around the central metal ion
• what is a monodentate ligand? give examples

  donates one pair of electrons to a central metal ion

  e.g. H2O, NH3, Cl-
• what is a bidentate ligand? give examples

  donates two pairs of electrons to the central metal ion

  e.g. 1,2-diamonethane (H2NCH2CH2NH2), ethanedioate ions (C2O42-)
• what is a multidentate ligand? give examples

  donates three or more electron pairs to the central metal ion

  e.g. EDTA4-, haemoglobin
• what shape contains 2 coordinate bonds?

  linear
• what shape forms 4 coordinate bonds?

  tetrahedral or square planar
• what shape forms 6 coordinate bonds?

  octahedral
• what is a ligand substitution reaction?

  where one ligand is substituted for another. this involves breaking the coordinate bond between the transition metal ion and the ligand
• what are substitution reactions of monodentate ligands?

  NH3 and H2O ligands are similar in size and are neutral.

  Cl- ligand is larger than the uncharged ligands and the repulsion between 6 Cl- ions is too great so only 4 bond to a central metal ion therefore complexes with CL- ligands form tetrahedral shapes
• what is the exception with [Cu(H2O)6]2+ and NH3

  incomplete substitution occurs unless you increase the concentration of ammonia solution. it forms [Cu(NH3)4(H2O)2]2+ complex. the ammonia ligands take the square planar position with the water ligands taking position above and below the metal ion
• How is oxygen transported in the blood?

  oxygen forms a 6th coordinate bond to the Fe2+ ion and this transports it around the blood
• why is carbon monoxide toxic.

  C-O bons at the site where O2 bonds. the CO forms a strong bond to the Fe2+ that is irreversible and therefore O2 can't bind so less RBC being transported around the body
• what is the chelating effect?


  as the multidentate ligands form several bonds with the same metal ion, meaning they act as a 'claw' around the central ion
• how does the ligand affect stability?

  the more bonds a single ligand can make with the metal ion the more stable the resulting complex will be
• how can you explain the stability of complexes with chelating ligands?

  the increase in entropy of the system

  △G = △H-T△S

  △G must be negative for a reaction to be feasible
• how does entropy increase?

  increase in the number of particles means an increase in disorder and therefore entropy
• what does the shape of a complex depend on?

  the number of coordinate bond the ligand can make with the central metal ion
• what type of isomerism can octahedral complexes display?

  cis-trans isomerism

  optical isomerism - when there are two or more bidentate ligands in the complex and exist as two non-superimposable mirror images
• what does the colour of a complex depend on?

  -oxidation state of metal centres

  -ligands attached

  - electron configuration of d sublevel electron

  -coordination number
• what are transition element ions coloured?


  colour arises when some of the wavelengths of visible light are absorbed and the remaining wavelengths of light are transmitted or reflected which appears the colour of this light . this results from the transfer of an electron between the orbitals in an unfilled d sub-level.
• how does an incomplete d sub-level occur?

  an electron can be 'promoted' from the ground state (lower energy state) to excited state (higher energy state) by absorbing certain frequencies of radiation from the visible light spectrum
• what factors can change the colour of a complex?

  -oxidation state

  -change in ligand

  -shape/co-ordination number
• what does the quantity of radiation absorbed depend on?

  -concentration - more ions for light to interact with

  -distance light has to travel

  (more light absorbed = less light reflected)
• why do transition metals undergo many redox reactions?

  the oxidation state of the metals within a complex change. in a redox reaction the transition metal ion is oxidised or reduced
• what are redox potentials?

  shows how easy it is to be reduced to a lower oxidation state. more +ve value, the more likely the ion is to be reduced. equations are shown as reduction so to show oxidation flip the equation around
• how does pH affect the size of the redox potential?

  more +ve in acidic conditions because the ion is more likely to be reduced
• Give an equation to show MnO4- being reduced to Mn2+.

  MnO4- + 5e- + 8H+ --> Mn2+ + 4H2O
• Give an equation where Fe2+ acts as the reducing AGENT

  Fe2+ --> Fe3+ + e-
• Give an overall equation to show Fe2+ reacting with MnO4-.

  MnO4- + 5Fe2+ + 8H+ --> Mn2+ + 5Fe3+ + 4H2O
• Give an equation where C2O42- acts as a reducing agent.


  C2O42_ --> 2CO2 + 2e-
• give the overall equation to show C2O42- reacting with MnO4-.

  2MnO4- + 5C2O42- + 16H+ --> 2Mn2+ + 10CO2 + 8H2O
• Whys is no indicator used in these titrations?

  MnO4- is self indicating so turns from colourless to pink

  (due to the presence of the first drop of MnO4- in excess)
• what suitable acid should be used?

  dilute H2SO4
• What acids are unsuitable and give a reason for each?

  -conc H2SO4: oxidises some of the Fe2+

  -conc HNO3: oxidises some of the Fe2+

  -HCl: oxidised by MnO4- to form Cl2

  -ethanoic acid: weak acid s not enough H+
• what do catalysts do?


  affect the rate of the chemical reaction without being chemically changed
• what allows transition metals to act as catalysts?


  ability to form more than one stable oxidation state means that they can accept and lose electrons easily