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Mixed-metal oxides have proven to be effective catalysts for the hydrogen evolution reaction, often outperforming either of the binary metal oxides. The reactivity of MnxMoOy− (x = 1, 2; y = 3, 4) clusters toward H2O was investigated via time-of-flight mass spectrometry with clear evidence of cluster oxidation and corresponding H2 production, specifically for MnxMoO3− (x = 1, 2) clusters. Unlike previously studied MoxOy− clusters, which assumed a broad distribution of stoichiometries (typically x ≤ y ≤ 3x), both MnMoOy− and Mn2MoOy− preferentially formed y = 3 and 4 compositions in significant quantities under our source conditions. The electronic and molecular structures of the MnxMoOy (x = 1, 2; y = 3, 4) anion and neutral clusters were probed with anion photoelectron spectroscopy and analyzed with supporting density functional theory calculations. Our studies suggest that both metal centers are involved in initial cluster–water complex formation, while Mo is the center that undergoes oxidation; hence, reactivity terminates when Mo is saturated in its highest oxidation state of +6. Across these four clusters, Mn remains relatively reduced and is stable in a high-spin electronic configuration. The preferential reactivity of water molecules toward the Mo center rather than Mn is rationalized by the much lower relative oxophilicity of Mn.

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