Variable oxidation states of 3d elements

Transition elements show variable oxidation states because the (n−1)d and ns electrons have very nearly the same energy, so both the ns electrons and a variable number of (n−1)d electrons can take part in bonding. Successive ionisation energies rise gradually (unlike the sudden jump after the s-electrons in s-block metals), so ions of several charges can form. In the 3d series the states usually differ by one unit (e.g. Fe²⁺, Fe³⁺), unlike p-block elements whose states differ by two (Sn²⁺, Sn⁴⁺).

Oxidation states of 3d elements

• Sc has one stable state (+3, loses 3 electrons to reach argon core) and Zn only +2 (3d¹⁰ stable), so they show little variability.
• The maximum state increases from Sc (+3) to Mn (+7), equal to the number of 3d + 4s electrons, then decreases after Mn because electrons are paired and the d orbitals become more stable.
• Examples: MnO₄⁻ (+7), Cr₂O₇²⁻ (+6), VO₂⁺ (+5).

Stability: the state with d⁰, d⁵ or d¹⁰ configuration is relatively stable (Sc³⁺, Mn²⁺, Fe³⁺, Zn²⁺). Higher states are stable in oxides and fluorides (high electronegativity), lower states in complexes with π-acceptor ligands such as CO.