Molybdenum 17- and 18-Electron Bis- and Tris(Butadiene) Complexes: Electronic Structures, Spectroscopic Properties, and Oxidative Ligand Substitution Reactions

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.