Transition Elements

The d block

Transition elements are elements which form ions with a partially-filled d-orbital – their outermost electron is found in a d sub-shell. This means that all the transition elements are found in the d-block in the middle of the periodic table.

However, not all d-block elements are technically transition metals.

Some elements, such as scandium and zinc, form ions that have an empty d-orbital (Sc) or a full d-orbital (Zn). Only elements that form ions with partially filled d-orbitals are classed as transition elements.

To the right are the electron configurations of the elements in the first row of the d-block. Notice how the configurations for chromium and copper aren’t what you’d expect. To increase stability, they will have just one electron in the 4s orbital so that they can have either five (Cr) or ten (Cu) electrons in their 3d orbitals.

Electron configuration of transition metal ions

If we have a look at the electron configurations of scandium and zinc, we can see why they are not classed as transition elements. Even though their atoms have an incomplete d sub-shell, their ions don’t so they are not technically transition metals.

Properties of transition elements

Transition elements have unique properties not found in other elements:

• They exist in various oxidation states e.g. iron (II) and iron (III) can exist in a +2 or +3 oxidation state respectively.

• They form coloured compounds e.g. copper sulfate is a blue solid.

• They act as catalysts e.g. nickel is used as a catalyst in hydrogenation reaction to convert alkenes into alkanes.

Various oxidation states

The 4s and 3d sub-shells are fairly close to each other and only slightly differ in energy. This means similar amounts of energy can be used to remove different numbers of electrons, resulting in variable oxidation states.

The table below shows the various oxidation states of transition metal ions:

Coloured compounds

The hydrated ion reacts with hydroxide ions (i.e. from sodium hydroxide) to form coloured precipitates of metal hydroxides. Coloured metal hydroxides are also formed when the hydrated ion reacts with ammonia.

Catalytic behaviour

Because transition metals can exist in a variety of oxidation states, they can switch oxidation state by gaining or losing electrons from their d-orbital. This means that they can transfer electrons to or from other molecules to speed up reactions - in other words, they act as catalysts.

They work by adsorbing molecules on their surface, lowering the activation energy of reactions. They are heterogeneous catalysts. Here’s some examples of some reactions that transition metals catalyse:

• Iron is a catalyst in the Haber process (the reaction between nitrogen and hydrogen to form ammonia in the manufacture of fertilisers).

• Nickel is a catalyst in hydrogenation reactions (adding hydrogen to alkenes to form alkanes).

• Manganese oxide is a catalyst in the decomposition of hydrogen peroxide into water and oxygen.

Transition metals are therefore useful from a commercial point of view since they allow industrial reactions to happen quickly and cheaply (since less energy is required). However, they can give rise to health problems since transition metals and their compounds are toxic. For example, excess exposure to manganese can lead to psychiatric problems.

Ligands and complex ions

Transition metals often form something called complex ions, which is where the transition metal ion is bound to ligands through coordinate bonds.

• Coordinate bond - a type of covalent bond in which both of the bonding electrons have come from the same atom. It is also called dative covalent bonding.

• Ligand - an atom, ion or molecule with a lone pair of electrons to donate to a central metal ion.

Ligands donate their lone pair electrons to the transition metal ion to form a coordinate bond. Several ligands are present in a complex ion, each forming a coordinate bond. The coordination number is the number of coordinate bonds that are formed with the transition metal ion.

For smaller ligands, such as water, ammonia or cyanide ions, six molecules can fit around the central metal ion, producing complex ions with a coordination number of 6. These have an octahedral geometry around the central metal ion, just like other types of molecules with six bond pairs. For larger ligands, such as chloride ions, only four molecules can fit around the metal ion, forming complex ions with a coordination number of 4. These complex ions usually have a tetrahedral geometry, but some assume a square planar shape (more on this below).

Ligands can have one or more lone pairs:

• Monodentate ligands are ligands with one lone pair. Each ligand forms one coordinate bond.

• Bidentate ligands are ligands with two lone pairs. Each ligand forms two coordinate bonds.

• Multidentate ligands are ligands with two or more lone pairs. Some of these ligands can form multiple coordinate bonds.

Isomerism in complex ions

Octahedral complex ions with three bidentate ligands, as shown in the image below, can exist as optical isomers. Optical isomers are isomers which are non-superimposable mirror images of each other. They rotate plane-polarised light in opposite directions (clockwise vs anticlockwise).

Other complex ions form cis-trans isomers, if two different groups are attached to the central metal ion. If the groups are on the same side of the complex ion, the complex ion exists in a cis conformation. Trans isomers have groups on opposite sides of the complex ion.

Cis-platin is an example of a complex ion which exists as a cis isomer with identical groups on the same side of the complex ion. It has a platinum (II) ion as the central ion, two ammonia ligands above the metal ion and two chloride ligands below the central metal ion. It is used as an anti-cancer drug and works by binding to DNA. The nitrogen atoms in DNA displace the chloride ligands of cis-platin. With cis-platin bound, DNA cannot replicate and the cancer cell cannot divide. The cis conformation is important to its anti-cancer properties as the trans conformation has a different effect on the body.

Ligand substitution

A complex ion can react with substances such as water, ammonia and chlorine in ligand substitution reactions, in which the ligands in the complex ion are replaced by different ligands. If the ligands are of the same size, the coordination number and geometry of the complex ion remains the same. If the ligands are a different size then the coordination and geometry of the complex ion will change. Sometimes all of the ligands are replaced and other times only some of them are – this is called partial substitution. Here’s some examples of ligand substitution reactions:

Reactions of transition metal ions

Transition metals form ions with various oxidation states, which means they can change between oxidation states by gaining electrons (reduction) or losing electrons (oxidation) in redox reactions. In redox reactions involving transition metal ions, a colour change is often seen. For example:

Identifying transition metal ions

When reacted with aqueous sodium hydroxide, most of the transition metal ions form a metal hydroxide which appears as a coloured precipitate.

This means that NaOH can be used to test for which transition metal ions are present in a solution.