Model structural types for rationalising stoichiometries and coordination numbers in lithium amidomagnesiates: syntheses and X-ray diffraction studies of [{Mg(NR 2 ) 2 } 2 ], [Li 2 Mg(NR 2 ) 4 ] and [LiMg(NR 2 ) 3 ·py](R = benzyl, py = pyridine)

Factors governing the intermetallic ratios and coordination geometries in lithium amidomagnesiates are discussed with reference to three model crystal structures containing the same (dibenzylamido) ligand, which remarkably reveal that an unsolvated lithium can induce coordination expansion about a magnesium centre.