Lithium Di- and Trimethyl Dimolybdenum(II) Complexes with Mo–Mo Quadruple Bonds and Bridging Methyl Groups

New dimolybdenum complexes of composition [Mo₂{μ-Me}₂Li­(S)}­(μ-X)­(μ-N^N)₂] (3a–3c), where S = THF or Et₂O and N^N represents a bidentate aminopyridinate or amidinate ligand that bridges the quadruply bonded molybdenum atoms, were prepared from the reaction of the appropriate [Mo₂{μ-O₂CMe}₂(μ-N^N)₂] precursors and LiMe. For complex 3a, X = MeCO₂, while in 3b and 3c, X = Me. Solution NMR studies in C₆D₆ solvent support formulation of the complexes as contact ion pairs with weak agostic Mo–CH₃···Li interactions, which were also evidenced by X-ray crystallography in the solid-state structures of the molecules of 3a and 3b. Samples of 3c enriched in ¹³C (99%) at the metal-bonded methyl sites were also prepared and investigated by NMR spectroscopy employing C₆D₆ and THF-d₈ solvents. Crystallization of 3c from toluene:tetrahydrofuran mixtures provided single crystals of the solvent separated ion pair complex [Li­(THF)₄] [Mo₂(Me)₂(μ-Me)­{μ-HC­(NDipp)₂}₂] (4c), where Dipp stands for 2,6-iPr₂C₆H₃. A computational analysis of the Mo₂(μ-Me)₂Li core of complexes 3a and 3b has been developed, which is consistent with a small but non-negligible electron-density sharing between the C and Li atoms of the mainly ionic CH₃···Li interactions.