Other oxidation states

Other complexes recently synthesized are listed in Table 6.12. 

If dilute solutions ( 10 M) of Cr are injected into 02-saturated HCIO4, then [(H20)5Cr02] (21) is formed according to equation (16). This complex [Amax, ( ) 290 (3100), 247 (7400)] is formulated as a superoxo complex of Cr(III), and has a formation rate constant k of about 1.6 x 10 s at 25 C. 

We note that in the formal sense, CrO and HCrO are equivalent in that they are simply related by hydrolysis reaction (19), as are HCr04 and Cr207 by equation (20). Nevertheless, the electron distributions within CrO and HCr04 

0 M) and via an inner-sphere mechanism with Fe and some Co macrocyclic ligand complexes.  

Oxo rather than hydroxo bridges are proposed in the original.  

Equation (25) represents the reaction between Cr(II) and [Co(dppd)(en)2]. ° The previously reported reaction between Cr(II) and 

Nickel(IIl) and nickel(IV) complexes are well documented, but have not been treated in this review. Whereas a transient Ni species presumably exists, there is no aqua ion representative of Ni(IV). 

After this review was completed, results from an extensive experimental study (from 278.15 to 393.15 K at 0.35 MPa) were reported [2004BRO/MER] for aqueous Ni(N03)2 solutions. The new apparent molar heat capacity values are markedly more negative than those reported by Spitzer et at. [79SPI/OLO]. 

Bhattacharya et al. [86BHA/MUK] synthesised the tris(2,2 -bipyridine 1,1 -dioxide) Ni(III) complex, Ni(bpyOj)3, and measured the potential of Reaction (V.21) in acetonitrile against the standard calomel electrode (SCE). 

These authors observed a linear correlation between the and M(bpy02)3 I M(bpyOj)3 potentials (M being Cr, Fe, Mn, Co) versus the SCE in aqueous and acetonitrile solutions, respectively. A correlation of similar quality is obtained when the standard electrode potentials of [89BRA] are linearly fitted 

A bridging ligand reduction model vs. the outer-sphere mechanism for electron transfer has been tested using rate constants from Cr(II) systems. A correlation between the rate constant and the gas-phase electron affinity of the bridging group implies an inner-sphere mechanism. If such a correlation is absent an outer-sphere mechanism is assumed.  

The most stable Cr(IV) complexes are the diperoxy(amine) chelates and [Cr(02)2(dien)] has now been investigated as an oxidant in glycine or acetate buffers. With Ti(III) and [Fe(CN)6] , both the peroxo groups and the Cr(IV) center are reduced but with only the Cr(IV) center. The Cr(III) products contain coordinated dien, but not all the N-donor atoms are coordinated. 

Another Cr(V) complex that has kinetic potential is [Cr(phen)2(0)2] (25), prepared by Pb02 or PhIO oxidation of ci5-[Cr(phen)2(H20)2]. In aqueous acidic solution, (25) can oxidize Cu(II)L to Cu(III)L and the second-order rate constant ( obs) varies with [H ] according to Eq. (3).  

93 X 10 M s in toluene. The reaction between Fe(II)aq and Cr(VI) is nearly instantaneous, but when the only source of Fe(II) is in hematite or biotite, the initial rate of Cr(VI) reduction is dependent on the rate of mineral dissolution. These dissolution rates can be increased by low pH or by the addition of anions that complex Fe(II).  

PCC pseudo-first-order with excess alcohol 136 

The pseudo-first-order rate constant (/robs) for the base hydrolysis of amino-carboxylato-complexes of Mo, and vary with [OH ] according to the general equation /robs=(/ri-l-/r2i ro[OH-] )/(l-l-J5 o[OH ] ) with n=2 for and n=l for Mo i and W i, where Ko is the outer-sphere association constant and k and k are the rate constants for reaction of the rapidly formed ion-pair with HgO and OH ion respectively. Complexes with ida , edda , nta , and edta ions were investigated, and non-bonded carboxylate arms were found to increase values of k and k by factors of 10 —10 . An analytic procedure has been proposed, based on these reactions, which allows concentrations of Mo, and to be estimated to an accuracy of ca. 5 % at molarities in the region of 10 M.  

NickeI(II) (rf ).— The mechanism of complex formation for some metal(ii) cations is now so well established, at least for simple ligands, that such reactions, particularly of nickel(n), are used to probe solvent and salt effects on kinetic patterns. Many of these studies are therefore dealt with in the Chapter on medium effects (Part II, Chapter 13). They include the reactions of nickel(n) and of magnes-ium(n) with chloride in aqueous alcohols, of nickel(ii) with imidazole in aqueous ethanol, with malonate in fructose-water solutions, with thiocyanate in methanol-DMSO mixtures, and with murexide or pada in various micellar media, and of several metal(n) cations with fluoride in aqueous salt solutions. In general, medium effects on observed rate constants (lit) for complex formation operate on the pre-association step (/fos) rather than on the interchange process (A i). The Eigen-Wilkins mechanism operates in all these media it has also been shown to operate for the bidentate ligand Etgdtc in DMSO, and even at the surface of a mercury electrode.  

Schultz-Grunow and T. A. Kaden, Helv. Chim. Acta, 1978, 61, 2291. 

Cobalt(II) (F).— The reaction of cobalt(ii) with pan (5) is slightly slower than that with par (6), allegedly owing to the greater bulk of the pan ligand. Several complex formation reactions show complicated kinetic patterns due to parallel redox processes. This is true for the cobalt(ii) reactions mentioned above  

The reaction of cobalt(n) with azide and mercaptoacetic acid has been studied. The rate law suggests equilibrium formation of C0N3+ followed by slow reaction with mercaptoacetate. 

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Complexes o/M". The absence of any other oxidation state of comparable stability for nickel implies that compounds of Ni" are largely immune to normal redox reactions. Ni" forms salts with virtually every anion and has an extensive aqueous chemistry based on the green [Ni(H20)6] + ion which is always present in the absence of strongly complexing ligands. 

From Other Oxidation States of the Preformed Ring System... 

B. Reactions of Carbolines in Other Oxidation States 1. At Carbon... 

Type B (redox) reactions are more complex. Sulfide in this reaction is converted into some other oxidation state of sulfur. For example, sulfides can be converted to a zero oxidation state of elemental sulfur by oxygen ... 

Rhodium(III) forms a wide range of complexes with tertiary phosphines and arsines [108, 109], though in some cases other oxidation states are possible. Table 2.5 summarizes the complexes produced from reaction of RhCl3 with stoichiometric quantities of the phosphine. 

Contrary to other oxidation states most of the compounds of trivalent plutonium that have been investigated show magnetic ordering at low temperatures. 

Studies of ligands which might provide specificity in binding to various oxidation states of plutonium seems a particularly promising area for futher research. If specific ion electrodes could be developed for the other oxidation states, study of redox reactions would be much facilitated. Fast separation schemes which do not change the redox equilibria and function at neutral pH values would be helpful in studies of behavior of tracer levels of plutonium in environmental conditions. A particularly important question in this area is the role of PuOj which has been reported to be the dominant soluble form of plutonium in some studies of natural waters (3,14). 

Cr3+ and Cr6+ are the most stable oxidation states of chromium, but with the only difference that while +3 oxidation state is cationic where as the +6 oxdation state is oxoanionic. However, the other oxidation states of +1, +2, +4 and +5 are also known for chromium, especially in aqueous solution at different pH. Inter-conversion of these oxidation states too is very frequent. With this view, an attempt is made here to examine the effect of ultrasound on the inter-convertibility of chromium among various oxidation states in aqueous solutions. The details of this study is reported in the literature [36]. 

Most biochemically relevant high-spin systems have such short 7j-relaxation times that their EPR is broadened beyond detection at ambient temperatures. An exception is the class of S = 5/2 Mn" systems with D hx. Also, S = 7/2 Gd"1-based MRI shift reagents exhibit readily detectable room-temperature EPR spectra. Otherwise, aqueous-solution transition ion bioEPR is limited to complexes of S = 1/2 metals, in particular Cu", and to a lesser extent VIV02+, NiIn, Ni1, Mov, and Wv. Cupric is the stable oxidation state of biological copper under aerobic conditions, however, the other metals are stable as Vv, Ni", MoVI, and WVI, and, therefore, the other oxidation states associated with S = 1/2 paramagnetism may exhibit oxidative or reductive reactivity and may thus require specific experimental precautions such as strict anaerobicity over the course of the EPR experiment. 

Nickel normally occurs in the 0 and +2 oxidation states, although other oxidation states are reported (NAS 1975 Nriagu 1980b Higgins 1995). In natural waters Ni2+ is the dominant chemical species in the form of (Ni(H20)6)2+ (WHO 1991 Chau and Kulikovsky-Cordeiro 1995). In alkaline soils, the major components of the soil solution are Ni2+ and Ni(OH)+ in acidic soils, the main solution species are Ni2+, NiS04, and NiHP04 (USPHS 1993). Most atmospheric nickel is suspended onto particulate matter (NRCC 1981). 

In a second type of experiment, oxidative quenching is achieved by use of [Co(NH3)5C1]2+ as the quencher. In the one example reported the ethyl-phenyl derivative of the substrate was used, and the Rum so generated oxidized the heme with k = 6x 103 s l. Prom spectroscopic studies it is believed that the heme is oxidized to a porphyrin n-cation radical and has an axial water ligand. One might anticipate the generation of other oxidized states with the use of other substrate derivatives. 

Manganese in soil has many characteristics that are similar to iron for instance, it exists in multiple oxidation states Mn2+, Mn3+, and Mn4+. Although manganese can exist in the laboratory in other oxidations states, from -3 to +7, the +2 to +4 species are the ones commonly found in soil. Manganese forms various oxide and hydroxide species and chelates with many soil components. Its low oxidation state (i.e., Mn2+) is more soluble and more available than its high oxidation state (i.e., Mn4+). 

Reduction of Disulfides and Other Oxidation States of Sulfur... 

Some other oxidation states of sulfur can be reduced but not sulfonic acids or, as mentioned above, sulfones. 

To date, the organometallic chemistry of copper, in terms of isolation and structural characterization of compounds, is essentially limited to the Cu(I) oxidation state. Only a very few examples of other oxidation states are known. The older literature offers a reported synthetic procedure for the synthesis of bis(aryl)copper(II) compounds [33, 34] (see Scheme 1.2), but this result has never been reproduced by others. 

There is currently a paucity of structural data for binuclear iron complexes in other oxidation states. Wieghardt et al. have reported the structure of the only triply bridged diferrous complex thus far, [(Me3tacnFe)20H(OAc)2] (7,8). Its structure is closely related to that of the corresponding difertic complex, with Fe-p.-OH bond lengths of 1.99 A and an Fe Fe separation of 3.4 A. The two metal centers are antiferrcMnagnetically coupled with a J value of -13 cm . ... 

Although the vast majority of coordination complexes of iron contain the metal in oxidation state two or three the lower oxidation states of one and zero are not uncommon, especially in areas bordering on organometallic chemistry. Oxidation state four is of relevance to bioinorganic electron transfer systems, while oxidation state six is represented by the ferrate(VI) anion, well known but rather little studied. Other oxidation states, from —1 to +8, have been at least mentioned in the past two decades. The more unusual oxidation states are briefly reviewed here, in ascending order. 

Symbol Fe atomic number 26 atomic weight 55.847 a Group VIII (Group 8) metallic element transition metal atomic radius 1.24A electron configuration [Ar]3d 4s2 most common valence states +2 and -i-3 other oxidization states -1, 0, -1-1, +4 and -i-6 are known but rare most abundant isotope Fe-56 natural isotopes and their abundances Fe-54 (5.90%), Fe-56 (91.52%), Fe-57 (2.245%), Fe-58 (0.33%). 

Symbol Nd atomic number 60 atomic weight 144.24 a rare earth lanthanide element a hght rare earth metal of cerium group an inner transition metal characterized by partially filled 4/ subshell electron configuration [Xe]4/35di6s2 most common valence state -i-3 other oxidation state +2 standard electrode potential, Nd + -i- 3e -2.323 V atomic radius 1.821 A (for CN 12) ionic radius, Nd + 0.995A atomic volume 20.60 cc/mol ionization potential 6.31 eV seven stable isotopes Nd-142 (27.13%), Nd-143 (12.20%), Nd-144 (23.87%), Nd-145 (8.29%), Nd-146 (17.18%), Nd-148 (5.72%), Nd-150 (5.60%) twenty-three radioisotopes are known in the mass range 127-141, 147, 149, 151-156. 

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