Transition elements cations

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. 

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. 

These superacidic solutions of d- and /-transition element cations can provide a controllable low-temperature route to compounds containing transition metals in unusually low oxidation states. Also they are perceived as the precursors to transition metal carbonyl cations, as indicated in Sec. 11.3.4.3 immediately below. 

The transition element cations, which comprise class C (2J, have 0-10 d subshell electrons in the M shell (first series) or N... 

The transition element cations, which comprise Schwarzenbach s class C, have 0 to 10 subshell electrons in the M shell (first series) or N shell (second series), etc. Examination of Table 3.5 shows that the class C cations are generally considered either hard or borderline hard-soft acids. These cations have partially filled 3d subshells either in the ground state or when ionized (Table 3.6). Moving across the periodic chart from Sc to Cu, protons are added to the nucleus and electrons to the unfilled inner 3d subshell. Attraction of these inner electrons to the nucleus leads to an overall decrease in cation radii (Table 3.7). The divalent ions are generally sixfold, coordinated in complexes. is an exception and, because of its small size and unique electronic configuration, tends... 

General Chemical and Physical Characterization. The x-ray diffraction data, chemical analyses by x-ray fluorescence and the effects of various synthesis parameters explored in this study lead to the conclusion that a new series of pyrochlores represented by formula 1 has been synthesized. The substitution of the larger post transition element cation for the noble metal cation on the octahedrally coordinated B-site leads to a considerable enlargement of the pyrochlore s cubic unit cell dimension. The relationship between lattice parameter (ag) and extent of substitution of ruthenium by either lead or bismuth is linear as shown in Figure 1. 

The thermal stability of all of these nonstoichiometric pyrochlores is limited and is inversely dependent upon the extent of substitution of noble metal cations on the B-site by post transition element cations (1). For example, Pb2[Ru2]05,5 is stable to 850 C in air while Pb2[RuPK]06,5 is only stable to about 400 C. 

A new family of high conductivity, mixed metal oxides having the pyrochlore crystal structure has been discovered. These compounds display a variable cation stoichiometry, as given by Equation 1. The ability to synthesize these materials is highly dependent upon the low temperature, alkaline solution preparative technique that has been described the relatively low thermal stability of those phases where an appreciable fraction of the B-sites are occupied by post transition element cations precludes their synthesis in pure form by conventional solid state reaction techniques. 

The preference of analcimc for the large Pb cation over the Na cation initially present in comparison to the smaller transitional element cations was thought to be due to the large polarisability of the lead cation. 

All the cited literature references to the above compounds have described solid-state syntheses at temperatures of 700-1200°. Such synthesis conditions will always lead to pyrochlore structure compounds in which all of the octa-hedrally coordinated sites are occupied by the noble metal cation, thus requiring the post-transition metal to noble metal molar ratio always to be 1.0. This paper focuses on solution medium syntheses at quite low temperatures (<75°), thereby stabilizing a new class of pyrochlore compounds in which a variable fraction of the octahedrally coordinated sites are occupied by post-transition element cations. The specific example here involves the Pb2[Ru2 cPbJ ]06.5 series. The synthesis conditions may be simply adapted, however, to accommodate preparation of a wider range of pyrochlores which can be described by the formula A2[B2 cAx]07 j, where A is typically Pb or Bi, B is typically Ru or Ir and 0 < jc < 1, and 0 < 1. 

La Nd) which crystallize in a filled C0AS3 structure. The polyanions form discrete X.i units which are squares in skutterudite but rectangles in the filled type (in LaFePi2 the P-P distances are 2.288 and 2.356 A, the P-P-P angle is 90.0°). The transition-element cations are in distorted octahedral coordination (LaFe4P,2 P-Fe-P = 97.9° and 82.1° instead of 90°) while the rare earth ions occupy the two nearly icosahedral voids at (0,0,0) and (5,5,5) of the body-centered cubic cell. 

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