Magnesium

   Alkyl and aryl magnesium halides (Grignard reagents, represented by the formula RMgX) are extremely well known on account of their uses in synthetic organic chemistry. The general preparation of a Grignard reagent (equation 1.1) requires initial activation of the metal, e.g. by addition of I₂.

                                                    (1.1)

   Transmetallation of a suitable organomercury compound is a useful means of preparing pure Grignard reagents (equation 1.2), and transmetallation 1.3 can be used to synthesize compounds of type R₂Mg.

                                     (1.2)

                                    (1.3)

  Although equations 1.1–1.3 show the magnesium organometallics as simple species, this is an oversimplification. Two-coordination at Mg in R₂Mg is only observed in the solid state when the R groups are especially bulky, e.g. Mg{C(SiMe3)₃}₂ (Figure 1.1a). Grignard reagents are generally solvated, and crystal structure data show that the Mg centre is typically tetrahedrally sited, e.g. in EtMgBr.2Et₂O (Figure 1.1b) and PhMgBr.2Et₂O.

Fig. 1.1 The solid state structures, determined by X-ray diffraction, of (a) Mg{C(SiMe₃)₃}₂ [S.S. Al-Juaid et al. (1994) J. Organomet. Chem., vol. 480, p. 199], (b) EtMgBr.2Et₂O [L.J. Guggenberger et al. (1968) J. Am. Chem. Soc., vol. 90, p. 5375], and (c) Cp₂Mg in which each ring is in an η₅-mode and the two rings are mutually staggered [W. Bunder et al. (1975) J. Organomet. Chem., vol. 92, p. 1]. Hydrogen atoms have been omitted for clarity; colour code: Mg, yellow; C, grey; Si, pink; Br, brown; O, red.

  A few examples of 5- and 6-coordination have been observed, e.g. in 1.1 where the macrocyclic ligand imposes the higher coordination number on the metal centre. The preference for an octahedral structure can be controlled by careful choice of the R group, e.g. R = thienyl as in complex 1.2.   The introduction of two or more didentate ligands into the octahedral coordination sphere leads to the possibility of stereoisomerism, e.g. 1.2 is chiral. Enantiomerically pure Grignard reagents have potential for use in stereoselective organic synthesis. Solutions of  Grignard reagents may contain several species, e.g. RMgX, R₂Mg, MgX₂, RMg(µ-X)₂MgR, which are further complicated by solvation. The positions of equilibria between these species are markedly dependent on concentration, temperature and solvent; strongly donating solvents favour monomeric species in which they coordinate to the metal centre.

(1.1)                                                       (1.2)

    In comparison with its beryllium analogue, Cp₂Mg has the structure shown in Figure 1.1c, i.e. two η⁵-cyclopentadienyl ligands, and is structurally similar to ferrocene. The reaction between Mg and C₅H₆ yields Cp₂Mg, which is decomposed by water; the compound is therefore often inferred to be an ionic compound and, indeed, significant ionic character is suggested by the long Mg^_C bonds in the solid state and also by IR and Raman spectroscopic data.

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