The results of spin-restricted SCF-Xalpha-SW and spin-unrestricted broken-symmetry SCF-Xalpha-SW calculations on the face-shared dimer complexes A3Mo2Cl9 (A = K, Rb, Cs, Me4N), A3Mo2Br9 (A = Cs, Me4N), and Cs3Mo2I9 are reported and used to discuss the electronic structure, metal-metal bonding, and magneto-structural correlations in these systems. The spin-restricted SCF-Xalpha-SW ground-state calculations on Cs3Mo2X9 (X = Cl, Br, I) show that the metal-metal sigma and pi bonding interactions are significantly reduced for the bromide and especially for the iodide complex relative to Mo2Cl9(3-), consistent with the increased metal-metal bond distances observed for both these complexes. The ground-state exchange interaction is shown to be almost entirely the result of direct overlap of magnetic orbitals with negligible contribution from superexchange effects. The Xalpha-SW calculations for the broken-symmetry ground state reveal that the magnetic orbitals involved in the metal-metal pi interaction are almost completely localized on the metal ions. The magnetic orbitals involved in the metal-metal sigma interaction, on the other hand, are partially delocalized between the two metals but still contribute significantly to the ground-state exchange interaction in agreement with earlier spectroscopic and theoretical studies. The calculated exchange coupling constants J(ab) for the complexes Cs3Mo2X9 (X = Cl, Br, I) support this conclusion and indicate that the effective maximum spin in the ground-state exchange levels lies between 2 and 3. A significant antiferromagnetic contribution arises from ligand --> metal spin-polarization effects which accounts for the unusually large-J(ab) value found for the iodide complex. The magneto-structural correlations observed for the chloride and bromide complexes have been successfully modeled by initially using Mo atomic sphere radii which reproduced the experimental J(ab) values for Cs3Mo2Cl9 and Cs3Mo2Br9 and then adjusting the Mo sphere radii for the remaining complexes in proportion to their metal-metal bond distances relative to the Cs salts.