New results on the electronic structures, spectroscopic properties, and reactivities of the molybdenum tris(butadiene) and tris(2,3-dimethylbutadiene) complexes [Mo(bd)(3)] (1(bd)) and [Mo(dmbd)(3)] (1(dmbd)), respectively, are reported. Importantly, the metal ligand bonding interaction can be weakened by oxidizing the metal center with ferrocenium salts. The addition of the bidentate phosphine ligand 1,2-bis(diphenylphosphino)ethane then leads to a new type of stable 17-electron complex, [Mo(dmbd)(2)(dppe)](X) (2; X = BF4-, PF6-, BPh4-), where one of the butadiene ligands is exchanged by a chelating phosphine. Reduction of the cationic complexes 2 generates the corresponding 18-electron complex [Mo(dmbd)(2)(dppe)] (3), thus establishing a new strategy for ligand substitution reactions in [Mo(bd)(3)] complexes via one-electron oxidized intermediates. The new heteroleptic molybdenum complexes are characterized by X-ray structure analysis; vibrational, NMR, and EPR spectroscopy; and electrochemistry. DFT calculations are performed to explain the structural and specroscopic trends observed experimentally. For compound 1(bd), a normal coordinate analysis is presented, providing additional information on the bonding situation in this type of complex.