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), square wave 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-1a2-/Fc-cnq-1b2- was not evolved in the presence of Be2+, Mg2+ and Ca2+, but Fc-cnq-1a.-/Fc-cnq-1b.- appeared with their cathodic 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=3.6ppb) 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+.Graphical abstractSynopsis Ultra-trace amount of beryllium ion (Be2+) sensing (LOD=3.6ppb) through the ion pair formation between the electrochemically reduced ferrocenyl naphthoquinone radicals (Fc-cnq-1a.-/Fc-cnq-1b.-) and Be2+ ions by cyclic voltammetry (CV), square wave voltammetry (SWV), and for the first time spectroelectrochemistry is reported in this study.