[PtAu24(SC2H4Ph)18]0 ([PtAu24]0) was fully and selectively converted to [PtAu24]-, having an open electronic structure with seven valence electrons, upon the addition of an equiamount of NaBH4. Stoichiometric production of [PtAu24]- by the reaction between an equal amount of [PtAu24]0 and [PtAu24]2- revealed that the above reaction proceeds via the spontaneous electron transfer (ET) from [PtAu24]2- nascently reduced by H--mediated reduction to [PtAu24]0 remaining in the solution. Theoretical calculation suggested that the driving force of this novel ET reaction was the larger adiabatic electron affinity of [PtAu24]0 compared to that of [PtAu24]-, partly associated with reduction-induced relief of the Jahn-Teller strain. We propose that ET proceeds via the dimer complex of [PtAu24]2- and [PtAu24]0 formed through the aurophilic interaction between Au(I) sites in the surface layer.