The chemistry of transition metal compounds
The chemistry of transition metal compounds is characterised by variable oxidation states, coloured complex ions and extensive redox reactions. Any one of the first row transition metals could be used to exemplify these types of reaction.
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The chemistry of copper
Copper is element number 29 in the periodic table. It has an electronic configuration of [Ar] 4s1 3d10, but, perhaps surprisingly, its most common oxidation state is +2.
Copper sulfate pentahydrate, CuSO4.5H2O, is probably the most well known of all of the compounds encountered in chemistry education. It is bright blue crystalline compound which loses water of crystallisation on heating to form the white anhydrous salt. This is a reversible reaction and addition of water caused a release of energy and the blue hydrated copper sulphate is regenerated.
CuSO4.5H2O CuSO4 + 5H2O
Dissolution of copper salts
Copper salts dissociate in aqueous solution and the metal ions are complexed by the water molecules:
Cu2+ + 6H2O(l) [Cu(H2O)6]2+(aq)
The octahedral hexaaquacopper(II) complex ions are blue in solution. This is the species that is produced by all copper salts when dissolved.
Addition of concentrated hydrochloric acid
Hydrochloric acid contains a high concentration of chloride ions. These undergo a ligand replacement reaction with the water ligands in the hexaaquacopper(II) complex.
[Cu(H2O)6]2+ + 4Cl- [CuCl4]2- + 6H2O(l)
The complex changes coordination number from 6 to 4 (the chloride ions cannot fit around the central atom in an octahedral arrangement) and the solution changes from blue to bright green.
Addition of potassium iodide solution
This is a redox reaction with the copper(II) complex being reduced to copper(I) while the iodide ions get oxidised to iodine. The colour change is from a blue solution to a white suspension of copper(I) sulfate mixed with the brownish iodine.
2Cu2+ + 2I- 2Cu+ + I2
This reaction may be used to determine copper solutions. The iodine released can be titrated against sodium thiosulfate using starch solution as an indicator.
2S2O32- + I2 S4O62- + 2I-
When the blue-black colour of the starch indicator disappears the end-point has been reached.
Addition of sodium hydroxide solution
Sodium hydroxide provides hydroxide ions that replace abstract protons from the water ligands around the copper ion. Once two protons have been removed the copper complex becomes neutral and precipitates from solution. This can be represented in a simple format:
Cu2+(aq) + 2OH-(aq) 2Cu(OH)2(s)
There is no further change on addition of excess hydroxide ions.
Addition of ammonia solution
Ammonia is also able to abstract hydrogen ions from the water molecule ligands around the central copper(II) ion:
[Cu(H2O)6]2+(aq) + 2NH3(aq) [Cu(H2O)4(OH)2](s) + 2NH4+(aq)
However, in this case, addition of more ammonia causes the blue precipitate to redissolves as the ammonia molecules replace the water (and hydroxide) ligands around the central copper(II) ion.
The final complex contains four ammmonia ligands arranged in a square planar arrangement (the equatorial positions of an octahedral structure) with two axial water ligands.
[Cu(H2O)4(OH)2](s) + 4NH3(aq) [Cu(NH3)4(H2O)2]2+(aq) + 2H2O(l) + 2OH-(aq)
The final solution is the deep blue tetraamminediaquacopper(II) ion solution. In many cases this is shortened to the tetraamminecopper(II) complex.
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