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

New dimolybdenum complexes of composition [Mo<sub>2</sub>{μ-Me}<sub>2</sub>Li­(S)}­(μ-X)­(μ-N^N)<sub>2</sub>] (<b>3a</b>–<b>3c</b>), where S = THF or Et<sub>2</sub>O 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 [Mo<sub>2</sub>{μ-O<sub>2</sub>CMe}<sub>2</sub>(μ-N^N)<sub>2</sub>] precursors and LiMe. For complex <b>3a</b>, X = MeCO<sub>2</sub>, while in <b>3b</b> and <b>3c</b>, X = Me. Solution NMR studies in C<sub>6</sub>D<sub>6</sub> solvent support formulation of the complexes as contact ion pairs with weak agostic Mo–CH<sub>3</sub>···Li interactions, which were also evidenced by X-ray crystallography in the solid-state structures of the molecules of <b>3a</b> and <b>3b</b>. Samples of <b>3c</b> enriched in <sup>13</sup>C (99%) at the metal-bonded methyl sites were also prepared and investigated by NMR spectroscopy employing C<sub>6</sub>D<sub>6</sub> and THF-<i>d</i><sub>8</sub> solvents. Crystallization of <b>3c</b> from toluene:tetrahydrofuran mixtures provided single crystals of the solvent separated ion pair complex [Li­(THF)<sub>4</sub>] [Mo<sub>2</sub>(Me)<sub>2</sub>(μ-Me)­{μ-HC­(NDipp)<sub>2</sub>}<sub>2</sub>] (<b>4c</b>), where Dipp stands for 2,6-<i>i</i>Pr<sub>2</sub>C<sub>6</sub>H<sub>3</sub>. A computational analysis of the Mo<sub>2</sub>(μ-Me)<sub>2</sub>Li core of complexes <b>3a</b> and <b>3b</b> 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 CH<sub>3</sub>···Li interactions.