Ion–molecule reactions of Mg+(H2O)n (n≈20–60) with CH3CN are studied by Fourier-transform ion-cyclotron resonance mass spectrometry. Collision with CH3CN initiates the formation of MgOH+(H2O)n−1 together with CH3CHN. or CH3CNH., which is similar to the reaction of hydrated electrons (H2O)n− with CH3CN. In subsequent reaction steps, three more CH3CN molecules are taken up by the clusters, to form MgOH+(CH3CN)3 after a reaction delay of 60 seconds. Density functional theory (DFT) calculations at the M06/6-31++G(d,p) level of theory suggest that the bending motion of CH3CN allows the unpaired electron that is solvated out from the Mg center to localize in a π*(CN)-like orbital of the bent CH3CN.−, which undergoes spontaneous proton transfer to form CH3CNH. or CH3CHN., with the former being kinetically more favorable. The reaction energy for a cluster with the hexacoordinated Mg center is more exothermic than that with the pentacoordinated Mg. The CH3CNH. or CH3CHN. is preferentially solvated on the cluster surface rather than at the first solvation shell of the Mg center. By contrast, the three additional CH3CN molecules taken up by the resulting MgOH+(H2O)n clusters coordinate directly to the first solvation shell of the MgOH+ core, as revealed by DFT calculations.