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The constitution of Grignard reagent, RMgCl (R = Me, tBu, Ph or benzyl), was investigated in the solid state by means of X-ray crystallography and in THF by coldspray ionization mass spectrometry (CSI-MS). Three types of crystal structures, (a) Mg2(mu-Cl3)(THF)6(+)*RMgCl2(THF)(-), (b) R2Mg4Cl6(THF)6, and (c) 2Mg2(mu-Cl3)(THF)6(+)*R4Mg2Cl22-, were identified, and MeMg2(mu-Cl3)(THF)4-6 were detected as major species of MeMgCl in solution.
Sakamoto et al. (2001) studied this question.