Metal ions, solvation state

One of the most promising areas of application of magnetochemistry is the study of transition metal complexes, in both the solid and dissolved states. Accordingly, magnetic susceptibility measurements may provide valuable information on the structures of solutions containing transition metal ions or complexes, on the symmetry conditions of transition metal ion solvates, etc. From magnetic measurements it is possible to establish the electronic structure of the central atom of the transition metal complex, its oxidation state and, in certain cases, its symmetry conditions. 

Stace AJ (2002) Metal ion solvation in the gas phase the quest for higher oxidation states. J Phys Chem A 106 7993-6005... 

Beryllium(II) is the smallest metal ion, r = 27 pm (2), and as a consequence forms predominantly tetrahedral complexes. Solution NMR (nuclear magnetic resonance) (59-61) and x-ray diffraction studies (62) show [Be(H20)4]2+ to be the solvated species in water. In the solid state, x-ray diffraction studies show [Be(H20)4]2+ to be tetrahedral (63), as do neutron diffraction (64), infrared, and Raman scattering spectroscopic studies (65). Beryllium(II) is the only tetrahedral metal ion for which a significant quantity of both solvent-exchange and ligand-substitution data are available, and accordingly it occupies a... 

So far, the reduction of metal ions into the metallic state was discussed involving a complete removal of the coordinated solvent molecules in the reduction process. We shall now consider such redox-systems in which both the oxidized and the reduced species are solvated. The polarographic reduction of Eu(III) to Eu(II) in different solvents occurs at such halt-wave potentials which are again related to the donicity of the solvent molecules118). In the Ei/j-DN plot a straight line is observed. Analogous results were obtained for the redox complexes Sm(III)-Sm(II) and Yb(III>Yb(II) 118> 120> (Fig. 27). 

Although the mechanism of the photo-induced generation of mono- and bimetallic metal clusters, except for the photographic application (Section 20.6), has been studied with considerably less detail than for the radiolytic route, some stable clusters, mostly of noble metals (Ag, Au, Pt, Pd, Rh), have also been prepared by UV excitation of metal ion solutions [129-141]. Generally, halides and pseudo-halides counter anions are known to release, when excited, solvated electrons, which reduce the metal ions up to the zerovalent state. Oxalate excitation yields the strong reducing carbonyl radical COO [30]. Photosensitizers are likewise often added [142]. Metal clusters are photo-induced as well at the surface of photo-excited semiconductors in contact with metal ions [143,144]. 

In these discussions, we have focused attention on details of the physical measurements which confer understanding of structure in pyridine based compounds bond lengths, bond angles, electronic levels in both ground and excited states, and the involvement of solvation, complexation and other environmental features. In the latter, we have, out of necessity, restricted ourselves generally to complexes in which the role of the pyridine structure is of paramount interest, for the whole of the available space could have been filled with lists of complexes incorporating these structures, but where they were of subordinate interest to some other feature, typically the mode of coordination of metallic ions. 

The qualitative elements of Marcus theory are readily demonstrated. For example, the process of transferring an electron between two metal ions, Fe2+ and Fe3 +, may be described schematically by Fig. 33 (Eberson, 1982 Albery and Kreevoy, 1978). The reaction may be separated into three discrete stages. In the first stage the solvation shell of both ions distorts so that the energy of the reacting species before electron transfer will be identical to that after electron transfer. For the self-exchange process this of course means that the solvation shell about Fe2+ and Fe3+ in the transition state must be the same if electron transfer is not to affect the energy of the system. In the second phase, at the transition state, the electron is transferred without... 

Highlights in the chemistry of cyclopentadienyl compounds have been reviewed.65 Trends in the metallation energies of the gas-phase cyclopentadienyl and methyl compounds of the alkali metals have been studied by ab initio pseudopotential calculations. Whereas there is a smooth increase in polarity of M-(C5H5) bonds from Li to Cs, lithium appears to be less electronegative than sodium in methyl derivatives. The difference between C5H5 and CH3 derivatives is attributed to differences in covalent contributions to the M-C bonds. In solution or in the solid state these trends may be masked by the effects of solvation or crystal packing.66 The interaction between the alkali metal ions Li+-K+ and benzene has also been discussed.67... 

It would have been difficult to find these kinds of relations with inorganic materials proper. The organic molecule is unique in that its physical properties may be changed in a continuous way by small modifications in its structure or by complexation with different metal ions. Another advantage is the fact that solid state effects are of smaller importance, and catalytic properties of the solid phase may be compared with physical properties in solution. In particular an extended jr-electron system works as a catalytic entity in itself, irrespective of whether it is surrounded by other molecules of its kind (solid phase) or solvating molecules (solutions). 

Ion formation mechanisms for silica gel matrices have never been studied for those elements that are not readily reducible to the metal. The solvation/desol-vation mechanism hypothesized previously may have a role in enhancing ion emission from these materials, but it would not be expected that an alkaline earth element could exist in the zero oxidation state in these glass matrices, which are oxide based. The species in the molten glass would be expected to be in the standard +2 oxidation state, but the experimentally observed species is +1. Indeed, there has never been a +2 species reported from thermal ionization, so there is the question of how the +2 species in the molten glass is converted to and emitted as a +1 ion. 

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