Pentavalent Species

Solid Me5Sb, obtained from Me3SbCl2 and MeLi in Et20, has a trigonal bipyramidal geometry 2 Jn t ie I3c NMR spectrum in solution, only 1 line is observed from RT to -90 C. The 

Breunig, S. Guelec, and R. Keller, Pho htmts Sulfur Silicon Relat-Eleih., 1992, 64, 107. 

Suzuki, T. Murafigi, and N. Azuma, J. Chem. Soc., Perkin Trans. 1,1992,1593. 

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Inorganic arsenic compounds are more toxic than organic arsenic compounds, and trivalent species are more toxic than pentavalent species. 

A cation in which significant positive charge is located on at least one carbon atom, itself having an even number of electrons. Both carbenium ions and pentavalent species (such as the methanonium ion, CH5+) are carbo-cations. However, radical cations and carbynium ions are not considered to be carbocations. See Carbenium Ion Bridged Carbocation Carbonium Ion... 

Neptunium - Neptunium is known to be present in oxic seawater predominantly as the highly soluble pentavalent species NpOj but, as would seem to be suggested... 

Several mechanisms satisfy the theoretical requirements of the Sn2 reaction (Fig. 3). We shall use the term classical Sn2 to refer to the reaction in which nucleophilic attack on covalent substrate leads directly to product. Two simple ion pair mechanisms will also be considered the Sn 2 (intermediate) is similar to the classical Sn2 mechanism except that the pentavalent species [5] is an intermediate rather than a transition state the ion-pair SN2 mechanism involves... 

The valencies of these metal atoms are limited to the tri- and pentavalent states. As can be seen in the earlier chapters in this volume, their true oxidation states can vary, depending on the ligands attached to the central atom. However, in nonorganic solvents and in the presence of air, the pentavalent state dominates. The biological effects of the trivalent and pentavalent species are entirely different and especially well-documented when it comes to arsenic. The trivalent compounds are almost always more toxic than the corresponding pentavalent derivatives . 

The toxicity of arsenic is well-documented. Trivalent arsenic (arsenite) is almost always more toxic than the pentavalent species (arsenate)The differential toxicity is due to their different modes of action. The primary biological targets of trivalent arsenic are thiol-containing molecules, e.g. in active sites of enzymes (see below). Arsenate, on the other hand, is better tolerated (detoxified) at low concentrations and, at higher levels, competes with phosphate, an ubiquitous biological anion. Indeed the toxicity of pentavalent arsenic in vivo may be partly due to its intracellular reduction to the trivalent form . ... 

The pentavalent oxidation state is accessible for the early actinides uranium, protactinium, neptunium, and plutonium. Pentavalent species with neutral Group 16 bases can include either adducts of AnXg or complexes incorporating oxo-containing cations, AnO " or An02". ... 

The preferential reduction of the trivalent species in hydride generation can be used for the speciation of As, while the pentavalent species is first reduced by hydrogen after converting it into the trivalent species by a reduction with cystein. A similar way can be followed for SelV and SeV This can be done both in the case of conventional as well as of electrochemical hydride generation [324]. 

Speciation studies have been carried out for methylated species of As, Sb, Ge, and Sn and on different redox states of these elements—As(III-V), Sb(III-V), or Cr(III-VI)— (Andreae and Froehlich, 1984), as shown in Fig. 13.2. In oxic Baltic waters the pentavalent species of As and Sb predominate, while in the anoxic basins the distribution shifts to the trivalent species and possibly also to polysulfide complexes. The methylated species of As, Sb, and Ge are detectable throughout the water column. Since the mid-1980s, an improvement in the understanding of the behavior of trace elements in the Baltic Sea has been achieved. First investigations into the speciation of mercury have been realized by Brugmann (1979) and Brugmann et al. (1991). A special emphasis has been made on the different physicochemical forms, such as dissolved Hg, weakly associated Hg with particles, and total Hg. 

What can we infer from the binding geometry of the pentavalent transition state analog, uridine vanadate, about the catalytic mechanism The binding of the inhibitor shows that the active site can accommodate pentavalent species postulated for a Sisi2-mechanism, in particular those involved in the formation (Scheme 13.8, step 1 - 2) and the breakdown (step 4 5) of the cyclic phosphodiester. The oxygen atom... 

C, respectively). These arsines, when formed in a reaction vessel, were trapped in a U-shaped tube immersed in liquid nitrogen. The liquid nitrogen was then removed and the tube warmed up for sequential evaporation, separation and detection of the arsines. At pH > 6, only the more toxic trivalent As species could be transformed into their arsines without interference from the pentavalent species (Andreae 1977, Norin and Vahter 1981). Avoiding the liquid nitrogen trap, the separation was also performed by ion-exchange chromatography (Tam et al. 1978). Since arsenobetaine is not transformed into a volatile hydride, these procedures allowed easy differentiation between toxic and practically... 

The bond between the nucleophile and the electrophilic carbon atom could form before the bond to the leaving group breaks, to give a discrete pentavalent reaction intermediate. Notice, in the previous mechanism (Section 4.2.1) the pentavalent species was an activated complex, but in this mechanism it is a reaction intermediate. The leaving group is then lost in the second step to give the expected product... 

A carbocation is a structure with a positive charge that is associated primarily with a carbon center or, in the case of delocalized systems, a collection of carbons. Heteroatom substitution is allowed, and it will inevitably lead to some positive charge being on the heteroatoms. There are two types of carbocations—carbenium ions and carbonium ions. Carbenium ions are trivalent species with a formula of RaC. Carbonium ions encompass two related types of structures. First are pentavalent species of the general formula R5C. While not common in solution phase chemistry, such pentavalent carbons are common in the gas phase. Also referred to as carbonium ions are carbocations that have an important contribution from three center—two electron bonding. The nomenclature will become more clear as we present examples below. 

In contrast to the high resihence of phosphites toward oxygen and peroxides, they react easily with water [128]. As a result, the corresponding pentavalent species are formed (Scheme 2.106). Stepwise substitution of all aryloxy/alkoxy groups eventually produces phosphorous acid, H3PO3. Because of tautomeric equUibrium, acids are continuously generated, which support the hydrolysis of the ester bonds by autocatalysis. 

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