We explored the mechanistic aspects of ion pair formation between electrochemically reduced radicals (Fc–cnq–1a•−/Fc–cnq–1b•−) and dianions (Fc–cnq–1a²−/Fc–cnq–1b²⁻) of ferrocenyl naphthoquinones (Fc–cnq–1a and Fc–cnq–1b) and several metal ions by cyclic voltammetry (CV), squarewave voltammetry (SWV) and spectroelectrochemistry, for the first time. The experiments demonstrated that Fc–cnq–1a²−/Fc–cnq–1b²⁻ were moderately affected with Na⁺, K⁺ and Cs⁺ by slightly shifting to the anodic side, but were strongly influenced with Li⁺ ion. Fc–cnq–1a•−/Fc–cnq–1b•− were not affected by alkali metal ions, indicating no ion pair formation between the radicals and these ions. Fc–cnq–1a^2-/Fc–cnq–1b^2- was not evolved in the presence of Be²⁺, Mg²⁺ and Ca²⁺, but Fc–cnq–1a•−/Fc–cnq–1b•− appeared with theircathodic waves, and participated with intermediates, [(Fc–cnq–1b)₂•−–Be²⁺ and [(Fc–cnq–1a)^.-–Cl]. The most pronounced effect on the ion-pair formation of the Fc–cnq–1a•−/Fc–cnq–1b•− was observed in Be²⁺,indicating that Fc–cnq–1a or Fc–cnq–1b can selectively sense ultra-trace amount of Be2+ (LOD=3.6 ppb)among the other metal ions with SWV titration, for the first time, based on the strong ion pair formation reaction between the radicals and Be²⁺.