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)(2x)(e )(2x) (0 <= x <= 2). This catalyst has low catalytic performance with an electron concentration (N-e) lower than 1 X 10(21) cm(3) and a high apparent activation energy (E-a) 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 X 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 metalinsulator transition of [Ca24Al28O64](4+)(O-2)(2x)(e )(2x) also occurs at Ne 1 X 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 metalinsulator 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.