Systematic variation of the 1,4-dioxane (dx) concentration during the precipitation of sparingly soluble [MgBr 2 (dx) 2 ] from ethereal Grignard solutions of RMgBr has allowed the structural investigation of crystallized [R 2 Mg(dx) n ] (n = 1, 1.5, 2, and 3), which form during this dioxane method, depending on the bulkiness of R. The numbering of the complexes explored in this study is based on the number n of dioxane molecules per magnesium atom, followed by the substituent R; an apostrophe denotes coordination polymers. The following derivatives were studied by X-ray crystal-structure determination and NMR spectroscopy :(1.5-oTol), and [(Me 3 Si-CC) 2 Mg(dx) 1.5 ] 1 (1.5'-C 2 SiMe 3 ); n = 2: [tBu 2 Mg(dx) 2 ] (2-tBu) and [oTol 2 Mg(dx) 2 ] (2-oTol); n = 3: [Ph 2 Mg(dx) 3 ] (3-Ph). In the structure types 1', 1.5, and 2, the magnesium atom exhibits the coordination number 4, whereas pentacoordinate metal atoms are observed in types 3 and 1.5'. The structure type 2' is realized for [(Ph-C C) 2 Mg(dx) 2 ] 1 (2'-C 2 Ph), [MgCl 2 (dx) 2 ] 1 (2'-Cl), and [MgBr 2 (dx) 2 ] 1 (2'-Br) with hexacoordinate metal atoms. The solubility of the dioxane adducts in common organic solvents strongly depends on the degree of aggregation with the solubility decreasing from molecular to strand to layer structures.[a] Dr.Scheme 2. Dioxane method to shift the Schlenk equilibrium of organylmagnesium halides toward soluble [R 2 Mg(dx) n ] and insoluble [MgX 2 (dx) 2 ] 1 by substitution of the Lewis base L (e.g., diethyl ether or thf) by 1,4-dioxane (dx).Scheme 3. Structural diversity of hitherto known 1,4-dioxane adducts of diorganylmagnesium complexes authenticated by X-ray crystal-structure determinations.