Shared interests: The structures of the title compounds (see example) were found to be directly comparable to those of lithium organocuprates, with the formal replacement of the lithium cation with a magnesium halide cation. Furthermore, both types of organocuprates form contact ion pairs in weakly coordinating solvents and solvent-separated ion pairs in strongly coordinating solvents. Supporting information for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2002/2008/z800684_s.pdf or from the author. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.