6.2.2 Oxidation States of Transition Metals

• Transition elements can have variable oxidation states
• These variable oxidation states can be formed as the 3d and 4s atomic orbitals are similar in energy
• This means that a similar amount of energy is needed to remove a different number of electrons
• When the transition elements form ions, the electrons of the 4s subshell are lost first, followed by the 3d electrons
• The most common oxidation state is +2, which is usually formed when the two 4s electrons are lost

Oxidation number at the start of the 3d transition elements

• At the start of the period, it is easier for the transition elements to lose the maximum number of electrons
• The maximum oxidation number of these transition elements involves all the 4s and 3d electrons in the atom
• For example, the maximum oxidation state of a titanium (Ti) ion is +3 or +4, as two 4s electrons and either 1 or 2 3d electrons are lost
• Ti atom = 1s² 2s² 2p⁶ 3s² 3p⁶ 3d² 4s²
• Ti³⁺ ion = 1s² 2s² 2p⁶ 3s² 3p⁶3d¹
  • Ti⁴⁺ ion = 1s² 2s² 2p⁶ 3s² 3p⁶

Oxidation number at the end of the 3d transition elements

• Towards the end, the 3d transition elements are more likely to adopt the +2 oxidation state
• This is because across the d block, the 3d electrons become slightly harder to remove as the nuclear charge increases
  • The 3d electrons are attracted more strongly to the nucleus
  • The higher oxidation states become less stable

• Therefore, the elements are more likely to lose their 4s electrons only
• For example, nickel (Ni) is a transition element at the end of the period which only forms ions with oxidation state +2, due to loss of the 4s electrons only
  • Ni atom = 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁸ 4s²
  • Ni²⁺ ion = 1s² 2s² 2p⁶ 3s² 3p⁶3d⁸

• Transition elements are often used as catalysts due to their ability to form ions with more than one stable oxidation state, and the fact that they contain vacant d orbitals

Oxidation states

• Transition element ions can adopt more than one stable oxidation state
• This means that they can accept and lose electrons easily to go from one oxidation state to another
• They can therefore catalyse redox reactions, by acting as both oxidising agents and reducing agents
• For example, iron (Fe) is often used as a catalyst due to its ability to form Fe(II) and Fe(III) ions, acting as an oxidising agent and a reducing agent
  • When Fe(II) acts as a reducing agent, it will reduce another species and become oxidised itself

Fe²⁺ → Fe³⁺ + e^-

• The Fe³⁺ formed in the catalytic cycle, can then also act as an oxidising agent by oxidising another species and getting reduced itself to reform the Fe²⁺ ion

Fe³⁺ + e^- → Fe²⁺

• Transition element ions with high oxidation states make powerful oxidising agents, because they will readily accept electrons
  • A common example of this is potassium permanganate (VII), where manganese has an oxidation state of +7

Vacant d orbitals

• When transition elements form ions, they have vacant d orbitals which are energetically accessible
  • The orbitals are not too high in energy

• This means that dative bonds can be formed between the transition element ion and ligands
  • Each ligand provides the pair of electrons required for the formation of a bond between the ion and the ligand
  • This pair of electrons is donated into the ion’s vacant d orbital

• The table below shows the electron configuration of the transition element atoms
• When they form ions, empty d orbitals are obtained which can be filled by the pairs of electrons donated by the ligands

Electronic configuration of transition elements table

• A complex is a molecule or ion formed by a central metal atom or ion surrounded by one or more ligands
  • A complex can have an overall positive or negative charge, or it can be neutral
  • If a complex is charged overall, it is often called a complex ion

• Transition elements can easily form complex ions, because they have empty d orbitals that are energetically accessible
  • The empty d orbitals are therefore not too high in energy and can accommodate a lone pair of electrons

• The transition element in the centre will accept pairs of electrons from the ligands into their empty d orbitals, forming dative bonds
  • The transition element in the centre is often referred to as the central metal ion, as all transition elements are metals, and it is often an ion in the centre

• For example, the titanium(III) (Ti³⁺) ion, has an electronic configuration of 1s² 2s² 2p⁶ 3s² 3p⁶3d¹
  • This means that there are vacant d orbitals that can be occupied by electrons, from ligands such as H₂O for example, to form a [Ti(H₂O)₆]³⁺ complex ion
  • 6 water ligands have each donated a pair of electrons, to form 6 dative bonds with the central metal ion