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23.3A: Beryllium

  • Page ID
    34547
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    Learning Objectives

    In this lecture you will learn the following

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

    Synthesis

    Organoberyllium compounds are best prepared via transmetallation reactions or by reaction of beryllium halides with other organometallic compounds e.g.,

    \[ \ce{HgMe2 + Be ->[\text{383 K}] Me2Be + Hg}\]

    \[\ce{2PhLi + BeCl2 ->[\text{diethyle ether}] Ph2Be + 2LiCl}\]

    In the vapor phase \(\ce{Me2Be}\) is monomeric with a linear C—Be—C (Be-C = 170 pm), but in the solid state it is polymeric and resembles that of \(\ce{BeCl2}\) with a bonding that is considered electron deficient with 3-center-2-electron bonds. With higher alkyls the amount of polymerisation decreases and the tert-butyl derivative is monomeric and linear in both solid and vapour phases.

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

    However, 1H NMR spectrum shows that all protons environments are equivalent even at 163 K. Also, the 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.

    Beryllocene

    The reaction of \(\ce{NaCp}\) with beryllium chloride leads to beryllocene (Cp2Be)

    \[\ce{2 K [C5Me5] + BeCl2 ->[\text{388 K}][ether/toluene] (C5Me5)2Be + 2 KCl}\]

    The solid state structure suggests that the two rings are bound to the Be differently such that 1 is designated η5 and the other η1.

    alt

    Structure of \(\ce{(η^{1}-Cp)(η^{5}-Cp)Be}\) also called Beryllocene

    The experimental 1H NMR spectrum adds to the confusion of the bonding since even at 163 K the protons all appear equivalent. This is accounted for by fluxional processes. Some variations of the compound have been prepared to see how general this effect is, for example, 4 protons on each ring replaced by methyl groups and all 5 protons replaced by methyl groups, (meCp)2Be, and even 4 on one ring and 5 on the other replaced with methyl groups. In the first case the fluxional process was observed down to 183K and in the second case the two rings were found to be coparallel and staggered. (Note that the structure of ferrocene is described as eclipsed when prepared at very low temperatures or in the gas phase but when formed at higher temperatures it is disordered and more staggered and since the barrier to rotation of the two rings is quite low, at 298 K in the solid state there is motion).

    However, Cp*2Be possesses a sandwich structure with both the rings are coplanar.

    References

    1. "Inorganic Chemistry" - C. Housecroft and A.G. Sharpe, Prentice Hall, 3rd Ed., Dec 2007, ISBN13: 978-0131755536, ISBN10: 0131755536, Chapter 17.
    2. "Chemistry of the Elements", Greenwood and Earnshaw, Elsevier.

    Contributors and Attributions


    23.3A: Beryllium is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by LibreTexts.

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