Structures of Organomagnesium Compounds in the Gas Phase

Though not belonging to the topic of this article in a strict sense, a brief discussion of gas-phase structure is imperative for reasons of complementarity and comparison, and it is facilitated by the scarcity of structures. By the use of gas-phase electron diffraction (GED), it is possible to determine the structure of molecules in the gas phase. Results obtained with this technique on organomagnesium compounds are of special interest for theoretical chemists, since calculations refer to molecules in the gas phase, too. In many cases, the structure of an organomagnesium compound in the gas phase will be different from that in the crystalline state. These differences often stretch beyond the effects of crystal packing. 

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. A second barrier for the use of GED may be the low 

The GED structure determination of 3 showed a centrosymmetric molecule with a linear C—Mg—C center (4). On evaporation of 3, which is trimeric in benzene solution and probably polymeric in the crystalline state, dissociation into monomeric molecules occurs. The structure obtained is in accordance with the considerations in Section I,C. Important geometric parameters of the structure are the distances Mg—C (2.126 A) and C—C (1.541 A) and the angle Mg—C—C (118°). By fusion of3 and 2, the mixed crystalline compound 4 (m.p. 70-72°C) was obtained. The structure of this compound, which is stable toward comproportionation to 

In this context, we mention also the gas-phase structures of MgBr2 (7) and Mglj ( 8) both are linear, and the bond lengths are Mg—Br 2.34 A and Mg—1 2.52 A. 

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