Cations cationic ammonia complexes

Metal cations in aqueous solution often form chemical bonds to anions or neutral molecules that have lone pairs of electrons. A silver cation, for example, can associate with two ammonia molecules to form a silver-ammonia complex ... 

The stoichiometiy of a metal complex is described by its chemical formula. For example, each cation of the silver-ammonia complex contains one Ag cation bound to two neutral NH3 ligands and carries a net charge of -i-l, as shown in Figure 18-11. The formula of a complex ion is enclosed in square brackets, as in [ Ag (NH3)2. The... 

Chelating ligands bind much more tightly to their metal cations than do ligands that possess only one donor atom. A good example is the ethylenediamine complex with. The ethylenediamine complex is much more stable than the analogous ammonia complex ... 

The fact that complex 38 does not react further - that is, it does not oxidatively add the N—H bond - is due to the comparatively low electron density present on the Ir center. However, in the presence of more electron-rich phosphines an adduct similar to 38 may be observed in situ by NMR (see Section 6.5.3 see also below), but then readily activates N—H or C—H bonds. Amine coordination to an electron-rich Ir(I) center further augments its electron density and thus its propensity to oxidative addition reactions. Not only accessible N—H bonds are therefore readily activated but also C—H bonds [32] (cf. cyclo-metallations in Equation 6.14 and Scheme 6.10 below). This latter activation is a possible side reaction and mode of catalyst deactivation in OHA reactions that follow the CMM mechanism. Phosphine-free cationic Ir(I)-amine complexes were also shown to be quite reactive towards C—H bonds [30aj. The stable Ir-ammonia complex 39, which was isolated and structurally characterized by Hartwig and coworkers (Figure 6.7) [33], is accessible either by thermally induced reductive elimination of the corresponding Ir(III)-amido-hydrido precursor or by an acid-base reaction between the 14-electron Ir(I) intermediate 53 and ammonia (see Scheme 6.9). 

The cationic gold complex with CAAC ligand 29 can also catalyze the unprecedented hydroamination reaction of alkynes and allenes using ammonia [59, 60]. It was also demonstrated that it can catalyze the simple hydroamination reaction. It... 

A (not-closed) heated solution of ammonia gradually loses ammonia. If a cation is complexed with ammonia, the free-cation concentration will gradually increase as ammonia is lost (a rare example of slow cation release rather than anion release). It will also increase with an increase in temperature, due to the decrease in stability of complexes with increasing temperature. As one example using this principle, thin films of mixed ZnO/Zn(OH)2, which converted to ZnO on heating over 200°C, were deposited from a heated aqueous Zn-ion/ammonia bath. 

There are a number of examples of homogeneous precipitation of hydroxides based on slow cation release, such as destruction of the Fe-EDTA complex with H2O2 (see Ref. 33). In CD, the only well-defined example of this is heating an ammonia complex (e.g. of Cd ). The loss of ammonia by volatilization will gradually increase the concentration of free Cd ions. 

Solutions of alkali metals in ammonia have been the best studied, but other metals and other solvents give similar results. The alkaline earth metals except- beryllium form similar solutions readily, but upon evaporation a solid ammoniste. M(NHJ)jr, is formed. Lanthanide elements with stable +2 oxidation states (europium, ytterbium) also form solutions. Cathodic reduction of solutions of aluminum iodide, beryllium chloride, and teUraalkybmmonium halides yields blue solutions, presumably containing AP+, 3e Be2, 2e and R4N, e respectively. Other solvents such as various amines, ethers, and hexameihytphosphoramide have been investigated and show some propensity to form this type of solution. Although none does so as readily as ammonia, stabilization of the cation by complexation results in typical blue solutions... 

In a similar manner the so-called Zintl salts composed of alkali metal cations and clusters of metals as anions (see Chapter 16) were known in liquid ammonia solution but proved to be impossible to isolate Upon removal of the solvent they reverted to alloys. Stabilization of the cations by complexation with macrocyclic ligands allowed the isolation and determination of the structures of these compounds. 

Finally, one of the first continuous ion-exchange plants installed used a weak-acid resin to recover copper from rayon-fibre spinning solutions. In the Bemberg or copper(II) ammonium process,357 the spinning takes place in an addic copper sulfate solution, and the fibre is then washed in ammonia solution. The wash water contains as much as 30% of the copper required for the spinning operation and its recovery is important in economic and environmental terms. The copper is extracted as the cationic amine complex by the weak-acid resin, and is then stripped from the resin with the acidic spinning solution. Zinc is recovered in a similar manner from vicrose rayon-spinning operations. 

In summary then, the appellation (53,84, 85) "loose ion-pair for the solvated electron-metal cation association complex in dilute metal-ammonia solutions is therefore a particularly apt description. 

From your previous knowledge, cite cases of departure from the Mellor-Maley order of eomplexing tendencies of dipositive cations (p. 342). Consider (a) ammonia complexes, (b) hydroxo complexes, (c) chloro complexes, (d) iodo complexes, and (e) cyano complexes. 

Analysis of the electron localization function for geometries of the same reaction path provides an alternative, but compatible, perspective on the evolution of electronic structure. The novel asynaptic basin of the DRA, which represents a pair of electrons that is delocalized over the periphery of the ammonium cation core, is transformed into a conventional, monosynaptic basin that is associated with the departing hydrogen at the geometries near the transition state. After the transition state, the NH system may be described as a hydride-ammonia complex. 

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