Supported σ-Complexes of Li-C Bonds from Coordination of Monomeric Molecules of LiCH3 , LiCH2 CH3 and LiC6 H5 to Mo≣Mo Bonds.

LiCH3 and LiCH2 CH3 react with the complex [Mo2 (H)2 (µ-AdDipp2 )2 (thf)2 ] (1⋅thf) with coordination of two molecules of LiCH2 R (R=H, CH3 ) and formation of complexes [Mo2 {µ-HLi(thf)CH2 R}2 (AdDipp2 )2 ], 5⋅LiCH3 and 5⋅LiCH2 CH3 , respectively (AdDipp2 =HC(NDipp)2 ; Dipp=2,6-i Pr2 C6 H3 ; thf=C4 H8 O). Due to steric hindrance, only one molecule of LiC6 H5 adds to 1⋅thf generating the complex [Mo2 (H){µ-HLi(thf)C6 H5 }(µ-AdDipp2 )2 ], (4⋅LiC6 H5 ). Computational studies disclose the existence of five-center six-electron bonding within the H-Mo≣Mo-C-Li metallacycles, with a mostly covalent H-Mo≣Mo-C group and predominantly ionic Li-C and Li-H interactions. However, the latter bonds exhibit non-negligible covalency, as indicated by X-ray, computational data and the large one-bond 6,7 Li,1 H and 6,7 Li,13 C NMR coupling constants found for the three-atom H-Li-C chains. By contrast, the phenyl group in 4⋅LiC6 H5 coordinates in an η2 fashion to the lithium atom through the ipso and one of the ortho carbon atoms.