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2.3: Organometallic Compounds of Alkaline Earth Metals (Beryllium and Magnesium)

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    172726

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    Objectives

    In this lecture you will learn the following

    • Organometallic compounds of beryllium and magnesium.
    • Structural features of alkyl lithium and beryllium sandwich compounds.

    Beryllium

    clipboard_e63ffe2fdd08bbf1e7507e776c3487cea.png

    \[\ce{HgMe2 + Be -> Me2Be + Hg (at \: 383K)} \nonumber \]

    \[\ce{2PhLi + BeCl2 -> Ph2Be +2LiCl (in\:diethyl\:ether)} \nonumber \]

    In vapor phase, Me2Be is monomeric with a linear C—Be—C (Be-C = 170 pm).
    The solid state structure is polymeric and resembles that of BeCl2.

    \[\ce{2NaCp + BeCl2 -> Cp2Be + 2NaCl} \nonumber \]

    The X-ray diffraction at 128 K suggested [(η1-Cp)(η5-Cp)Be].
    However, 1H NMR spectrum shows that all protons environments are equivalent even at 163 K.

    Also, solid state structure shows the Be atom is disordered over two equivalent sites and NMR data can be interpreted in terms of fluxional process in which the Be atom moves between these two sites.
    However, Cp*2Be possesses a sandwich structure with both the rings are coplanar.

    clipboard_e950e647887ad675e8fc0529410ef32ff.png

    Magnesium

    Alkyl and aryl magnesium halides (Grignard reagents, RMgX) are extremely well-known on account of their uses in synthetic chemistry.

    \[\ce{Mg + Rx -> RMgX (in\:diethyl\:ether)} \nonumber \]

    Transmetallation is useful means of preparing pure Grignard reagents.

    \[\ce{Mg + RHgBr -> Hg + RMgBr} \nonumber \]

    \[\ce{Mg + R2Hg -> Hg + R2Mg} \nonumber \]

    Two-coordination at Mg in R2Mg is observed only when the R groups are sufficiently bulky, e.g. Mg{C(SiMe3)3}2.
    RMgX are generally solvated and Mg centre is typically tetrahedral.
    e.g. EtMgBr.2Et2O; PhMgBr.2Et2O.

    Cp2Mg has a staggered sandwich structure.

    Solutions of Grignard reagent may contain several species, e.g. RMgX, R2Mg, MgX2, RMg(μ-X)2MgR, which are further complicated by solvation. The position of equilibrium between these species is markedly dependent on concentration, temperature and solvent; strongly donating solvents favour monomeric species in which they coordinate to the metal centre.

    clipboard_e9c7a8fe3b20f56c1a6ee72f2a27d3037.png

    Treatment with dioxane results in the precipitation of MgCl2(dioxane) leaving behind pure R2Mg in the solution.

    Problems:

    1. The compound (Me3Si)2C(MgBr)2.nTHF is monomeric. Suggest a value of ‘n’ and propose a structure for this Grignard reagent.

    Solution

    clipboard_eec42cb9e7a1dcaeed2d61e7606c0176c.png

    2. If a typical Grignard reagent exists as an equilibrium mixture of dialkylmagnesium and magnesium halide, give a method of isolating pure dialkyl magnesium. Your answer should be in the form of balanced chemical equations only.

    Solution

    \[\ce{2RMgX <=> R2Mg + MgX2} \nonumber \]

    Treatment of equilibrium mixture with dioxane results in the precipitation of, say, MgCl2(dioxane) (if, X = Cl), leaving behind pure R2Mg in the solution.

    clipboard_efa410328b540f4dcf761ddba8fc0d69d.png

    This page titled 2.3: Organometallic Compounds of Alkaline Earth Metals (Beryllium and Magnesium) is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by M. S. Balakrishna & Prasenjit Ghosh via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.