transition metals

• the electron configuration of chromium is 1s2 2s2 2p6 3s2 3p6 3d5 4s1
• the electronic configuration of copper is 1s2 2s2 2p6 3s2 3p6 3d10 4s1
• when transition metal atoms become ions they lose their 4s electrons first as the 4s orbital is at a slightly higher energy level once the 3d orbital starts to fill with electrons
• properties of transition metals:

  • variable oxidation states
  • catalytic properties
  • form coloured compounds
  • form complex ions
• a transition metals is an element that has an ion with an incomplete d sub-shell
• a ligand is a molecule or ion that donates a pair of electrons to from a dative covalent bond to a central metal ion
• a complex ion is a transition metal ion that is bonded to one or more ligands
• a ligand can be monodentate, bidentate or polydentate depending on how many dative covalent bonds it forms and how many pairs of electrons it has available to donate
• a bidentate ligand can donate two pairs of electrons and can form two dative covalent bonds
• in octahedral complexes like hexaaquacopper (II) the bond angles are 90 degrees
• in tetrahedral complexes the bond angles between ligands are 109.5 degrees
• in square planar complexes the bond angels are 90 degrees
• complexes can show cis/trans isomerism like cisplatin

  
• cis-platin is an anti-cancer drug that binds to the DNA of cancer cells, preventing cell division.
• complexes are able to show optical isomerism in any molecule that doesn't have a line of symmetry, optical isomers are non-superimposable on their mirror images, an example is [Co(en)3]2+

  
• haemoglobin is a complex molecule found in red blood cells and transports oxygen around the body, it has an Fe2+ ion at its centre giving the molecule an octahedral shape
• in haemoglobin 5 dative covalent bonds are already taken up so there is space for one more ligand which is normally taken up by an oxygen molecule during gaseous exchange

  
• if carbon monoxide is inhaled a very stable complex called carboxyhaemoglobin is formed in the red blood cells, carbon monoxide is a very toxic gas as it forms a much stronger dative covalent bond to Fe2+ than oxygen, this is a ligand exchange reaction and this is an irreversible reaction and leaves less haemoglobin to which oxygen can bind depriving the cells of oxygen
• when adding concentrated hydrochloric acid to copper sulphate solution it turns from blue through green until the reaction is complete when it turns yellow (ligand exchange)

  
• when slowly adding dilute ammonia solution to copper (II) sulphate solution the blue solution turns into a pale blue precipitate in an acid base reaction

  
• if excess ammonia is then added to precipitate from copper (II) sulphate solution the pale blue precipitate dissolves and forms a dark blue solution due to ligand exchange

  
• when dilute ammonia is slowly added to a solution containing Cr3+ ions there is a colour change from a blue-green solution to a green precipitate through an acid base reaction

  
• when excess ammonia is added to the precipitate formed from solution of Cr3+ ions the green precipitate dissolves and a violet solution is formed by ligand exchange

  
• the decomposition of hydrogen peroxide speeds up considerably when manganese (IV) oxide is added, this shows the catalytic properties of transition metal compounds

  
• zinc normally reacts slowly with acid giving off hydrogen gas, the rate of reaction is increased by adding copper (II) sulphate solution which catalyses the reaction

  
• the oxidation of iron (II) to iron (III) has a colour change of purple to orange

  
• the reduction of iron (III) back to iron (II) is done by adding iodide ions from potassium iodide which is a mild reducing agent, the colour change is orange to green/brown (the final colour is dominated by iodine)

  
• hydrogen peroxide is a strong enough oxidising agent to convert chromium (III) all the way to chromium (VI) the colour change is blue/green to yellow

  
• the conversion of CrO4- to Cr2O72- is completed by adding acid in the form of H+ ions the colour change is orange to yellow

  
• the reduction of chromium (VI) back to chromium (III) is done by adding a reducing agent like zinc the colour change is orange to blue/green

  
• reduction of copper (II) to copper (I) can be done with potassium iodide, the colour change is blue to brown, CuI is a white compound as a Cu+ ion does not have an incomplete d sub-shell

  
• conversion of copper (I) back to copper (II) can only be done by disproportionation when sulphuric acid is added to copper (I) oxide and an acid base reaction occurs and produces Cu+ ions which are unstable in solution

  
• the reaction of Cu2+ with sodium hydroxide forms a pale blue precipitate from a blue solution and the precipitate doesn't dissolve if excess sodium hydroxide is added

  
• the reaction of Fe2+ with sodium hydroxide produces a green precipitate from a green solution, the precipitate doesn't dissolve when excess sodium hydroxide is added

  
• the reaction of Fe3+ with sodium hydroxide produces a brown precipitate from a yellow/orange solution, the precipitate doesn't dissolve when excess sodium hydroxide is added

  
• the reaction of Mn2+ with sodium hydroxide produces a yellow/brown precipitate which darkens as it oxidises from a colourless solution, the precipitate does not dissolve when excess sodium hydroxide is added

  
• the reaction of Cr3+ with sodium hydroxide produces a green precipitate from a blue/green solution and the precipitate dissolves in excess sodium hydroxide

  
• the reactions of transition metal ions with ammonia are the same as with sodium hydroxide as the ammonia solution is an alkali so there will be some OH- ions available to form the hydroxide precipitates
• only hydroxide precipitates of Cu2+ and Cr3+ dissolve in excess ammonia, Cu2+ forms a dark blue solution and Cr3+ forms a violet solution
• the catalytic properties of transition metals are often associated with the variable oxidation states of the elements and empty readily available electron orbitals