Pentavalent state

The reaction of iodine heptafluoride is accompanied by reduction to the pentavalent state (S15). The intercalation process can thus be described in terms of the reaction... 

Braman et al. [34] used sodium borohydride to reduce arsenic and antimony in their trivalent and pentavalent states to the corresponding hydrides. Total arsenic and antimony are then measured by their spectral emissions, respectively, at 228.8 nm and 242.5 nm. Limits of detection are 0.5 ng for antimony and 1 ng for arsenic, copper, and silver. Oxidants interfere in this procedure. 

Although purely thermodynamic considerations suggest that arsenic should exist in oxic seawaters practically entirely in the pentavalent state, equilibrium... 

If necessary a preconcentration was carried out on this solution to lower the detection limits of the method. Preconcentration was achieved by a method involving co-precipitation of the antimony with hydrous zirconium oxide in which the digest is stirred with 150mg zirconyl chloride and the pH adjusted to 5 with ammonia to coprecipitate antimony and hydrous zirconium oxide. The isolated precipitate is dissolved is 7M hydrochloric acid and 30% sulphuric acid. Antimony is then converted to the pentavalent state by successive treatment with titanium III chloride and sodium nitrite and excess nitrite destroyed by urea. 

Americium may be separated from other elements, particularly from the lanthanides or other actinide elements, by techniques involving oxidation, ion exchange and solvent extraction. One oxidation method involves precipitation of the metal in its trivalent state as oxalate (controlled precipitation). Alternatively, it may be separated by precipitating out lanthanide elements as fluorosilicates leaving americium in the solution. Americium may also he oxidized from trivalent to pentavalent state by hypochlorite in potassium carbonate solution. The product potassium americium (V) carbonate precipitates out. Curium and rare earth metals remain in the solution. An alternative approach is to oxidize Am3+ to Am022+ in dilute acid using peroxydisulfate. Am02 is soluble in fluoride solution, while trivalent curium and lanthanides are insoluble. 

Sulfur is oxidatively fluorinated up to its highest valence state, six. For instance, alkyl thiols give perfluoroalkyl-sulfurpentafluorides (Fig. 40) [104] and sulfides give perfluorodialkyl-sulfurtetrafluorides (Fig. 41) [105,106]. Similarly, phosphorous is oxidatively fluorinated up to the pentavalent state (Fig. 42) [107]. 

Increasing Vanadium and ADA Content. Raising the concentration of vanadium in the Stretford solution can increase the rate of elemental sulfur formation in the absorber/reactor, and thus lower the concentration of sulfur compounds that are converted to thiosulfate in the oxidizer (Moyes 1974 and Nicklin 1977). It may also be necessary to increase the ADA concentration as well, in order to convert the additional tetravalent vanadium that will be formed back into its pentavalent state. 

Detection.—Apart from naturally occurring ores of vanadium, vanadium steels, and ferrovanadium, the commonest compounds of vanadium are those which contain the element in the pentavalent state, viz. the pentoxide and the various vanadates. The analytical reactions usually employed are, therefore, those which apply to vanadates. Most vanadium ores can be prepared for the application of these reactions by digesting with mineral acids or by alkaline fusion with the addition of an oxidising agent. When the silica content is high, preliminary treatment with hydrofluoric acid is recommended. Vanadium steels and bronzes, and ferrovanadium, are decomposed by the methods used for other steels the drillings are, for instance, dissolved in sulphuric acid and any insoluble carbides then taken up in nitric acid, or they are filtered off and submitted to an alkaline fusion. Compounds of lower valency are readily converted into vanadates by oxidation with bromine water, sodium peroxide, or potassium permanganate. 

In PCI and other molecules of this type, the pentavalent state is formed by the promotion of an s electron to the vacant d shell. The valence configuration is therefore dsp and the bipyramidal structure is to be expected, in agreement with the results of electron diffraction studies. Table XI shows that two of the remaining d orbitals are capable of forming strong ir bonds the other two, two weak r bonds by accepting pairs of electrons. Neglecting weak bonds, the structure of PCI is best written... 

The hydrolysis of p-trifluoromethylphenyldimethylethoxysilane was carried out at four different acid concentrations and the rate constants were calculated (Table 8). If the reaction is first order in acid, the rate constant should not change as shown in Table 8. Each of the alkoxysilanes was hydrolyzed with a ten-fold excess of water. If the rate equation is correct, water does not react until after the rate determining step and should not affect the rate constants (Table 9).- Again, this is the case. If the mechanism proceeded through a pentavalent state such as Smith suggested, we would expect to see water involved in the rate equation. At this point, Jada s mechanism seems more likely. 

Abbasi [243] described a spectrophotometric method based on N(-pN, N-dimethyl anilo-3-methoxy-2-naphtho)hydroxaminic acid for the determination of vanadium in soil. The soil sample was digested with 8 M hydrochloric acid and potassium permanganate (to oxidise vanadium to the pentavalent state). This extract was retrieved with a chloroform solution of the chromogen and the violet colour was evaluated spectrophotometrically. 

Both in their inorganic and their organic compounds these metals can occur in the trivalent or in the pentavalent state. Consequently the following types of organometallic compounds are known ... 

In these compounds the metal-carbon bonds are highly covalent and chemically rather stable. The chemical reactivity of the trivalent compounds, in particular those of the type R3M is associated with an easy oxidizability to the pentavalent state rather than with rupture of metal-carbon bonds. Thus, the vigorous reaction of the trialkyl compounds R3M with air depends on the following oxidative transformation ... 

In electrophilic reactions bis-2,2 -biphenylylene-organylarsoranes react like the corresponding phosphoranes exclusively under cleavage of one arsafluorene system i2i,i23) tjjUS reflecting the presence of considerable ring strain at least for the pentavalent state. 

For instance in the synthesis of dimethyl arsinic acid (cacodylic acid) trivalent arsenic is isomerised to the pentavalent state... 

The accurate analysis of the problem is complicated as, under reaction conditions (presence of oxygen), all redox equilibria between V5+ and the lower oxidation states are shifted towards the pentavalent state. The generation of realistic model systems in which, for example, conductivity experiments can be performed, thus requires the exact control of the gas phase in contact with the melt. 

In an acidic solution, iodate oxidizes trivalent antimony to the pentavalent state. This reaction may be used to estimate the antimony in solution. A typical procedure consists of the following steps ... 

II. The Pentavalent State in Phosphorus Stereochemistry A. General Considerations... 

These considerations are illustrated by an analysis of the stereochemistry of the oxaphosphetane (59), and by observation of its variable-temperature 1H n.m.r. spectrum. Phosphorane (59) has four oxygen ligands which provides considerable stability to the pentavalent state of the phosphorus. 

For those actinides stable in the pentavalent state, ions of the form An02(H20)x+ (An = Np, Pu x = 4, 5, 6) have been postulated. Pa is not included in this list since it readily hydrolyzes in aqueous solutions to form Pa205-xH20. All of the structural data for these complexes come from XAFS. The bond distances for both An=0 (1.83 A for both Np and Pu) and An-OH2 (2.51 and 2.5 A, respectively) expand in the... 

Pentavalent. A limited number of pentavalent actinide complexes have been isolated with N-donor ligands. The primary reason for this is the inability of Th to attain the pentavalent state and the propensity of U complexes to disproportionate into and complexes. Some of the typical complexes that have been reported are adducts of U(0R)5 and UX5 with the hgands described for the tri- and tetravalent actinide complexes. 

Protactinium oxides can be stabilized in the tetravalent and pentavalent state. The most stable oxide phase obtained by the burning of metal or protactinium compoimds is the white pentoxide, Pa20s. The structme of the pentoxide is related to fluorite and has cubic symmetry. Pa02 is a black solid that crystallizes in the cubic fluorite structure. 

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