Oxidation states

Transition elements show many oxidation states. The widest range is in or near the middle of the series: Mn shows every state from +2 to +7.

Few states appear at the two ends. Sc and Ti have too few electrons to give up or share; Cu and Zn have so many d electrons that little room is left to bond with. So Sc(II) is almost unknown, Ti(IV) outlasts Ti(III) and Ti(II), and zinc sticks to +2.

Scandium is the transition element that shows no variable oxidation state (only +3).

As far as Mn, the top stable state is the sum of the 4s and 3d electrons: TiO₂ has Ti at +4, VO₂⁺ has V at +5, CrO₄²⁻ has Cr at +6 and MnO₄⁻ has Mn at +7. Beyond Mn the high states drop off quickly: iron and cobalt show +2 and +3, nickel +2, copper +1 and +2, zinc +2.

Transition-metal oxidation states usually differ by one (V(II), V(III), V(IV), V(V)); in main-group elements they typically differ by two.

In groups 4 to 10 the heavier members favour the higher states, the reverse of the inert pair effect in the p-block. Mo(VI) and W(VI) are more stable than Cr(VI), so dichromate in acid is a strong oxidant while MoO₃ and WO₃ are not.

Ligands that accept π electrons as well as donate σ electrons allow very low oxidation states: Ni in Ni(CO)₄ and Fe in Fe(CO)₅ are in the zero state.