Ammoniated electron

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. 

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... 

In the century since its discovery, much has been learned about the physical and chemical properties of the ammoniated electron and of solvated electrons in general. Although research on the structure of reaction products is well advanced, much of the work on chemical reactivity and kinetics is only qualitative in nature. Quite the opposite is true of research on the hydrated electron. Relatively little is known about the structure of products, but by utilizing the spectrum of the hydrated electron, the reaction rate constants of several hundred reactions are now known. This conference has been organized and arranged in order to combine the superior knowledge of the physical properties and chemical reactions of solvated electrons with the extensive research on chemical kinetics of the hydrated electron. 

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 ammoniated electron, e, has been described by Ogg (35) and various other authors as an electron-in-a-cavity. In this model, the electron is stabilized in a large cavity by polarization and orientation of... 

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 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. 

Blandamer et al. (4) have recently made a half-hearted effort to resurrect the e2 2 species. They state that if a species such as M2 or M contains two ammoniated electrons with sufficient overlap of the electronic wave functions to cause the species to exist in a singlet state, then only by a coincidence could the absorption spectrum be similar to that of the far-separated ammoniated electrons. They suggest that a comparable coincidence could just as well occur in the case of the e2 2 species. Some clarification of the ionic aggregate model is therefore needed. It should be recognized that the optical absorption peak does shift slightly... 

The perturbation of the Si-H vibration of ammoniated trichlorosilylated silica (figures 12.14 and 12.16) consists at first sight of at least 3 distinct bands, indicating that at least 3 different species with an Si-H band exist on the surface. The strong electron donating effect of the amine functions will cause a low wavenumber shift of the Si-H band. 

Although agreement among workers in the field is not complete, it now appears that the predominant species in concentrated solution is the solvated Na2 molecule with the two 3a electrons delocalized over a number of surrounding solvent molecules, but still paired. The blue species is probably an ammoniated electron—that is, the electride ion. ... 

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. 

Ion-dipole complexes ( n), from an ion combined with dipole molecules such as water and ammonia. As far as the type of bonding is concerned, we meet in this case also both the electrostatic bonding, as in the hydrates [Mg(H20)6]2+, and the electron pair bond in the ammoniates, for example, such as that of trivalent cobalt [Co(NH3)6]3+. 

Shkrob lA. (2006) Ammoniated electron as a solvent stabilized mnltimer radical anion. JPhys Chem A 110 3967-3976. 

For example, the one-electron models incorrectly predict (even at a qualitative level) the Knight shifts in and NMR spectra of ammoniated electron, and solvated electrons in amines (Sec. 4.1). The same problem arises in the explanation of magnetic (hyperfine) parameters obtained from ESEEM spectra of trapped (hydrated) electrons in low-temperature alkaline ices. The recent resonance Raman spectra of also appear to be incompatible with the one-... 

The metal-NHs reductions of carbonyl groups are exceedingly fast reactions for the reaction of acetone with an ammoniated electron the rate is 9 x 10 M" s". Although many, particularly older, published experimental procedures for the metal-NHs reduction of ketones employ prolonged reaction times with excess metal, these conditions are unnecessarily harsh. The reactions of carbonyl compounds with metals in NH3 are effectively instantaneous and by using short reaction times it appears that reduction of terminal alkenes and disubstituted alkynes can be avoided.In addition to the functional groups mentioned above, alcohols, amines and ethers, other than epoxides, are usually stable to reductions of aldehydes and ketones by dissolving metals. " ... 

The binding of ammonia to the cluster induces a change in electronic structure relative to that of the bare cluster. This is probed by reacting ammoniated clusters with hydrogen and comparing the reaction rate constants as a function of cluster size with the naked iron clusters. The absolute reaction rate constants toward H 2 for the fully ammoniated clusters are about an order of magnitude smaller than those for the bare clusters. The minima in reactivity observed for bare iron clusters are shifted to smaller cluster size for the ammoniated species for example, Fe,3 is reactive with H2, but upon... 

The diffusion coefficient for the hydrated electron has been calculated from conductivity measurements [32] to be (4.9 0.25) x 10 cm sec". The equivalent conductance is much higher than that of all other ions except OH" and but considerably lower than the ammoniated... 

Several models have been proposed for the structure of the ammoniated electron. One which has shown qualitative agreement with experimental... 

It is interesting that the greater stability of the ammoniated electron makes its observation easier in continuous radiolysis than that of the hydrated electron. The ESR spectrum of dilute solutions shows a single, narrow line. The g factor is 2.0012 which is close to the free electron value and indicates only a weak interaction between electron and solvent [84]. 

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