Stabilizing Magnesium Hydride Complexes with Neutral Ligands

The reaction of Mg[N(SiMe 3 ) 2 ] 2 with PhSiH 3 in benzene at room temperature gave a white precipitate of approximate constitution MgH 1.5 N′′ 0.5 . This undefined hydride rich Mg salt can be dissolved by addition of well‐known neutral donors, such as PMDTA, DABCO or the N‐heterocyclic carbene I Me i Pr ( N , N′ ‐diisopropyl‐2,3‐dimethylimidazol‐2‐ylidine). This allowed for crystallization and structural characterization of five new magnesium hydride complexes: Mg 2 (µ‐H) 2 N′′ 2 · (PMDTA) ( 1 ), Mg 4 (µ‐H) 6 N′′ 2 · (PMDTA) 2 ( 2 ), Mg 5 (µ‐H) 7 N′′ 3 · (PMDTA) 2 ( 3 ), Mg 2 (µ‐H) 2 N′′ 2 · (DABCO) 2 ( 4 ) and Mg 2 (µ‐H) 2 N′′ 2 · (I Me i Pr) 2 ( 5 ). The PMDTA complexes 1 – 3 were only isolated as mixtures that could not be separated but complexes 4 and 5 have been fully characterized. The structures of the Mg hydride complexes 1 , 4 and 5 are reproduced well by DFT calculations (B3PW91/6‐311++G**). Bonding between Mg and hydride is largely ionic (circa 80 %) and there is very little charge transfer from the neutral donor to Mg 2+ . The calculated solvation enthalpies (Δ H solv ) per ligand are –12.4 kcal/mol (PMDTA), –15.9 kcal/mol (DABCO) and –20.8 kcal/mol (I Me i Pr). In complexes 4 and 5 Bond‐Critical‐Points are found between the two negatively charged hydride ligands. The very high electron densities of 0.15–0.16 e/Å 3 in these unusual BCP's are related to the short H – ··· H – distances of circa 2.55 Å.