The interaction between a spherical metal ion M+ and a rare gas atom Rg is analyzed in terms of the long-range effects of polarization and dispersion. The inclusion of higher-order linear polarization effects is found to enhance significantly the polarization contribution to binding compared to a simple charge-induced dipole interaction. The discrepancy between experimental or ab initio binding energies and the long-range energy of polarization and dispersion is found to be much reduced when higher-order effects are included, indicating a negligible role for covalency in these ions.