The substitution of electrons for O(2-) anions in the crystallographic cages of [Ca24Al28O64](4+)(O(2-))2 was investigated to clarify the correlation between the electronic properties and catalytic activity for ammonia synthesis in Ru-loaded [Ca24Al28O64](4+)(O(2-))2-x(e(-))2x (0 ≤ x ≤ 2). This catalyst has low catalytic performance with an electron concentration (Ne) lower than 1 × 10(21) cm(-3) and a high apparent activation energy (Ea) for ammonia synthesis comparable to that for conventional Ru-based catalysts with a basic promoter such as alkali or alkaline earth compounds. Replacement of more than half of the cage O(2-) anions with electrons (Ne ≈ 1 × 10(21) cm(-3)) significantly changes the reaction mechanism to yield a catalytic activity that is an order higher and with half the Ea. The metal-insulator transition of [Ca24Al28O64](4+)(O(2-))2-x(e(-))2x also occurs at Ne ≈ 1 × 10(21) cm(-3) and is triggered by structural relaxation of the crystallographic cage induced by the replacement of O(2-) anions with electrons. These observations indicate that the metal-insulator transition point is a boundary in the catalysis between Ru-loaded [Ca24Al28O64](4+)(O(2-))2 and [Ca24Al28O64](4+)(e(-))4. It is thus demonstrated that whole electronic properties of the support material dominate catalysis for ammonia synthesis.