A series of new molybdenum(VI) dioxo complexes of the type [MoO2ClLX] with potentially monoanionic phenolate ligands L-X(L = 4,6-di-tert-butyl-2-{[(X)methylamino]methyl}-phenolate; L-OMe, where X = 2'-methoxyethyl; L-SEt, where X = 2'-ethylthioethyl; L-NEt2, where X = 2'-diethylaminoethyl; and L-Nme2, where X = 2'-dimethylaminoethyl) have been synthesized as models for molybdoenzymes. All molybdenum complexes were readily accessible by employing the eta(2)-coordinate pyrazolate complex [MoO2Cl(eta(2)-t-Bu(2)pz)]. The nitrogen ligand can easily be exchanged by the monoanionic phenolate ligands L-X in toluene at room temperature, leading to monosubstituted complexes 1-4 as yellow to red powders in good yields. Suitable single crystals for X-ray diffraction analysis of complexes 2 and 3 were obtained from a concentrated benzene solution. Both complexes reveal a six-coordinate molybdenum atom in a distorted octahedral surrounding, with a tridentate fa-c coordination of the ligand. Additionally, the complexes were characterized by elemental analysis; IR, UV/vis, and NMR spectroscopy; and mass spectrometry. For complexes 1 and 2, only one isomer can be detected in solution, whereas complexes 3 and 4 reveal the formation of two isomers in a 1:1 ratio for 3 and in a 1:3 ratio for 4. Optimized geometries and relative free energies of all possible isomers have been established by DFT calculations, indicating, two isomers in solutions of 3 and 4 to be two types of facially coordinated complexes. Furthermore, this is supported by gauge-independent atomic orbital calculations of the H-1 NMR shifts with a high correlation of experimental and calculated shifts for the fac compounds. Oxygen atom transfer reactions of 1 to PMe3 quickly form monooxo molybdenum compound cis, mer-[MoOCl2(PMe3)(3)]