X-Ray Structural Analyses of Organomagnesium Compounds

Publisher Summary This chapter discusses the X–ray structural analyses of organomagnesium compounds. Organomagnesium compounds are well-established tools in organic chemistry. Compared to the long history of the synthetic applications of organomagnesium compounds, the investigation of their structure and bonding covers a much shorter period. By the use of gas-phase electron diffraction (GED), it is possible to determine the structure of molecules in the gas phase. A major drawback of GED is that only small molecules can be resolved completely; for larger molecules, the degrees of freedom must be limited during the calculations by assuming and optimizing values for many of the structural parameters. In almost all structure determinations of simple Grignard or diorganylmagnesium compounds, the magnesium atoms are surrounded by four ligands. In the absence of coordinating solvents such as ethers, most organomagnesium compounds tend to form polymeric structures. Solid and volatile organomagnesium compounds can be slowly sublimed in a static high vacuum. On the symmetrization of di-Grignard reagents to the analogous halogen-free compounds, both polymeric and cyclic species may be expected. Although the tetrahedral coordination geometry is preferred by organomagnesium compounds, many examples have been structurally characterized, which show deviating coordination numbers. Increased coordination numbers for magnesium are also found for an ethylmagnesium-chloride/magnesium-chloride adduct. Fascinating structures can be obtained on the complexation of organomagnesium compounds with polyether ligands.