5.3. Transition Elements: Module 5: Physical Chemistry & Transition Elements: Chemistry OCR A A Level

Transition Metals

The transition metals are elements in the d-block of the periodic table that form one or more stable ions with a partially filled d-orbital. Transition metals lose electrons to form positive ions, with the s-orbital electrons being removed first.

Copper and chromium

Copper and chromium are exceptions to the rule that the 4s subshell is filled before the 3d subshell.

Ligands

A ligand is a molecule or ion with a lone electron pair that is able to form a dative (coordinate)

bond to the central metal ion by donation of this electron pair. Common ligands include:

● Cl-

● H2O

● NH3

Monodentate

Monodentate ligands each form one coordinate bond.

Examples:

H2O, Cl-, NH3.

Bidentate

Bidentate ligands each form two coordinate bonds.

Example: NH2CH2CH2NH2.

Coordination Number

The coordination number of a transition metal complex is the total number of coordinate bonds formed with the central metal ion. Silver complexes have a coordination number of 2 and platinum complexes commonly have a coordination number of 4. The coordination number determines the shape of the complex ion.

Colourless Ions

Some metal ions and complexes are colourless. Colour arises when some wavelengths of light are absorbed while others are reflected. This involves the excitation of electrons. Ions and complexes where there are no available electrons to excite cannot absorb light and are therefore colourless.

Octahedral Complexes

Transition metal complexes with H2O and NH3 ligands commonly form octahedral complexes with a bond angle of 90°. Octahedral complexes are formed when there is six-fold coordination.

Tetrahedral Complexes

When complexes form with larger ligands such as Cl-, they form tetrahedral complexes with a bond angle of 109.° and 4 ligands coordinated. Tetrahedral complexes can show optical isomerism.

Square Planar Complexes

Platinum and nickel complexes form in a square planar shape. This consists of four coordinate bonds with a bond angle of 90°.

Optical Isomerism

Optical isomerism is seen in some octahedral complexes with bidentate ligands. These complexes have non-superimposable mirror images.

Chelate Effect

In ligand substitution reactions, a positive entropy change is favourable as it means a more stable complex is being formed. Therefore, it is favourable to have more moles on the right of the reaction than on the left. This is achieved by substituting monodentate ligands with bidentate or polydentate ligands. This is known as the chelate effect.

Enthalpy Change

The enthalpy change (∆H) for ligand substitution reactions is very small as the bonds being formed are very similar to the bonds that were broken. Therefore, overall enthalpy change isnear to zero.

Iron

The most stable oxidation states of iron are Fe^2+ and Fe^3+.

Copper

The most stable oxidation states of copper are Cu^+ and Cu^2+.

Cu^2+

Can be reduced to Cu by zinc.

Cu^+

Readily disproportionates to Cu and Cu^2+.

Ammonium

If ammonium ions are present, adding NaOH and gently warming results in the formation of ammonia gas, which is basic. Therefore, the presence of ammonium ions can be tested by holding damp red litmus paper over a petri dish of the substance being tested. It will turn blue if ammonium ions are present.

Transition Metal Ions

Transition metals form coloured compounds. This property can be used to determine which transition metal ions are present in a compound. The previous section explores the colours of chromium(III), iron(II), iron(III), manganese(II) and copper (II) metal-aqua ions and their precipitation reactions with sodium hydroxide and ammonia.