Characteristic Properties of Transition Elements (HL) (HL IB Chemistry)

• Although the transition elements are metals, they have some properties unlike those of other metals on the periodic table, such as:
  • Variable oxidation states
  • High melting points
  • Have magnetic properties
  • Behave as catalysts
    • Use in catalytic converters and as biological catalysts
  • Form coloured compounds
  • Form complex ions with ligands
• These properties are a result of having an incomplete d sublevel

For more information about the electrical conductivity and high melting points of transition metals, see our revision note on the Physical Properties of Transition Elements

Variable Oxidation States

• Like other metals on the periodic table, the transition elements will lose electrons to form positively charged ions
• However, unlike other metals, transition elements can form more than one positive ion
  • They are said to have variable oxidation states
• Because of this, Roman numerals are used to indicate the oxidation state of the metal ion
  • For example, the metal sodium (Na) will only form Na⁺ ions (no Roman numerals are needed, as the ion formed by Na will always have an oxidation state of +1)
  • The transition metal iron (Fe) can form Fe²⁺ (Fe(II)) and Fe³⁺ (Fe(III)) ions

Magnetic Properties

• Magnetism in transition metals is due to the presence of unpaired electrons in the d-orbitals
• Spinning electrons create a tiny magnetic dipole
• When paired electrons orientate themselves, the magnetic dipoles act in opposite directions, which means that there is no overall magnetic effect
• Most materials have paired electrons arranged like this, making them non-magnetic
• Some transition elements have unpaired electrons 
• These unpaired electrons can be aligned in an external field resulting in magnetic properties
• The transitions elements iron, cobalt and nickel have strong magnetic properties
  • The alloy steel also has strong magnetic properties because it contains iron
• They contain unpaired electrons in their d orbitals

Arrangement of electrons in orbitals for iron, cobalt and nickel

Iron, cobalt and nickel have strong magnetic properties because they contain unpaired electrons in their d orbitals 

• If iron, cobalt and nickel are heated and cooled in a magnetic field, the magnetic field of the electrons remains
• Magnetic regions within the metal that are aligned magnetically are known as domains
• Banging or heating a permanent magnet will weaken the magnetism

Transition elements as catalysts

• Transition metals are often used as catalysts in the elemental form or as compounds
• The ability of transition metals to form more than one stable oxidation state means that they can accept and lose electrons easily
• This enables them to catalyse certain redox reactions
  • They can be readily oxidised and reduced again, or reduced and then oxidised again
  • This is a consequence of transition metals having variable oxidation states 
• There are two types of catalyst:
  • A heterogeneous catalyst is in a different physical state (phase) from the reactants
    • The reaction occurs at active sites on the surface of the catalyst
    • An example is the use of iron, Fe, in the Haber process for making ammonia

N₂ (g) + 3H₂ (g) ⇌ 2NH₃ (g)

• • A homogeneous catalyst is in the same physical state (phase) as the reactants

Further examples of transition metal catalysts

• The hydrogenation or reduction of alkenes makes use of a nickel catalyst

CH₂=CH₂ (g) + H₂ (g) → CH₃CH₃ (g)

• • The same reaction is used in the hydrogenation of vegetable oils 
• The decomposition of hydrogen peroxide is a common reaction in the study of chemical kinetics and uses manganese(IV) oxide as the catalyst

2H₂O₂ (g) →  2H₂O (aq) + O₂ (g)

Catalytic converters

• Catalytic converters are used in car exhaust boxes to reduce air pollution
• They usually consist of a mixture of finely divided platinum and rhodium supported on a ceramic base

Catalytic converter diagram

The transition metal catalyst is on an inert support medium in a vehicle catalytic converter

• Carbon monoxide, nitrogen dioxide and unburnt hydrocarbons are sources of pollution in car exhaust
• The transition metal catalysts facilitate the conversion of these pollutants into less harmful products:

2NO (g) + 2CO (g) → N₂ (g) + 2CO₂ (g)

CH₃CH₂CH₃ (g) + 5O₂ (g) → 3CO₂ (g)  + 4H₂O (g)  

• Some transition metals are precious metals so they can be very expensive
  • In order to minimise the cost and maximise the efficiency of the catalyst the following measures can be taken:
    • Increasing the surface area of the catalyst
    • Coating an inert surface medium with the catalyst to avoid using large amounts of the catalyst
  • This is achieved by spreading the catalyst over a hollow matrix such as a honeycomb-like structure

Biological catalysts

• Many of the enzyme catalysed reactions in the body make use of homogeneous transition metal catalysts
• An example of this is haemoglobin, abbreviated to Hb, which transports oxygen around the blood:

Haemoglobin structure diagram

Haemoglobin contains haem units that are responsible for transporting oxygen

The structure of haem

The haem unit contains an iron(II) ion

• The iron(II) ion is in the centre of a large heterocyclic ring called a porphyrin
• The iron has a coordination number of four, is square planar and can bind to one oxygen molecule
• The Hb molecule contains four porphyrin rings so each Hb can transport four oxygen molecules

Forming coloured compounds

• Another characteristic property of transition elements is that their compounds are often coloured
  • For example, the colour of the [Cr(OH)₆]^3- complex (where the oxidation state of Cr is +3) is dark green
  • Whereas the colour of the [Cr(NH₃)₆]³⁺ complex (the oxidation state of Cr is still +3) is purple

For more information about transition metals as coloured compounds, see our revision note on Colour in Transition Metal Complexes

Forming Complex Ions

• Another property of transition elements caused by their ability to form variable oxidation states is the ability to form complex ions
• A complex ion consists of a central metal atom or ion, with a number of molecules or ions surrounding it
  • A molecule or ion surrounding the central metal atom or ion is called a ligand
• Due to the different oxidation states of the central metal ions, a different number and wide variety of ligands can form bonds with the transition element
  • For example, the chromium(III) ion can form [Cr(NH₃)₆]³⁺, [Cr(OH)₆]^3- and [Cr(H₂O)₆]³⁺ complex ions

For more information about complex ions and transition metals, see our revision note on Coordinate Bonds