Quantum mechanical ab initio calculations at the Hartree-Fock and MP2 level of theory using relativistic effective core potentials for the metal atoms are reported for neutral and negatively charged oxo and nitrido complexes of molybdenum, tungsten, rhenium, and osmium with the general formula MXL(n) (L = F, Cl; n = 3-5). The calculated geometries and vibrational frequencies at the HF level are in good agreement with experimental data. The analysis of the electronic structure of the complexes shows that the M-O and M-N bonds are strongly covalent but the M-L bonds have mainly ionic character. The a and n contributions to the M-O bonds are strongly polarized toward oxygen. The M-N bonds are much less polarized and should be considered as triple bonds. The metal bonding in the M-X and M-L bonds has mainly hl(d) character.