Transition elements
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In older textbooks, the hydrated ion of Cu2+ was the tetrahydrate and still described as so in coppersulfate pentahydrate. Nowadays it is described as the hexahydrate. Yet the complex ion formed with ammonia as ligand has 4 ammonia molecules. Does it also have 2 water molecules (and why not 6 ammonia) i.e. maintain a coordination number of 6 and if so does this mean then that 2 isomers could exist a cis and trans with water molecules sharing a face of the octahedron in the former. If this were so one would expect two different absorption spectra - as far as I know this is not so yet I have seen nothing reported on it.
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Ulex writes
A good deal depends on the state in which the copper(II) ion exists. If we start with the situation in hydrated copper(II) sulphate, the copper(II) ion is complexed with not only water molecules but also the sulphate ion as ligands. As you point out, current thinking favours the copper(II) ion in aqueous solution as being hexahydrated. There is evidence, however, that the ammonia complex has four ammonia molecules arranged in a square-planar manner with water molecules in the other two octahedral positions. When this complex crystallises, this arrangement is maintained – there are no isomers.
Why four ammonias and not six? The only answer one can give is that the square planar arrangement with the two water molecules is thermodynamically more stable than the hexamine.
If you delve deeper into this you find something called the Jahn-Teller effect, which the Cu2+ ion exhibits as a result of its d9 electronic configuration. I won't go into detail here, suffice it to say that the result is that the perfect octahedral arrangement becomes distorted and the ligand-metal distance for two of the ligands opposite each other is longer (i.e. weaker bonding) than the four arranged in a plane.
The situation is yet more strange in the copper(II) ion in solution with chloride ions. The formula then is CuCl42- with the four chloride ions arranged tetrahedrally. The solution is green in colour, but on dilution the chloride ions are replaced by water molecules and the colour turns to blue, indicating the hexahydrate.
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updated: 28 July 2008
