Inspired by the fact that Mo is a key element in biological nitrogenase, a series of gas-phase MoxSy- cluster anions are prepared and their reactivity toward N2 is investigated by the combination of mass spectrometry, photoelectron imaging spectroscopy, and density functional theory calculations. The Mo5S2- and Mo5S3- cluster anions show remarkable reactivity compared with the anionic species reported previously. The spectroscopic results in conjunction with theoretical analysis reveal that a facile cleavage of N≡N bonds takes place on Mo5S2- and Mo5S3-. The large dissociative adsorption energy of N2 and the favorable entrance channel for initial N2 approaching are proposed as two decisive factors for the superior reactivity of Mo5S2- and Mo5S3-. Besides, the modulation of S ligands on the reactivity of metal centers with N2 is proposed. The highly reactive metal-sulfur species may be obtained by the coordination of two to three sulfur atoms to bare metal clusters so that an appropriate combination of electronic structures and charge distributions can be achieved.