Elements cations

Fig. 7.4 Potential energy/Iattice position diagram for occupation of interstitial sites in Fe304 lattice alloying element cations...

Modern nomenclature includes Element Cation Old-style name Modern name... 

In the course of our investigations to develop new chiral catalysts and catalytic asymmetric reactions in water, we focused on several elements whose salts are stable and behave as Lewis acids in water. In addition to the findings of the stability and activity of Lewis adds in water related to hydration constants and exchange rate constants for substitution of inner-sphere water ligands of elements (cations) (see above), it was expected that undesired achiral side reactions would be suppressed in aqueous media and that desired enanti-oselective reactions would be accelerated in the presence of water. Moreover, besides metal chelations, other factors such as hydrogen bonds, specific solvation, and hydrophobic interactions are anticipated to increase enantioselectivities in such media. 

Several reviews appeared on the heavier congeners of the carbenium ions. Clearly, the silylium ion problem has received the most attention, and both theoretical as well as experimental aspects have been reviewed. The chemistry of cationic germanium, tin and lead is covered by a recent review by Zharov and Michl. We will concentrate in this review on the description of the progress made during the last 4 years and will try to give an account on the synthesis, the properties and the structure of organosubstituted three-coordinated, tiivalent group 14 element cations and closely related species in the condensed phase. 

Mineral series Minerals that have an identical basic chemical and structural unit in which small amounts of chemical substitution of similar elements (cations of similar size, stereochemical, and bonding character) in the same site in the crystal structure are usual and predictable. Mineral series are usually defined by the end member species, that is, those compounds that contain only one of the possible cations. Intermediate members may have specific names or be identified by the ratio of the cations (see chapter 2). 

Cocalia, V.A., Jensen, M.P., Holbrey, J.D., Spear, S.K., Stepinski, D.C., Rogers, R.D., Identical extraction behavior and coordination of trivalent or hexavalent f-element cations using ionic liquid and molecular solvents, /. Chem. Soc., Dalton Trans., 11,1966-1971,2005. 

Enthalpies of Hydration of Some Transition Element Cations... 

Similar sorts of results may be found with the nitrate anion. In this case, the nitrate ion itself has a characteristic absorption in the ultraviolet. When paired with a transition-element cation, in alcoholic solution, this absorption is markedly altered (2). It also shows alterations with other cations. In certain ketone and ether solutions, it has been possible to demonstrate further that the vibrational spectrum of the nitrate ion has been altered in such a pattern as to be consistent with a binding of one of the nitrate oxygens to the cation (2), so that major vibration now occurs between this oxygen and the rest of the bound nitrate group. 

The hexahydrated or hexasolvated transition-element cation is coordinate saturated, so that in its hypothetical vapor state it may be expected... 

Uniformly, within this group of cations, perchlorate ion accompanying the transition-element cation is replaced by nitrate (7,31), thiocyanate (7,52), or halide (7,6). Nitrate is probably replaced by thiocyanate, but a secondary change takes place in many systems, which makes direct comparison difficult (see below). If one then makes the further reasonable assumption that solvent interference can be used as an inverse measure of tendency to bind to the central metal cations, thiocyanate, whose competition with alcohol is less efficient (52) than that of chloride (6), should be somewhat replaceable with chloride. Comparisons between chloride and thiocyanate in acetonitrile show also that the formation of a complex with a given anion/cation ratio takes place much more readily with chloride than with thiocyanate (55, 34). By the same criterion, from experiments in alcoholic solution (55), bromide should replace chloride, and an extrapolation of the behavior to iodide seems reasonable. 

Yaita, T., Herlinger, A.W., Thiyagarajan, P., Jensen, M.R 2004. Influence of extractant aggregation on the extraction of trivalent f-element cations by a tetraalkyldiglycolamide. Solvent Extr. Ion Exch. 22 (4) 553-571. 

Jensen, M.R, Yaita, T., Chiarizia, R. 2007. Reverse micelle formation in the partitioning of trivalent f-element cations by biphasic systems containing a tetraalkyldiglycolamide. Langmuir 23 (9) 4765 1774. 

Stacking of anion octahedra leads to close-packed anion arrays. Distortions of these octahedra leading to Jahn-Teller stable cation coordinations are easily possible. Moreover, the octahedral coordination is electrostatically slightly favoured over the trigonal-prismatic one. It is therefore understandable that the octahedral coordination is so frequent with transition-element cations. Undistorted anion octahedra are possible... 

Thus it is evident that in compounds with early transition-element cations (Group IV, V and VI) the less tightly-bound d electrons often prefer to be collective and their structures therefore are less easily predictable. 

Richards, J. and N. Elliott The Magnetic Susceptibilities of Some Complex Cyanides with Transition Element Cations. J. Am. Chem. Soc. <52, 3182 (1940). 

Further, it is observed experimentally that electron-pair bonds are frequently associated with anisotropic, i.e. directed, atomic orbitals. This gives rise to open structures. However, the electrostatic (Madelung) energy associated with ionic crystals favors close packing Therefore largely ionic crystals favor more close-packed, two-sublattice structures such as rock salt versus zinc blende. In the case of two-sublattice structures induced by d electrons, electron-pair bonds are generally prohibited by the metallic or ionic outer s and p electrons that favor close packing. Nevertheless, it will be found in Chapter III, Section II that, if transition element cations are small relative to the anion interstice and simultaneously have Rti RCf electron-pair bonds may be formed below a critical temperature. 

Electron Configurations and Net Spins for Transition Element Cations in Strong Cubic and Tetragonal Octahedral Fields... 

As has been pointed out previously, ionic compounds are characterized by a Fermi level EF that is located within an s-p-state energy gap Ef. It is for this reason that ionic compounds are usually insulators. However, if the ionic compound contains transition element cations, electrical conductivity can take place via the d electrons. Two situations have been distinguished the case where Ru > Rc(n,d) and that where Rlt < Rc(n,d). Compounds corresponding to the first alternative have been discussed in Chapter III, Section I, where it was pointed out that the presence of similar atoms on similar lattice sites, but in different valence states, leads to low or intermediate mobility semiconduction via a hopping of d electrons over a lattice-polarization barrier from cations of lower valence to cations of higher valence. In this section it is shown how compounds that illustrate the second alternative, Rtt < 72c(n,d), may lead to intermediate mobility, metallic conduction and to martensitic semiconductor metallic phase transitions. 

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