An unexpected structural principle for crystalline MgBr2 center dot OEt(2): Chains of trigonal bipyramids

MgBr2 . OEt(2) (1), which is commercially available and has been examined by different methods in the past, was obtained in crystalline form as a by-product of the reaction of amine-stabilised AlCl solution with mesitylene magnesium bromide. The following unexpected structure was obtained by X-ray techniques: five-coordinate magnesium centres form trigonal bipyramids, which are connected by two common bromine atoms into chains. Structural details and especially the relationship with solid MgBr,, MgBr2 .(OEt(2))(2) and MgI2 .(OEt(2))(2) (2) are discussed in order to illuminate the unusual bonding in 1.