• 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