Different Oxidation States

At ambient condition these materials act more as isolated Pt(II) and Pt(IV) materials than as homogeneous Pt(III) substances. This is confirmed crystal-lographically as the M X distance is longer than the M -X distance, e.g., 75. 

In addition to the above mentioned mixed valent Pt complexes the black platinum(III) halides, PtX8(X = Cl, Br) have been shown to be mixtures of Pt and Pt halides (393, 417) although they do not possess a columnar structure. 

Several mixed valent complexes containing gold(lll) have been reported. Table XV. Several of these dichroic diamagnetic complexes exhibit a onedimensional structure in the solid with alternating square planar and linear M X anions, (476, 480) (M = Au, X = Cl, I M = Ag, X = Cl) with a (Au - -X -X )j, linkage (476). A similar linkage is also 

The electrical conductivity of these mixed valent gold(III) complexes has been reported. Table XV. Pollard s Salts (X = Cl, Br), 16, exhibit low con- 

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In spite of the diverse nature of alkaloid structures, two structural units, i.e. fused pyrrolidine and piperidine rings in different oxidation states, appear as rather common denominators. We therefore chose to give several examples for four types of synthetic reactions which have frequently been used in alkaloid total synthesis and which provide generally useful routes to polycyclic compounds with five- or six-membered rings containing one nitrogen atom. These are ... 

Oxidation can also occur at the central metal atom of the phthalocyanine system (2). Mn phthalocyanine, for example, can be produced ia these different oxidation states, depending on the solvent (2,31,32). The carbon atom of the ring system and the central metal atom can be reduced (33), some reversibly, eg, ia vattiag (34—41). Phthalocyanine compounds exhibit favorable catalytic properties which makes them interesting for appHcations ia dehydrogenation, oxidation, electrocatalysis, gas-phase reactions, and fuel cells (qv) (1,2,42—49). 

Plutonium was the first element to be synthesized in weighable amounts (6,7). Technetium, discovered in 1937, was not isolated until 1946 and not named until 1947 (8). Since the discovery of plutonium in 1940, production has increased from submicrogram to metric ton quantities. Because of its great importance, more is known about plutonium and its chemistry than is known about many of the more common elements. The metallurgy and chemistry are complex. MetaUic plutonium exhibits seven aUotropic modifications. Five different oxidation states are known to exist in compounds and in solution. 

The main synthetic route to high nuclearity metal carbonyl clusters involves a condensation process (/) a reaction induced by coordinatively unsaturated species or (2) a reaction between coordinatively saturated species in different oxidation states. As an example of (/), Os2(CO)22 can be condensed to form a series of higher coordinated species (89). 

The interaction of species in different oxidation states can lead to higher coordinated molecules (54,116). 

When an element is present on the surface of a sample in several different oxidation states, the peak characteristic of that element will usually consist of a number of components spaced close together. In such cases, it is desirable to separate the peak into its components so that the various oxidation states can be identified. Curve-fitting techniques can be used to synthesize a spectrum and to determine the number of components under a peak, their positions, and their relative intensities. Each component can be characterized by a number of parameters, including position, shape (Gaussian, Lorentzian, or a combination), height, and width. The various components can be summed up and the synthesized spectrum compared to the experimental spectrum to determine the quality of the fit. Obviously, the synthesized spectrum should closely reproduce the experimental spectrum. Mathematically, the quality of the fit will improve as the number of components in a peak is increased. Therefore, it is important to include in a curve fit only those components whose existence can be supported by additional information. 

Fluorine and nitrogen may be added to olefins with the nitrogen in different oxidation states Fluorine and a nilro group are added by reaction of an olefin with nitryl fluoride [131], nitronium tetrafluoroborate [195] (equation 32), or a combination of nitric acid and hydrogen fluoride [131, 196] (equation 33)... 

Folic acid derivatives (folates) are acceptors and donors of one-carbon units for all oxidation levels of carbon except that of CO2 (where biotin is the relevant carrier). The active coenzyme form of folic acid is tetrahydrofolate (THF). THF is formed via two successive reductions of folate by dihydrofolate reductase (Figure 18.35). One-carbon units in three different oxidation states may be bound to tetrahydrofolate at the and/or nitrogens (Table 18.6). These one-carbon units... 

The ability of N to exist in its compounds in at least 10 different oxidation states from —3 to +5 poses certain thermodynamic and mechanistic problems that invite systematic treatment. Thus, in several compounds N exists in more than one oxidation state, e.g. [N- "H4] + [N "02] , [N-" H4] + [N 03] , [N-"2H5] + [N 03]-, [N- "H4] + [N-3 3]-, etc. Furthermore, we have seen (p. 423) that, under appropriate conditions, NH3 can be oxidized by O2 to yield N2, NO or NO2, whereas oxidation by OCl yields N2H4 (p. 427). Likewise, using appropriate reagents, N2H4 can be oxidized either to N2 or to HN3 (in which the average oxidation number of N is — ). The thermodynamic relations between these various hydrido and 0x0 species containing N can be elegantly codified by means of their... 

This latter reaction is most unusual in that it simultaneously involves an element (N) in four different oxidation states. Use of N-enriched reagents shows that all the N from HNO2 goes quantitatively to the internal N of N20 ... 

As in the preceding transition-metal groups, the refractory behaviour and the relative stabilities of the different oxidation states can be explained by the role of the (n — l)d electrons. Compared to vanadium, chromium has a lower mp, bp and enthalpy of atomization which implies that the 3d electrons are now just beginning to enter the inert electron core of the atom, and so are less readily delocalized by the formation of metal bonds. This is reflected too in the fact that the most stable oxidation state has dropped to +3, while chromium(VI) is strongly oxidizing ... 

Inletconvetsion between different oxidation states occurs easily for Tc (see Section 24.2.4.), and its control often requires careful adjustment of pH and the relative excess of reductant u.sed. 

Because of the differing focus of interest in these elements their chemistries have not developed in parallel and the data on which strict comparisons might be based are not always available. Nevertheless many of the similarities and contrasts expected in the chemistry of transition elements are evident in this triad. The relative stabilities of different oxidation states in aqueous, acidic solutions are summarized in Table 24.2 and Fig. 24.1. 

The oxidative and reductive interconversions of different oxidation states of the carboline ring system are systematically discussed in Section III. 

This reaction shows that the hydrated oxide SiCV-xHjO is acidic, since it reacts with a base-As we mentioned earlier, phosphorus can be found in four different oxidation states. The hydroxides of the +1, +3, and +5 states of phosphorus are hypophosphorous acid, H3P02, phosphorous acid, H3P03, and phosphoric acid, H3P04. Their structures are shown in Figure 20-4. As suggested by their names, these compounds are distinctly acidic, and are of moderate strength. The equilibrium constant for the first ionization of each acid is approximately 10-2 hypophosphorous acid ... 

Sometimes the metal may be transformed into a different oxidation state thus copper(II) may be reduced in acid solution by hydroxylamine or ascorbic acid. After rendering ammoniacal, nickel or cobalt can be titrated using, for example, murexide as indicator without interference from the copper, which is now present as Cu(I). Iron(III) can often be similarly masked by reduction with ascorbic acid. 

Potentiometric titration using a bright platinum-saturated calomel electrode system this can be used when the reaction involves two different oxidation states of a given metal. 

Metabolites that are composed of structures of quite different oxidation states. Certain secondary metabolites and biosurfactants fell into this dass since they have both carbohydrates and fatty adds in their structures. 

An important source of reference is another excellent review by Stern [844] this one is concerned with the high temperature properties of oxy-halides. The following general trends are found in salts containing an XO anion (X = Cl, Br and I) there are variations in stabilities in the sequences (i) (X =) Cl > Br > I for halogens in the same oxidation state, and (ii) XO4 > XOJ > XO2 > XCT for the different oxidation states of a particular halogen. 

As it turns out, a direct experimental measure of kEE for Mn(sar)3+/Mn(sar)2+ was later reported,35 making use of an ingenious method36 for evaluating electron exchange by the mixing of the enantiomeric forms in different oxidation states ... 

The chemistry of plutonium is unique in the periodic table. This theme is exemplified throughout much of the research work that is described in this volume. Many of the properties of plutonium cannot be estimated accurately based on experiments with lighter elements, such as uranium and neptunium. Because massive amounts of plutonium have been and are being produced throughout the world, the need to define precisely its chemical and physical properties and to predict its chemical behavior under widely varying conditions will persist. In addition to these needs, there is an intrinsic fundamental interest in an element with so many unusual properties and with so many different oxidation states, each with its own chemistry. 

The magnetic properties of Pu compounds in different oxidation states are reviewed. New measurements on Pu(C8H8)2, PuFi, [(C2Hs)itN]2PuCl6, and [ (C2H5)itN]itPu(NCS)s are presented. The interpretation of the data is based on intermediate, j-j mixed crystal field states and orbital reduction due to covalency. Especially in the case of the organometallic compounds a large orbital reduction is found. 

The effect of pH and complexation on the relative stabilities of the oxidation states of Pu is discussed. A set of ionic radii are presented for Pu in different oxidation states and different coordination numbers. A model for Pu hydration is presented and the relation between hydrolysis and oxidation state evaluated, including the problem of hydrous polymerization. 

Complexation of Pu is discussed in terms of the relative stabilities of different oxidation states and the "effective" ionic charge of Pu0 and Pu02+2. An equation is proposed for calculating stability constants of Pu complexes and its correlation with experimental values demonstrated. The competition between inner v outer sphere complexation as affected by the oxidation state of Pu and the pKa of the ligand is reviewed. Two examples of uses of specific complexing agents for Pu indicate a useful direction for future studies. 

As with all such generalizations, this one can be negated by other factors such as complexing which can even reverse the trends and the relative stability of the different oxidation states. For example, the greater strength of complexation of... 

Plutonium cations in whatever oxidation state can be described as hard acids and interact with anionic species by ionic bonding. As a result certain generalizations can be made about the relative complexing tendencies of the different oxidation states. 

The uncertainty of the proper coordination number of any particular plutonium species in solution leads to a corresponding uncertainty in the correct cationic radius. Shannon has evaluated much of the available data and obtained sets of "effective ionic radii" for metal ions in different oxidation states and coordination numbers (6). Unfortunately, the data for plutonium is quite sparse. By using Shannon s radii for other actinides (e.g., Th(iv), U(Vl)) and for Ln(III) ions, the values listed in Table I have been obtained for plutonium. These radii are estimated to have an uncertainty of 0.02 X ... 

In studies where different oxidation states of plutonium have been complexed by the same ligand, the sequence of complex-ing strength most commonly observed is that described for the... 

Ideally, this information should be made available in the form of easy-to-use nomographs or empirical equations which can be quickly and rapidly solved on a programmable desk calculator. New instrumentation which can be used on an in-line basis to analyze process streams for the concentrations of plutonium in different oxidation states is also needed. 

Calculate the standard potential of a redox couple from two others relating to different oxidation states (Example 12.6). 

A central theme in our approach, which we believe to be different from those of others, is to focus on the changing chemistry associated with higher, middle and lower oxidation state compounds. The chemical stability of radical species and open-shell Werner-type complexes, on the one hand, and the governance of the 18-electron rule, on the other, are presented as consequences of the changing nature of the valence shell in transition-metal species of different oxidation state. 

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