The structural, thermodynamic, and spectral characteristics of four-coordinate MX^- ₄ anions (M = Al, Ga, In; X = F, Cl, Br, I) and the corresponding trihalides MX₃ were calculated by ab initio methods (self-consistent field SCF and density functional B3LYP) in combination with the method of effective core potentials. The calculated spectral characteristics of MX₃ and MX^- ₄ are well consistent with the published experimental data. The calculated affinities for the halide anion correlate with the lengthening of the M-X bond in going from the planar neutral halide MX₃ to the tetrahedral MX^- ₄ anion. The bond lengthening changes in parallel with the frequency of the fully symmetrical stretching vibration v₁. In the gas phase the affinity for the halide anion X^- is much larger than the energy of interaction with neutral donor molecules, and the four-coordinate anionic form can be expected to be stable even in strongly electron-donor solvents.