Complexes of the Heavier Alkali Metals

There are many examples known of n bonding between the heavier alkali metals and carbanionic fragments. In most cases evidence for n complexation stems from spectroscopic studies in solution (2) here we restrict ourselves to those situations where n interactions have been proved by X-ray structural 

The crystal structure of the red triphenylmethylsodium TMEDA complex (compound XVI in Fig. 3), published by Weiss and Koster (41), resembles that of the red triphenylmethyllithium-TMEDA complex (42) and can be described as a n complex between a triphenylmethyl carbanion with an sp2-hybridized central carbon atom and a sodium cation coordinated to the bidentade ligand TMEDA. The sodium atom has close contacts to several carbon atoms of the triphenylmethyl ligand, which possesses twisted phenyl groups. An additional short distance exists between sodium and a p-C (phenyl) atom of a neighboring n system. 

The crystal structure of the fluorcnylpotassium-TMEDA complex (compound XlXa in Fig. 3) has been solved by Stucky et al. (45). The coordination sphere of the potassium atom is made up of two tertiary amines and two unsaturated groups instead of two tertiary amines and one unsaturated organic group as found for the lithium atom in the fluorenyllithium complex VI (12). 

The structure of a potassium salt of a cyclooctatetraene dianion has been determined by Raymond and co-workers (269). In this bright yellow complex (XlXb in Fig. 3) a diglyme molecule is coordinated to each of the potassium atoms through all three oxygen atoms. Two potassium-diglyme units lie on either side of the planar carbocyclic ring equidistant from the ring center. 

Finally, two other types of n coordination to sodium documented by crystal structure data will be mentioned. rc-Type bonding interactions between bis(THF)sodium units and the benzene rings of complex aluminate anions derived from naphthalene or anthracene have been found in the compounds [Na(THF)2]2[Me2AlC10H8]2 and [Na(THF)2]2[Me2AlC14H10]2 (46). Even more complex coordination patterns between sodium, transition metals, and n systems have been reported by Jonas and Kruger (5). 

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The complexes of the heavier alkali metals are probably essentially ionic in character. In thf the potassium and pentadienyl anions form contact ion pairs. The bonding is thought to be rj5 not only with the compact U form but also with the W and S forms. The preferred conformation is U except where there are bulky 1- and 5-substituents. In liquid ammonia, which is a better solvating medium than thf, solvent-separated ion pairs are present and the anion adopts a W form. 

Fig. 3. Simplified structures of n complexes of the heavier alkali metals.

The bonding interactions within organoalkah metal complexes of the heavier alkali metals are generally considered to be strongly electrostatic or ionic in natnre. This is snpported by a large collection of evidence, consisting primarily of solution NMR data, single-crystal X-ray analyses, and gas-phase computational studies. 

To evaluate conclusively the hypotheses presented here, additional solution and solid-state studies are needed, particularly for trisolvated organolithium compounds, solvent-separated complexes, and unsaturated complexes of the heavier alkali metals. The results do, however, furnish the first glimpse of the stereochemistry of a chemically important class of compounds and should also provide working models for understanding the role of Group la metals in polymerization reactions and in their reactions with electrophilic reagents. 

A family of recently published Cp complexes sheds a more detailed light on the role of the donors on the overall structural pattern. Demonstrated with a group of Cp derivatives in the presence of the crown ethers 15-crown-5 and 18-crown-6, monomeric complexes may be obtained in the form CpNa(15-crown-5) 84. In a parallel fashion, 18-crown-6 has been shown to be effective in supporting monomeric structures of the heavier alkali metals bound to Cp. Examples include CpM(18-crown-6) (M = K 85, Rb 86, Cs 87).100 101... 

The extensive use of alkyllithium initiators is due to their solubility in hydrocarbon solvents. A common example is n-butyllithium which is usually available as a solution in n-hexane. The C-Li bond is not ionic in hydrocarbon media where the initiator molecules exist as aggregates. Initiation is thus fairly slow in hydrocarbon media. Addition of tetrahydrofuran to this solvent increases the concentration of unaggregated initiator (which is more active for initiation) by forming a 1 1 complex with this compound. Alkyls and aryls of the heavier alkali metals, such as Na and K, are poorly soluble in hydrocarbons because of the greater ionic character of the Na—C and K-C bonds. 

Alkali oxides are thermodynamically stable up to very high temperatures, and even hydrides have saline character and considerable stability. Lithium nitride is a compound which can be isolated from the solution in the metal in crystalline form. Dissolved oxides have the ability to react with transition metal oxides to form complex oxides, or with hydrogen to form hydroxides of the heavier alkali metals. Lithium cyanamide is formed by means of the reaction between nitrogen and carbon dissolved in the molten metal. The reaction product in liquid sodium is sodium cyanide. 

Orzechowsld L, Jansen G, Harder S (2009) Methandiide complexes (R2CM2) of the heavier alkali metals (M=potassium, rubidium, cesium) reaching the limit Angew Chem hit Ed 48 3825-3829. doi 10.1002/anie.200900830... 

Similar structural diversity has been established for the heavier alkali metals also but it is unnecessary to deal with this in detail. The sUTictural chemistry of the organometallic compounds in particular, and of related complexes, has been well reviewed. 

Sulfur diimides react quantitatively with organolithium reagents at the sulfur centre to produce lithium sulfinimidinates of the type Li[RS(NR )2] A. The lithium derivatives may be hydrolysed by water to R NS(R)NHR which, upon treatment with MH (M=Na, K) or the metal (M=Rb, Cs) in THF, produces the heavier alkali-metal derivatives.132 The structures of these complexes are influenced by (a) the size and electronic properties of the R group, (b) the size of the alkali metal cation, and (c) solvation of the alkali-metal cation. 

The different type of n bonding in the lithium and in the heavier alkali metal complexes can be deduced from the metal-ligand centroid distances, which are collected for some comparable alkali metal n complexes in Table I. In the complexes with the heavier metals these distances are greater by the value <5A than expected from the differences in the ionic radii AM+. As already pointed out by Stucky (7), the bonding in the sodium and potassium n complexes can... 

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