The binding energies of M+ and MX+ ions to benzene and mesitylene were measured by the radiative association kinetics approach in the Fourier transform ion cyclotron resonance (FT-ICR) spectrometer, where M was Mg, Ca, or Sr and X was Cl or Br. MCl+ binds to the aromatic face roughly half again as strongly as the corresponding M+ ion. (For instance, 2.6 eV vs 1.6 eV for MgCl(benzene)(+) vs Mg(benzene)(+).) This is true for all three metals, although the absolute magnitude of the binding energy decreases sharply in going from Mg to Ca to Sr as a consequence of decreasing electrostatic interaction. Bromide has a smaller effect on the binding ability of the metal center than chloride, in keeping with its lower electron-withdrawing ability. Quantum chemical calculations for MgCl+ are in accord with the measured binding energy and also show the expected decrease of the metal-ring bond distance in going from Mg+ to MgCl+. The qualitative picture is presented that MX+ behaves as a metal ion center with the charge of a monovalent ion but the electronic character of a divalent alkaline earth cation. The calculations indicate that the gas-phase binding of MX+ ions to the aromatic face is intermediate between the binding abilities of M+ and M2+. Tn the particular case of MgCl+, quantitative analysis via quantum chemical calculations indicates that this ion binds in a manner equivalent to a hypothetical magnesium center with fractional charge of +1.3 to +1.4. A second aromatic ligand normally adds to the monomer complex. Radiative association kinetics analysis indicates that the second ligand is bound with energy comparable to, or somewhat less than, the first ligand, and substantially less in the magnesium cases.