Alkaline earth metal complexes 1,2-dimethoxyethane

The simplest chelates of this type, ethyleneglycol (eg) and 1,2-dimethoxyethane (glyme-2), give complexes with alkali and alkaline earth metal cations. The complexes MX2-2eg = Mg, Ca X = NOi-), MXz-Seg (MJ = Mg, Ca X = Cr, Br-), CaX2-4eg (X = Cr, Br ) and SrBr2 2eg have been isolated and IR studies suggest that the majority are likely to contain monodentate The Sr complex is, however, believed to have bidentate eg present. 

A motif found in the majority of alkali metal stabilized carbanion crystal structures is a nearly planar four-membered ring (13) with two metal atoms (M ) and two anions (A ), i.e. dimer. This simple pattern is rarely observed unadorned as in (13), yet almost every alkali metal and alkaline earth carbanion aggregate can be built up from this basic unit The simplest possible embellishment to (13) is addition of two substituents (S) which produces a planar aggregate (14). Typically the substituents (S) in (14) are solvent molecules with heteroatoms that serve to donate a lone pair of electrons to the metal (M). Only slightly more complex than (14) is the four coordinate metal dimer (15). Often the substiments (S) in (15) are joined by a linear chain. The most common of these chains are tetramethylethylenediamine (TMEDA) or dimethoxyethane (DME) so that the spirocyclic structure (16) ensues. Alternatively the donors (S) in (16) have been observed as halide anions (X ) when the metal (M ) is a divalent cation, e.g. (17) or (18). Obviously, the chelate rings found in (16) are entropically favorable relative to monodentate donors (S) in (14), (15), (17) or (18) (Scheme 2). 

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