Various possible structures for neutral ML complexes (M = Al, Ga; L = CO2, CS2, and COS) are investigated using density functional methods and the Moller-Plesset perturbation approach. Formation of C-2v and C-s species is observed. Obtained vibrational frequencies are compared with experimental data. Differences between CO2, and CS2 adducts are discussed. Absence of C-2v species for MCS2 complexes, as seen in experiments, is consistent with calculated relative energies. Erratic behavior of the MP2 method when excluding core electrons from correlation suggests that for gallium 3d semi-core electrons of GaCO2 complex play an important role in complex formation. Very small (cat -3 kcal/mol) binding energies explain major difficulties in experimental observation of GaCO2 complexes. To investigate this matter further, calculations for experimentally unknown MCOS complexes have been performed.