Group 1: alkali metal organometallics

   Organic compounds such as terminal alkynes (RC≡CH) which contain relatively acidic hydrogen atoms form salts with the alkali metals, e.g. reactions 1.1and 1.2 .

           (1.1)

           (1.2)

    Similarly, in reaction 1.3, the acidic CH₂ group in cyclopentadiene can be deprotonated to prepare the cyclopentadienyl ligand which is synthetically important in organometallic chemistry  Na[Cp] can also be made by direct reaction of Na with C₅H₆. Na[Cp] is pyrophoric in air, but its air-sensitivity can be lessened by complexing the Na‏ ion with 1,2-dimethoxyethane (dme). In the solid state, [Na(dme)][Cp] is polymeric (Figure 1.1).

                               (1.3)

Fig. 1.1 Part of a chain that makes up the polymeric structure of [Na(dme)][Cp] (dme=1,2-dimethoxyethane); the zig-zag chain is emphasized by the hashed, red line. The structure was determined by X-ray diffraction [M.L. Coles et al. (2002) J. Chem. Soc., Dalton Trans., p. 896]. Hydrogen atoms have been omitted for clarity; colour code: Na, purple; O, red; C, grey.

A pyrophoric material is one that burns spontaneously when exposed to air.   Colourless alkyl derivatives of Na and K are obtained by transmetallation reactions starting from mercury dialkyls (equation 1.4).

                                                   (1.4)

   Organolithium compounds are of particular importance among the group 1 organometallics. They may be synthesized by treating an organic halide, RX, with Li (equation 1.5) or by metallation reactions (equation 1.6) using nbutyllithium which is commercially available as solutions in hydrocarbon (e.g. hexane) solvents.

                     (1.5)

                                          (1.6)

   Solvent choices for reactions involving organometallics of the alkali metals are critical. For example, ⁿBuLi is decomposed by Et₂O to give nBuH, C₂H₄ and LiOEt.  Alkali metal organometallics are extremely reactive and must be handled in air- and moisture-free environments; NaMe, for example, burns explosively in air.

   Lithium alkyls and aryls are more stable thermally than the corresponding compounds of the heavier group 1 metals (though they ignite spontaneously in air) and mostly differ from them in being soluble in hydrocarbons and other nonpolar organic solvents and in being liquids or solids of low melting points. Sodium and potassium alkyls are insoluble in most organic solvents and, when stable enough with respect to thermal decomposition, have fairly high melting points. In the corresponding benzyl and triphenylmethyl compounds, Na‏⁺[PhCH₂]^-­ and Na⁺‏[Ph₃C]^- (equation 1.7), the negative charge in the organic anions can be delocalized over the aromatic systems; this enhances stability and the salts are red in colour.

                                 (1.7)

   Sodium and potassium also form intensely coloured salts with many aromatic compounds (e.g. reaction 1.8). In reactions such as this, the oxidation of the alkali metal involves the transfer of one electron to the aromatic system producing a paramagnetic radical anion.

                (1.8)

Table 1.1 Degree of aggregation of selected lithium alkyls at room temperature (unless otherwise stated).

    A radical anion is an anion that possesses an unpaired electron. Lithium alkyls are polymeric both in solution and in the solid state. Table 1.1 illustrates the extent to which MeLi, ⁿBuLi and ^tBuLi aggregate in solution. In an (RLi)₄ tetramer, the Li atoms form a tetrahedral unit, while in an (RLi)₆ hexamer, the Li atoms define an octahedron. Figures 1.2a and b show the structure of (MeLi)₄; the average Li^_Li bond length is 261 pm compared with 267pm in Li₂ , the bonding in lithium alkyls is the subject of problem 1.2 at the end of the chapter. Figures 1.2c and d show the structure of the Li₆C₆-core of (LiC₆H₁₁)₆ (C₆H₁₁ = cyclohexyl); six Li^_Li bond distances lie in the range 295–298pm, while the other six are significantly shorter (238–241pm). The presence of such aggregates in solution can be determined by using multinuclear NMR spectroscopy. Lithium possesses two spin-active isotopes and the solution structures of lithium alkyls can be studied using ⁶Li, ⁷Li and ¹³C NMR spectroscopies as worked example 1.1 illustrates. The alkyls of Na, K, Rb and Cs crystallize with extended structures  or are amorphous solids.