The anharmonicity of weakly bound complexes is studied using the vibrational self-consistent field (VSCF) approach for a series of metal cation dihydrogen (M+ -H-2) complexes. The H-H stretching frequency shifts of M+ -H-2 (M+ = Li+, Na+, B+, and Al+) complexes are calculated with the coupled-cluster method including all single and double excitations with perturbative triples (CCSD(T)) level of theory with the cc-pVTZ basis set. The calculated H H stretching frequency of Li+ -H-2, B+-H-2, Na+ -H-2, and Al+ -H-2 is red-shifted by 121; 202, 74, and 62 cm(-1), respectively, relative to that of unbound H-2. The calculated red shifts and their trends are in good agreement with the available experimental and previously calculated data. Insight into the observed trends is provided by symmetry adapted perturbation theory (SAPT).