Alkali metals ionic compound formation

The production of n-butylbenzene may be attributed to an inherent lack of complete selectivity in carbanion reactions, because the greater stability of an intermediate does not exclude the formation of the less stable product. This stability is only important when the step in forming intermediates is slow or when energy differences are large. On the other hand, the formation of n-butylbenzene from toluene and propylene may be due to a partial radical character of benzyl alkali metals. The latter would not seem to be the case because the potassium compounds should have greater ionic character, but they yield more n-butylbenzene. This agrees with the idea that lack of selectivity may be due to greater rate of reaction of potassium compounds with olefins. 

The highly electropositive character of the lanthanide metals, which is comparable to that of the alkali and alkaline earth metals, leads as a rule to the formation of predominantly ionic compounds, Ln(III) being the most stable oxidation state [58]. Scheme I outlines this and other intrinsic properties of the lanthanide series and will serve as a point of reference in this section [59-62]. In the following, electronic and steric properties are treated separately. 

Around 1928, Zintl had begun to investigate binary intermetallic compounds, in which one component is a rather electropositive element, e.g., an alkali- or an alkaline earth metal [1,2]. Zintl discovered that in cases for which the Hume-Rothery rules for metals do not hold, significant volume contractions are observed on compound formation, which can be traced back to contractions of the electropositive atoms [2]. He explained this by an electron transfer from the electropositive to the electronegative atoms. For example, the structure of NaTl [3] can easily be understood using the ionic formulation Na Tl" where the poly- or Zintl anion [TF] forms a diamond-like partial structure - one of the preferred structures, for a four electron species [1,2], Zintl has defined a class of compounds, which, in the beginning, was a somewhat curious link between well-known valence compounds and somehow odd intermetallic phases. 

The reason that different types of oxides are formed when alkali metals react with oxygen has to do with the stability of the oxides in the solid state. Since these oxides are all ionic compounds, their stability depends on how strongly the cations and anions attract one another. Lithium tends to form predominantly lithium oxide because this compound is more stable than lithium peroxide. The formation of other alkali metal oxides can be explained similarly. 

Favorable thermodynamics (AG° (298K) <0) for obtaining acetylene diolates from M-M bonded complexes occurs when 2(M-0)>(M-M)+ 147 kcal while single metal units require that the M-0 bond energy exceed 78 kcal (Table I, entries u,v). This limiting type of CO coupling is best known for reactions of alkali metals with CO which form solid ionic acetylene diolate compounds.Rhodium macrocycle complexes have Rh-0 bond energies 50-60 kcal and thus are excluded as potential candidates for acetylene diolate formation. 

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