Account for the following: (i) Manganese shows the maximum number of oxidation states in the 3d series. (ii) The Eo value for the Mn³⁺/Mn²⁺ couple is much more positive than that for Cr³⁺/Cr²⁺. (iii) Ti⁴⁺ is colorless whereas V⁴⁺ is colored in an aqueous solution.

i) Because Mn has a 3d⁵4s² electronic configuration, it has a maximum number of electrons (5d electrons and two 4s electrons) to lose or share. So it can exhibit oxidation states from +2 to +7, which is the maximum number.

ii) Mn³⁺ (3d⁴) → Mn²⁺ (3d⁵) As Mn³⁺ → Mn²⁺ reduction is more feasible because Mn²⁺ is more stable (half-filled d-orbital). (Ref. image)

iii) Ti⁴⁺ (3d⁰) has no electrons in the d orbital, so no d-d transition occurs. Therefore, it is colorless. (Ref. image) V⁴⁺ has one unpaired electron, enabling a d-d transition. Therefore, V⁴⁺ is colored.