Transition Metals - Formation of complex ions

Today, we will discuss the fomation of complex ions, specifically aqua ions.

Aqua ions occur when d-block cations dissolve in water and become hydrated (i.e. surrounded by water molecules). The oxygen atom in the water molecule has a lone pair of electrons that forms a bond with an empty 3d or 4p orbital in the metal ion.

The theory behind the exact nature of the bonding is beyond the scope of A-level. All you need to know is that a dative covalent bond forms with the oxygen atom as the donor atom.
There are other theories that tend to explain the bonding such as: the electrostatic forces of attraction between the delta negative oxygen and the positive metal ion.

An example of an aqua ion is the hydrated chromium (III) ion, [Cr(H₂O)₆]³⁺
The water molecules are called ligands

One of the lone pairs of electrons on the oxygen atom of each water molecule forms a dative covalent bond with an empty orbital in the Cr³⁺ ion. Six dative bonds form so the hydrated ion has the coordination number 6.

The ion, with its water molecules bonded to the central metal ion, is called a complex ion.

The coordination number is the number of near neighbouring atoms that are bonded to the central ion.

Ligands are organic molecules that donate the necessary electrons to form dative (coordinate) covalent bonds with metallic ions. (e.g. H₂O, NH₃)

Note: When drawing the hydrated ions make sure that the dative covalent bonds all start from the O of the H₂O and not from the H.

There are six dative bonds, each containing a pair of electrons. These six pairs of bonding electrons repel each other to the position of maximum separation and minimum repulsion. The shape is therefore a octahedral.

All complex ions with coordination number 6 are octahedral.

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Copper:

In the solid state, hydrated copper (II) ions have four water molecules arranged in a plane around Cu²⁺ion. However, in aqueous solution, two more water molecules are weakly bonded at right angles, forming an octahedron. The two non planar water molecules are further from the copper ion than are the four planar water molecules. Because of this, the formula of the hydrated ion in solution is sometimes written as [Cr(H₂O)₄]²⁺ instead of [Cu(H₂O)₆]²⁺