Oxidation state of the cations

Using carbon monoxide as the probe, its frequency turns out to be sensitive to the oxidation state of the cation, since the latter determines the withdrawal from, as well as the backdonation of electronic charge into, antibonding orbitals. The vibrational frequency of CO adsorbed to a metal surface or in front of a cation is classified as follows  

With variable confines this scheme is also applicable to zeolites. 

Calzaferri et al. [109] investigated the reduction of partially dehydrated AgNaA zeolites with hydrogen. After admission of CO, bands at 2174 and 2153 cmr were assigned to Ag(I)-CO complexes of different rates of adsorption, which disappeared with increasing degree of reduction since reduced silver does not form chemical bonds with CO. 

Formation of palladium carbonyl clusters entrapped in faujasites and consistent with the geometric constraints in the supercages are reported by Sachtler et al. [110]. Terminal, doubly and triply bridged CO species could be distinguished. After argon purging they were able to classify the terminal complexes into Pd +-CO, Pd -CO and Pd°-CO in the sequence of decreasing CO frequency. 

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In a series of papers Palenik and his coworkers (Palenik 1997a,6,c Kanowitz and Palenik 1998 Wood and Palenik 1998, 1999a,6 Wood et al. 2000) have determined bond valence parameters for transition metals. Some of these have been chosen to be independent of oxidation state in an attempt to provide values of Rq that can be used when the oxidation state of the cation is not known. While these parameters are not as accurate as those that take the oxidation state into account, they can be used to make an approximate determination of the oxidation state, after which the correct value of Rq can be substituted. 

In recent studies See et al. (1998) and Shields et al. (2000) suggest that Rq sometimes depends on factors other than the oxidation states of the cation and anion. To obtain correct bond valence sums around transition metals with nitrogen ligands, it is necessary to use different values of Rq depending on the coordination number of as discussed in Section 9.2. 

However, the extent of the activity enhancement cannot be related to the higher surface area of this material. Two possible explanations were proposed to account for the effect of mirror plane composition on combustion activity one is related to the different oxidation state of the cation in the mirror plane the other is associated with the crystal structure of layered-alumina materials (i.e., magne-toplumbite and (3-Al203) which have different population and co-ordination of the ions in the mirror planes. Both these electronic and structural factors can, in principle, affect the redox properties. 

In this case, further assumptions are necessary—namely, that no hydroxo complexes other than the two monovalent species play a significant potential-determining role and that the equilibrium constants for the two adsorption reactions are identical. The following relationship among the IEP(s) of an oxide or hydroxide, the charge or oxidation state of the cation and its radius, was derived on this basis in an earlier paper (72),... 

Inversion of phase relationships induced by spin-pairing in Fe2+ ions provides one mechanism for possibly enriching this transition element in the Lower Mantle. Other, more general mechanisms influencing element fractionations, are the effects of pressure on relative sizes, crystal field stabilization energies, bond-types and oxidation states of the cations. 

There is a tendency towards a lower oxidation state of the cation in sylvanite AgAuTe4. The sylvanite structure is almost identical with the calaverite structure, except that the two different cations are ordered. The occupied anion octahedra are twisted in such a way that the interatomic distances between a Te atom and its three Te neighbours are 2.87 A, 3.55 A and 3.65 A, respectively. The short Te—Te distance... 

Table 11 Complexation constants for hydroxide, carbonate, and sulfate complexes (data from WATEQ4F and ( ) CHEMVAL data base) Me = metal cations, n = oxidation state of the cations (n = 1, 2,3)...

Adsorbed NO can be even more sensitive than adsorbed CO to the oxidation state of the cations. The use of NO as a probe allows, for example, the determination of the oxidation state of Co ions in zeoHtes better than CO [159], showing the existence of both Co and Co species. In fact, the bands observed at 1901, 1817cm are due to [Co(NO)2] gem-dinitrosyl species, while that at 1945cm is assigned to [Co-NO] mononitrosyl. The three bands due to adsorbed CO at 2204,... 

Vanadium oxides of various stoichiometries exist, related to the different oxidation states of the cation. V2O5 has the vanadium atoms with the highest valency and a (f configuration. It is insulating at low temperature, with an orthorhombic layered structure made of VO5 pyramids sharing corners and edges. A transition towards a metallic phase takes... 

Fig. 4.9 Energies of free cations and of ionic compounds as a function of the oxidation state of the cation. Top Lines represent the ionization energy necessary to form the +1. +2, +3, and + 4 cations of sodium, magnesium, and aluminum. Note that although the ionization energy increases most sharply when a noble gas configuration is broken, isolated cations are always less stable in Itiifher oxidation states. Bottom Lines represent the sum of ionization energy and ionic bonding energy for hypothetical molecules MX, MXj, MXj, and MX in which the interatomic distance, r, has been arbitrarily set at 200 pm. Note that the most stable compounds (identified by arrows) arc NaX, MgXj, and AlXj. (All of the.se molecules will be stabilized additionally to a small extent by the electron affinity of X.)...

The parameters in these equations are as follows x> a> P, and n are the oxidation state of the cation in the barrier layer the polarizability of the film/solution interface (i.e., the dependence of the potential drop across the film/solution interface on the applied voltage) the dependence of the potential drop across the film/solution interface on the pH and the kinetic order of the film dissolution reaction with respect to hydrogen ion concentration, respectively. Note that, in deriving Eqs (12) and (13), the oxidation state of the cation in the barrier layer (x) is set equal to the oxidation state of the same cation in the solution/outer layer. The standard rate constants, k , and a, correspond to the reaction shown in Fig. 4, e is the electric field strength, Y = F/RT, and K = sy. The three terms on the right side of Eq. (12) arise from the transmission of cation interstitials, the transmission of cation vacancies, and the transmission of oxygen vacancies (or dissolution of the film), respectively. Values for these parameters are readily obtained by optimizing the PDM on... 

Oxygen in the reaction vessel may influence kinetic behaviour by participating in these equilibria. There is also the possibility that changes in oxidation state of the cation may occur during the decomposition (e.g. FeS04 and CrSOJ. A number of sulfates (e.g. CaS04 and rare earth sulfates) decompose to yield intermediate oxysulfates. The possibility of melt formation before or during reaction cannot always be excluded and is a further factor that complicates the kinetic interpretations. 

These relations apply for any metal cation in aqueous solution. Obviously, the hydrolysis ratio of a given precursor depends mainly on both the oxidation state of the cation (z+) and the pH of the solution. A valency-pH diagram may then be drawn (Figure 8.14) where three domains are separated by two lines represented hy h = I and h = 25 - 1, respectively, originally drawn by Jorgensen. - ... 

The solid state chemist approaches the problem in a different way(2). His main interest focus on the phase composition of the solid, type of crystal planes exposed, presence of additives and impurities, oxidation states of the cations and their changes in the course of the reaction, type of defects in the oxide lattice, etc. Correlation is sought between these parameters and the activity and selectivity of the oxide system in the given reaction, but little attention is usually paid to the type of interactions between the hydrocarbon molecule and the surface and to the possible transition states. When these two approaches are integrated, several general conclusions may be formulated, but also a number of important yet unanswered questions emerge. 

In all of the above systems (1-4) reduction can be regarded to occur stepwise. Carbon monoxide may become coordinated as reduction proceeds with partial loss of the original anionic ligands and the oxidation state of the cation decreases. The oxidation state at which the metal-CO bond acquires some stability is a function of the metal. 

As discussed in greater detail in Chap. 6, the oxidation states of the cations in spinel need not be restricted to +2 and +3, but may be any combination as long as the crystal remains neutral. This important class of ceramics is revisited in Chap. 15, when magnetic ceramics are dealt with. 

Attention has also been focused on the oxidation of thiols in the presence of solid catalysts. One of the more comprehensive investigations into systems of this type has been made by Wallace et al. [133,145, 146] with a view to the possible use of phthalocyanine type complexes as commercial sweetening catalysts. Comparisons were drawn with metal pyrophosphates, phosphomolybdates, phosphotungstates, and phosphates. Pyrophosphates were found to be effective catalysts, possible due to the existence of six-membered rings involving the cobalt cation [147], which enhances the ability of the cation to donate an electron to oxygen and stabilises each oxidation state of the cation. For a series of pyrophosphates, the order of activity was Co > Cu > Ni > Fe, an activity pattern which was explained in terms of the stability of the 3d electron shells. 

Another possibility of redox reactions involving surface hydroxyl groups is the change of the oxidation state of the cations to which they are bound. In CoAPO-18 catalysts, a fraction of the framework aluminum is iso-... 

Additional areas of lanthanide halide chemistry that have been reviewed include the synthesis, phase studies, and structures of complex lanthanide halides - compounds formed between one or more group 1 cation and lanthanide element halides (Meyer 1982). Halides in combination with lanthanide elements in the II, III, and IV oxidation states were considered with the chemistry of the heavier halides being emphasized. More recently the reduced ternary lanthanide halides (Meyer 1983) and the reduced halides of the lanthanide elements were reviewed (Meyer 1988). The latter review considered lanthanides in which the formal oxidation state of the cation was 2 and included hydride halides, oxide halides, mixed-valence ternary halides, and reduced halide clusters. Corbett et al. (1987) discussed the structures and some bonding aspects of highly reduced lanthanide halides and compounds stabilized by a second-period element bound within each cluster, e.g., SC7CIJ2B, SC5CI5B, YjCljC. 

When considering ceramic sintering processes, you should always remember that the oxidation state of the cations may vary depending on the environment. For example, when CeOi is sintered at high temperatures the Ce may be partially reduced to Ce, which can cause cracking in the compact. Such cracking will, of course, change the available diffusion paths. 

Reagent (1) has been condensed with allylic alcohols in the presence of a catalytic quantity of a Lewis acid (such as boron trifluoride etherate) to achieve a new synthesis of a./S-unsaturated aldehydes (eq 2). This reaction is analogous to the widely used Mtiller-Cunradi-Pieroh reaction except that the oxidation state of the cationic species is one unit lower and that of the nucleophilic species is one unit higher. Thus condensation with 8-ionol leads to an a-bromo aldehyde which was transformed into S-ionylideneacetaldehyde by dehydrobromination, using 1,8-diazabicyclo[5.4.0]undec-7-ene. ... 

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