Inspired by the fact that the active centers of natural nitrogenases are polynuclear iron-sulfur clusters, the reactivity of isolated iron-sulfur clusters toward N2 has received considerable attention to gain fundamental insights into the activation of the N≡N triple bond. Herein, a series of gas-phase iron-sulfur cluster anions FexSy- (x = 1-8, y = 0-x) were prepared and their reactivities toward N2 were investigated systematically by mass spectrometry. Among the 44 investigated clusters, only Fe5S2- and Fe5S3- showed superior reactivity toward N2. Theoretical results revealed that N2 binds molecularly to the iron sites of Fe5S2,3- in a common end-on coordination mode with an unprecedented back-donation interaction from the localized d-d bonding orbitals of Fe-Fe sites to the π* antibonding orbitals of N2. This is the first example to disclose the significant contribution of the dual metal sites rather than the single metal atom to N2 adsorption in the prevalent end-on binding mode.