Alkali metals chelated complexes

The molecules producing coordinative saturation need not be chelating. As a by-product to the study of synergism in the extraction of alkali metal chelate salts, Healy (14) isolated crystalline complexes Li(PhCO-CHCOPhJSa where S = tri-n-octylphosphine oxide or N, N-dibutyl-acetamide. 

The general structure of alkali metal alcoholates of polyhydroxy compounds is probably very similar to those proposed by Martell and Calvin1 for the alkali metal chelates of o-salicylaldehyde (see Figs. 9 and 10). 148 1M>IM Unfortunately, because of the highly amorphous nature of nearly all pf the alcoholates and adducts formed by the interaction of metal hydroxides with carbohydrates, x-ray diffraction studies have failed to furnish information regarding the precise location of the metal in these complexes. 

The data in Table V were obtained by sequential treatment of the initial sample with sodium iodide and lithium chloride. Complexation with sodium iodide was done in a heptane-benzene slurry. The sparingly soluble sodium iodide chelate was isolated by filtering the mixture. The remaining solution was concentrated, and the residue obtained was contacted with lithium chloride in pentane. After stirring this heterogeneous mixture, a solid lithium chloride chelate complex was isolated by filtration. Decomposition of the alkali-metal salt complexes followed by recovery and analysis of the polyamine components showed that the sodium iodide complex contained 82.6% n-HMTP while the LiCl complex contained 94.8% N,N -c-PMPP. Table V shows that the initial polyamine sample contained 48.8 and 11.4% of these ligands, respectively. 

Becker et al. reported the first structurally authenticated alkali metal arsenide complex in 1982 (102). The complex [Li As(SiMes)2 (DME)]2, synthesized by the reaction of As(SiMe3)s with BuLi or MeLi in DME, crystallizes as centrosymmetric dimers containing a planar Li2As2 rhombus-shaped core [Li-As = 2.59(2) A As-Si = 2.307(7) A As-Li-As = 99(1)°]. Tetrahedral coordination of the lithium atoms is achieved by coordination to the two 0 atoms of the chelating DME hgands. Cryoscopic measurements indicate that the dimeric structure of this complex is preserved in solution. (Selected bond lengths and angles of alkali metal arsenides are listed in Table III.)... 

A. W. Laager, ed., "Polyamine Chelated Alkali Metal Complexes," Chem. Ser. 130 (1974). 

Oxygen chelates such as those of edta and polyphosphates are of importance in analytical chemistry and in removing Ca ions from hard water. There is no unique. sequence of stabilities since these depend sensitively on a variety of factors where geometrical considerations are not important the smaller ions tend to form the stronger complexes but in polydentate macrocycles steric factors can be crucial. Thus dicyclohexyl-18-crown-6 (p. 96) forms much stronger complexes with Sr and Ba than with Ca (or the alkali metals) as shown in Fig. 5.6. Structural data are also available and an example of a solvated 8-coordinate Ca complex [(benzo-l5-crown-5)-Ca(NCS)2-MeOH] is shown in Fig. 5.7. The coordination polyhedron is not regular Ca lies above the mean plane of the 5 ether oxygens... 

In the skeleton of many chelating diphosphines, the phosphorus atoms bear two aryl substituents, not least because the traditional route to this class of compounds involves the nucleophilic substitution with alkali metal diarylphosphides of enantiopure ditosylates derived from optically active natural precursors, approach which is inapplicable to the preparation of P-alkylated analogs. The correct orientation of these aryl substituents in the coordination sphere has been identified as a stereo chemically important feature contributing to the recognition ability of the metal complex [11,18-20]. 

The low affinity of the alkali metals for neutral P-donor ligands has hampered efforts to synthesize complexes in which there is a genuine R3P-M interaction (see Section I). However, this poor affinity may be overcome by incorporating a remote phosphine functionality into a potentially chelating anionic ligand, such as a phosphine-substituted alkoxide, amide, or aryl, and several alkali metal complexes of such ligands have been isolated. 

A triply chelating ligand is the purpurate anion, (/), the ammonium salt of which is the indicator murexide. The kinetics of its complex formation with alkali metals have been reviewed (75). In the lithium salt the cation is 5-coordinated in a square pyramid with the base formed by the three donor atoms, 0, N, O from one anion and a carbonyl oxygen from... 

Since non-bound or non-coordinated nucleophiles are even more reactive, crown-ethers [138] and cryptands (polyaminoethers) [139,140] have been used to chelate the alkali metal cations, notably the potassium ion of K[ F]F. This allows the [ F]fluoride anion to be less tightly paired with the cation and therefore to be more reactive, which has been coined the naked ion effect. In practice, the crown-ether (e.g. 18-crown-6) or better the polyaminoether Kryptofix-222 (4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane) is added to the aqueous K[ F]F/K2C03 solution which is then concentrated to dryness [139,140]. The complex (KP FIF-K ) can be further dried, if needed, by one or more cycles of addition of dry acetonitrile and azeotropic evaporation. 

These results clearly indicate that the chelate ligation is driven primarily by the enthalpic factor and the entropy plays merely a trivial role in determining the complex stability. This is quite reasonable since the structures of these chelate complexes are strictly defined by the number and direction of the coordination sites of given heavy/transition metal ions, and therefore there is little room for the entropic term to adjust flexibly the complex structure and stability. On the contrary, alkali and alkaline earth metal ions also have the formal coordination numbers, but the actual number and direction of ligand coordination are highly flexible in the weak interaction-driven ligation by hard donors like glyme and crown ether. 

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