Potential energy curves for the X-2 Sigma(1/2)(+), A(2) Pi(1/2,3/2,) and B-2 Sigma(1/2)(+) states of the Mg+-RG type complexes (where RG = Ar, Xe) were generated using relativistic spin-orbit configuration-interaction calculations based on effective core potentials. The anomalous behavior of the spin-orbit splitting of the A(2) Pi state has been investigated and explained as an effect of the mixing of rare-gas valence pn orbital into the Mg+ 3p pi orbital in the wave function of the complex. A similar mixing causes a different behavior for the Mg+-CO2 complex, and this has also been investigated.