• Category:Transition Metals Revision Notes

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These have been grouped br the type of complex, but you may prefer to group them by metals. I found it a lot easier to remember these when I realised Fe (II) is always green, Al (III) is always white as a ppt and colourless as a sol. Cu (II) is almost always blue etc. etc. The reason that aluminium complexes are never coloured is that they do not have a d-orbital, as Aluminium is not a transition metal. This means that its complexes cannot absorb light in the visible region.

Hexa-aqua complexes

[Co(H2O)6]2+ - Pink sol. [Cu(H2O)6]2+ - Blue sol. [Fe(H2O)6]2+ - Green sol. [Fe(H2O)6]3+ - Violet sol. (often looks orange in lab experiments, due to impurities) [Cr(H2O)6]3+ - Violet sol. [Al(H2O)6]3+ - Colourless sol.

These may sometimes be written as Co2+ (aq) but it is more correct to write the full complex

Hydroxide Complexes

When a small amount of OH- is present (or dilute ammonia), the 2+ ions form a [M(H2O)4(OH)2] precipitate. All 3+ ions form [M(H2O)3(OH)3] ppt. When OH- is added to the hexa-aqua complex, the OH- ions DO NOT substitute the water molecules. It is an acid-base reaction, where the OH- ions in solution take H+ ions away from the water molecules in the complexes. In the case of dilute ammonia, the ammonia molecules take in an extra proton to become NH4+. This can happen because the metal ions are highly polarizing. The following complexes are written in shorthand, as I have already shown you the longhand version.

Co(OH)2 - Blue ppt Cu(OH)2 - Blue ppt Fe(OH)2 - Green ppt Fe(OH)3 - Brown ppt Cr(OH)3 - Green ppt Al(OH)3 - White ppt

It is worth noting that Fe (II) hydroxide can be oxidised by the air to make brown Fe (III) hydroxide.

In the presence of excess hydroxide ions, two of these complexes can redissolve. This is because they react further with hydroxide ions to produce a charged complex. This means they are amphoteric.

[Cr(OH)6]3- Green sol. [Al(OH)4]1- Colourless sol.

Ammonia Complexes

Of the 6 main metals we currently need to know about for the A-level spec, three of them react with excess concentrated ammonia to form ammonia complexes. Dilute ammonia will only yield a hydroxide complex. The three which react, all conveniantly begin with the letter 'C' which is how I always remembered it. Please note that the copper complex is slightly different to the other two.

[Cu(NH3)4(H2O)2]2+ Deep blue sol. [Co(NH3)6]2+ Yellow sol. [Cr(NH3)6]3+ Purple sol.

The Cobalt complex here can also be oxidised by the air and turn brown.


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