Ammoniates, metal

Physical chemical studies of dilute alkali metal-ammonia solutions indicate the principal solution species as the ammoniated metal cation M+, the ammoniated electron e , the "monomer M, the "dimer" M2 and the "metal anion" M. Most data suggest that M, M2, and M are simple electrostatic assemblies of ammoniated cations and ammoniated electrons The reaction, e + NH3 - lf 2 H2 + NH2 is reversible, and the directly measured equilibrium constant agrees fairly well with that estimated from other thermodynamic data. Kinetic data for the reaction of ethanol with sodium and for various metal-ammonia-alcohol reductions of aromatic compounds suggest that steady-state concentrations of ammonium ion are established. Ethanol-sodium reaction data allow estimation of an upper limit for the rate constant of e + NH4+ 7, H2 + NH3. 

The dimer species, M2, was described by Huster (21) as a simple dissolved diatomic molecule. Such a species would be very unstable with respect to the ammoniated species, M + and e and may be ruled out by thermochemical data. The expanded metal dimer model of Becker, Lindquist, and Alder, in which two ammoniated metal ions are held together by a pair of electrons in a molecular orbital located principally between the two ions, is just as difficult to reconcile with optical, volumetric, and NMR data as the expanded metal monomer. In order to account for the similar absorption spectra of e, M, M2 (and any other species such as M or M4 that might exist at moderate concentrations of metal), Gold, Jolly, and Pitzer (16) assumed that species such as M and M2 consist of ionic aggregates in which the ammoniated electrons remain essentially unchanged from their state at infinite dilution. 

Pharmaceuticals. A variety of mercury compounds have had pharmaceutical appHcations over the years, eg, mercury-containing diuretics and antiseptics. Whereas some mercury compounds remain available for use as antiseptics such as merbromin [129-16-8] mercuric oxide, and ammoniated mercury [10124-48-8] or as preservatives such as thimerosal [54-64-8] in dmgs and cosmetics, most have been supplanted by more effective substances. A detailed discussion of mercury-containing antiseptics is available (37). Many hospitals use mercury metal to serve as weight for keeping nasogastric tubes in place within the stomach. 

As first demonstrated by Stork,the metal enolate formed by metal-ammoni reduction of a conjugated enone or a ketol acetate can be alkylated in liquic ammonia. The reductive alkylation reaction is synthetically useful since ii permits alkylation of a ketone at the a-position other than the one at whicf thermodynamically controlled enolate salt formation occurs. Direct methyl-ation of 5a-androstan-17-ol-3-one occurs at C-2 whereas reductive methyl-... 

The heavier alkaline earth metals Ca, Sr, Ba (and Ra) react even more readily with non-metals, and again the direct formation of nitrides M3N2 is notable. Other products are similar though the hydrides are more stable (p. 65) and the carbides less stable than for Be and Mg. There is also a tendency, previously noted for the alkali metals (p. 84), to form peroxides MO2 of increasing stability in addition to the normal oxides MO. Calcium, Sr and Ba dissolve in liquid NH3 to give deep blue-black solutions from which lustrous, coppery, ammoniates M(NH3)g can be recovered on evaporation these ammoniates gradually decompose to the corresponding amides, especially in the presence of catalysts ... 

See Gold(III) chloride Ammonia Mercury Ammonia Potassium triamidothallate ammoniate Silver azide Ammonia Silver chloride Ammonia Silver nitrate Ammonia Silver(I) oxide Ammonia See N-METAL DERIVATIVES... 

Lewis Bases. A variety of other ligands have been studied, but with only a few of the transition metals. There is still a lot of room for scoping work in this direction. Other reactant systems reported are ammoni a(2e), methanol (3h), and hydrogen sulfide(3b) with iron, and benzene with tungsten (Tf) and plati num(3a). In a qualitative sense all of these reactions appear to occur at, or near gas kinetic rates without distinct size selectivity. The ammonia chemisorbs on each collision with no size selective behavior. These complexes have lower ionization potential indicative of the donor type ligands. Saturation studies have indicated a variety of absorption sites on a single size cluster(51). 

Because ammonia is the solvent, the solvated species is known as an ammoniate. The solvent molecules are not always bound in the solid in the same way. For example, some solids may contain water of hydration, but in other cases the water may be coordinated to the metal ion. In classifying materials as hydrates or ammoniates, the mode of attachment of the solvent is not always specified. 

Although beryllium and magnesium salts do not form stable mctal-ammines yet they unite with ammonia, forming additive compounds of the hydrate type which are sometimes referred to as ammoniates or ammonio-compounds. These appear to be of the same type as the metal-anunines, and the difference seems to be merely one of stability. The ammonio-compounds are formed by the addition of ammonia gas to dry or fused salt, and most of them decompose with liberation of ammonia when dissolved in water. 

These uncertainties are increased when the existence of compounds of CO with metals, the carbonyls such as Fe(CO)5, for which no electrostatic model is conceivably possible are considered. The iron obviously is not present in the compound as an ion how then can the attraction for the CO molecules be explained Even in the straight ammoniates there is some doubt regarding the validity of the simple electrostatic representation of the structure for it is found experimentally that the magnetic properties of halides are radically altered by the taking up of molecules of ammonia. This shows that - some of the electrons of the positive ions are influenced by the ammonia molecules in a way which an electrostatic picture cannot explain. 

There are ammoniates of PtCl2, of halides of other platinum metals and of cobalt and nickel, too, some of which have been mentioned before in, Section 50. The cobalt complexes clearly show the importance of the completed d shells for the stability of the complex. Non complex compounds of trivalent cobalt are very unstable. Solutions of divalent cobalt in ammonia, however, are readily oxidized by air, because the NH3 complex of trivalent cobalt Co(NH3)6 3+ClT has eighteen electrons used in bond formation, whereas the ion Co(NH3) + would have nineteen electrons. 

Ammonia can form many "coordinated complexes with the above mentioned metals and these complexes, called "ammines or "ammoniates" are among the most important. Usually divalent metals form complexes with four NH, groups, while trivalent metals usually coordinate with six NH, groups. There are, however, many exceptions... 

Kraus Johnson(Ref 2) reinvestigated the solns of lithium in liq ammonia and came to the conclusion that there is no evidence indicating that Li is joined to the nitrogen of ammonia, forming a substituted ammonium ion Note The compds formerly called "metal-ammoniums are now called amminesf,qv) or ammoniates ... 

The alkali metals are soluble in liquid ammonia, and certain amines, to give solutions which are blue when dilute. The solutions ate paramagnetic and conduct electricity, the carrier being the solvated electron. In dilute solutions the metal is dissociated into metal ions and ammoniated electrons. The metal ions are solvated in the same way that they would be in a solution of a metal salt in ammonia, and so comparison can be made with, for example, [Na(NH3)4]+I-, the IR and Raman spectra of which indicate a tetrahedral coodination sphere for the metal.39... 

Heavy Metals. -- Dissolve 1 uni. iwith hydrogen sulphide water nor with ammoni. i water in excess accompanied by 2 to 3 drops of ammonium sulphide solution. 

The monomer species, M, has been described by Kraus (31) as an ion pair. Although he did not elaborate on its possible structure, one may assume that he pictured this species as two ammoniated ions held together by electrostatic forces. Douthit and Dye (12) pointed out that such a picture is consistent with the absorption spectra of sodium-ammonia solutions. Becker, Lindquist, and Alder (2) proposed an expanded metal model in which an electron was assumed to circulate in an expanded orbital on the protons of the coordinated ammonia molecules of an M + ion. The latter model is difficult to reconcile with optical, volumetric, and NMR data (16). 

The initially formed ammoniates Ln(NH3)6 (Eq. la) can be isolated by evaporation of NH3 as gold metallic solids and low temperature X-ray studies at 200 K reveal a body-centered cubic array of octahedral molecules [38a]. Eu(NH3)6 can easily be converted to pure Eu(NH2)2 by catalytical (Fe203) [40a] and thermal (50 °C) treatment [40b] (Eq. lb,c). In the case of ytterbium... 

The ternary systems display a variety of structural chemistry depending on the sizes of the alkaline and lanthanide metals (Scheme 3 Fig. 3 [43, 45-57]). The smaller alkali cations determine the expected coordination structures as found in salt-like compounds, e.g., Na3Y (NH2)6 or KY(NH2)4. Layer structures are observed in alkali metal poor systems like MLa2(NH2)7 while cesium derivatives, apart from the lanthanum compounds, form perowskit-like arrangements as in CsEu(NH2)3 and Cs3Ln2(NH2)9. The mono ammoniates of some Cs-systems are probably metastable. Preparation of analogous ternary systems with Li were unsuccessful in contrast to, e.g., LiAl(NH2)4 [58]. 

The hydroxides dissolve in NH4OH, not because of any acidic character but because of the ability of the metal ions to form ammoniates (see pages 118-119). Addition of ammonia to the simple metal radical seems to strengthen its metallic character so that it can exist more easily as a positive ion. Thus the hydroxides of the ammonio-metal radicals, excepting those of mercury, are soluble and as highly ionized as the hydroxides of the alkali metals. 

This potassium salt, K4Ni2(CN)6, may be further reduced by potassium in liquid ammonia to yield a yellow substance, K4Ni(CN)4. This has nickel in the zero-valent state and is thus comparable to the metal carbonyls, Fe(CO)5 and Ni(CO)4 (p. 157), to cobalt nitrosyl carbonyl Co(CO)4NO, and to the metal ammoniates Ca(NH3)6 and Pt(NH3)2. However, K4Ni(CN)4, and the closely related acetylene derivative, K4Ni(C=CH)4, are especially unusual, for in them, the zero-valent metal has been incorporated into an anion, whereas in the carbonyls and metal ammoniates, the zerovalent metals are present as uncharged species. 

Based on the data presented in Figures 12.20-12.22, it is clear that the potential of NH3 to solubilize heavy metals depends on metal softness and on the concentration of NH3. Soft metals (see Chapter 1) are metals that are electron rich with high polarizability (e.g., Cd2+, Ni2+, Hg2+, Co2, Cu2+, Zn2+, and Ag+. Hard or intermediate metals such as Fe2+, Mn2, Al3+, Fe3+, Ca2+, and Mg2+ do not solubilize in ammoniated waters because of their inability to form metal-ammine complexes. 

The data in Figures 12.25 and 12.26 show that NH4 adsorption by Cu2+-resin and Cu2+-bentonite (a clay mineral known to form metal outer-sphere complexes) exhibits two major adsorption plateaus, whereas K+ adsorption exhibits a single titration plateau. The data in Figures 12.27-12.29 also show that adsorption of metals by surfaces in the presence of ammoniated solutions was greater than adsorption of metals at similar pH values but in the absence of NH3. These data are also exhibited in the form of Freudlich plots (Fig. 12.30 and 12.31) and the adjustable parameters are summarized in Table 12.12. 

Three metals, Zn2+, Cu2+, and Cd2+, were discussed in some detail with respect to their potential to form metal-ammine complexes. Which metal would dissolve the most in an ammoniated solution If this represented areal problem, how would... 

If traces of gold are suspected, the metal is now dissolved in aqua regia, repeatedly evaporated with hydrochloric acid to expel nitric acid, and the chloride dissolved in dilute hydrochloric acid. Pure sulphur dioxide is passed through the solution, and the whole allowed to stand. If there is no precipitate, gold is absent. When such is the case the chloride is converted into the di-ammoniate and reduced to the pure metal. 

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