Transition Metals- Colours, Substitution and Precipitation

Transition metal elements form ions with variable oxidation states. They all form compounds with a +2 oxidation state due to the loss of electrons from the 4s orbital as it is at the highest energy level. 

The highest oxidation state of a transition element is often found in strong oxidising agents. Managanese forms potassium permanagante.

When white light passes through a solution containing transition metal ions, some wavelengths of visible light are absorbed. A solution of copper (II) sulfate appears pale blue because the solution absorbs the red/orange region of the electromagnetic spectrum and reflects or transmits blue. 

A precipitation reaction is one in which soluble ions, in separate solutions, are mixed together to form an insoluble compound- that settles out of solution as a solid.

Transition metals in aqueous solution react with aqueous sodium hydroxide to form coloured precipitates.A blue solution of copper (II) ions reacts with aqueous sodium hydroxide to form a pale blue precipitate of copper (II) hydroxide:

Cu2+ (aq) + 2OH- (aq) ---> Cu(OH)2 (s)

Pale blue solution               Pale blue precipitate

Cobalt (II), iron (II) and iron (III) also form precipitates with sodium hydroxide.

A ligand substitution reaction is one in which a ligand in a complex ion is replaced with another ligand. An aqueous solution of copper (II) ions contains [Cu(H2O)6]2+ complex ions which have a characteristic pale blue colour. When an excess of aqueous ammonia solution is added, the pale blue solution changes colour and a deep blue solution forms. 

[Cu(H2O)6]2+ + 4NH3 -><- [Cu(NH3)4(H2O)2]2+ + 4H2O

The product has six ligands and is an octahedral shape. The copper-oxygen bonds are longer than the copper-nitrogen bonds, so the shape is described as a distorted octahedral shape.

When concentrated hydrochloric acid is added to solution containing copper (II) ions, the pale blue solution initially forms a green solution before turning yellow. This reaction exists in equilibrium and can be reversed by adding water to return it to the original blue colour.

[Cu(H2O)6]2+ + 4Cl- -><- [CuCl4]2+ + 6H2O

The [CuCl4] complex only has four ligands as there is a stronger repulsion and the chloride ligands are larger, so fewer chloride ions can fit around the central metal ion. [CuCl4] has a tetrahedral shape.

An aqueous solution of cobalt (II) ions contains [Co(H2O)6]2+ complex ions, which have a characteristic pink colour. If concentrated hydrochloric acid is added, the solution forms a dark blue solution. The concentrated hydrochloric acid provides a high concentration of chloride ions, replacing the six water molecules with four chloride ions.

[Co(H2O)6]2+ + 4HCl- -><- [CoCl4]2- + 6H2O