OCR Advanced GCE in Chemistry/Chemistry of Transition metals


Vanadium is a silver-grey metal which is very resistant to corrosion.

It can form compounds in the +5,+4,+3 and +2 oxidation states



Cobalt (Atomic no 27 relative mass no 59) is an unreactive white metal with a slight blue appearance and forms compounds with +2 and +3 oxidation states. The +2 state is usually the most stable.


Co2+ forms the complex molecule [Co(H2O)6]2+ which is a pink octahedral complex ion.

Co2+ also forms the tetrahedral complex ion [CoCl4]2- which is blue.

Cobalt chloride paper can therefore be used to test for water because the complex [Co(H2O)6]2+ is formed. This causes the blue cobalt chloride paper to turn pink.

This reaction is an equilibrium reaction:

[CoCl4]2-(aq) + 6H2O(l) <---> [Co(H2O)6]2+(aq) + 4Cl-(aq)

The reaction is exothermic in the forward reaction. By adding either heat or a high concentration of Cl- the reverse ligand substitution reaction will occur.


Co3+ forms the aqueous complex [Co(H2O)6]3+ which is blue but is so easily reduced to [Co(H2O)6]2+ that we do not really look at it. Other simple compound of Co3+ become hydrated and are reduced in the same way.

Co3+ does however make other complex ions with different ligands and they prove to be very stable indeed:


[Co(H2O)6]3+ + e- <---> [Co(H2O)6]2+; E= +1.81V

[Co(NH3)6]3+ + e- <---> [Co(NH3)6]2+; E= +0.11V

Here the Electrode potential values show that the Co3+ complex without H2O ligands (Hexaamminecobalt ll) is the more stable. This is because ammonia has only one lone pair of electrons resulting in stronger dative bonds.


Last modified on 14 May 2009, at 00:42