Magnesium, tetrahedral coordination

At the same time the structures in the solid state of Mc2Mg and Et2Mg were determined by X-ray powder diffraction studies . Both compounds form polymeric chains as the result of the bridging of two methyl groups between two magnesium atoms, rendering the magnesium atoms almost perfectly tetrahedrally coordinated. 

The first structure, unambiguously established by an X-ray crystal-structure determination, is MegAFMg (3), that has the structural motif of four monoanionic carbon ligands bound to magnesium in a distorted tetrahedral coordination geometry (Figure 4). ... 

From the reaction of the radical anion of l,2-bis[(2,6-diisopropylphenyl)imino]acen-aphthene (dpp-bian) with i -PrMgCl, the persistent radical complex isopropylmagnesium dpp-bian (253) was isolated in yields up to 60% (equation 20). An X-ray crystal-structure determination of 253 showed that the magnesium atom has distorted tetrahedral coordination geometry as the result of the cr-bonded isopropyl group, one coordinate-bonded diethyl ether molecule and A, A -chelate bonding of the dpp-bian radical anion. The radical anionic character of the dpp-bian moiety is indicated by the relatively long Mg—N bond distances [Mg-N 2.120(2) and 2.103(2) A]. 

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