Two phosphaadamantane-substituted, new mononuclear Mn(I) carbonyl complexes, fac-[Mn(CO)₃(κ²-S₂NC₇H₄)(PTA)] 1 and fac-[Mn(CO)₃(κ²-SN₂C₇H₅)(PTA)] 2, were synthesized by the aerobic reaction of dinuclear {MnMn} complexes [Mn₂(CO)₆(µ-S₂NC₇H₄)₂] A and [Mn₂(CO)₆(µ-SN₂C₇H₅)₂] B with 1,3,5-triaza-7-phosphaadamantane (PTA) ligand in dichloromethane at room temperature. The complexes were characterized using SC-XRD, FTIR, NMR, mass spectrometry and elemental analysis. The cage-like monodentate phosphaadamantane ancillary ligand was incorporated with the aim of increasing the solubility of the Mn carbonyl complexes in aqueous media. Complexes 1 and 2 effectively catalyzed electrochemical proton reduction in CH₃CN and the CH₃CN/water (1 : 1) mixture with trifluoroacetic acid and acetic acid as proton sources. Complexes 1 and 2 displayed a turnover frequency (TOF/s⁻¹) (O.P., η/V) of 718 (1.11) and 254 (1.55), respectively, in CH₃CN, and a TOF of 280 (1) and 61 (2) in CH₃CN/water mixture for the catalytic process.