VI Aqua Ions

Solutions containing monomer and dimer aqua ions can be prepared from sodium molybdate(VI) by addition of dilute perchloric acid. As indicated above, the composition of solutions (0.3-9.0) X 10 3 M in Mo(Vl), in 0.5 M HC104 (I = 3.0 M NaC104), has been specified.27 Equilibration processes are known to be rapid. 

All Mo(VI) aqua ions are colorless. Peaks are observed for the dimeric species in the 240-250 nm region.27 

TABLE I Summary of Single Oxidation State Aqua Ions of Molybdenum 

Rudolf, and B. Jezowska-Trezbiatowska, Inorg. Chim., Acta, 7, 365 (1973). 

Submitted by RICHARD A. HENDERSON,t WASIF HUSSAIN,t G. JEFFERY LEIGH,t and FRED B. NORMANTONf 

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In this section we focus on the thermodynamics of two fundamental reaction mechanisms that determine the ability of metal cations to form complex species cation hydrolysis and solvent exchange. The thermodynamic modeling strategy is first discussed, followed by the hydrolysis of the U(IV), U(V), and U(VI) aqua ions. Finally, the solvent exchange thermodynamics of Cm(in) in dilute, perchlorate (CIO4), chloride (CF), and bromide (Br ) solution will be discussed in which the effect counter anions on changes in the Cm(ni) primary hydration number. 

Atta-Fynn R, Bylaska EJ and de Jong WA 2013 Importance of counteranions on the hydration structure of the curium ion. The Journal of Physical Chemistry Letters 4(13), 2166-2170. Atta-Fynn R, Johnson DF, Bylaska EJ, llton ES, Schenter GK and De Jong WA 2012b Structure and hydrolysis of the u (iv), u (v), and u (vi) aqua ions from ab initio molecular simulations. 

Aqueous Chemistry. Molybdenum has weU-characterized aqueous chemistry in the five oxidation states, VI, V, IV, III, and II. A listing of aqua ions is given in Table 2. Except for the Mo(VI) species all of the aqua ions are only soluble or stable in acidic media (17). The range of aqueous ions known for molybdenum is far broader than that of other elements. 

None of the Cr(III) products from Equations 6 or 7 are effective crosslinkers since a chromic aqua ion must be hydrolyzed first to form olated Cr to become reactive. Colloidal and solid chromium hydroxides react very slowly with ligands. In many gelation studies, this critical condition was not controlled. Therefore, both slow gelation times and low Cr(VI) Cr(III) conversion at high chromate and reductant concentrations were reported (9,10). 

Molybdenum has an extensive aqueous solution chemistry for oxidation states II through VI. It is unique in having aqua or aqua/oxo ions for all five states in acidic solution (pH < 2). These are well defined in all but the Mg 1 case, the study of which is complicated by the existence of rapid equilibria involving protonated/deprotonated monomer/dimer (and higher) forms. The VI state is without question the most stable and in contrast to Crvi is only the mildest of oxidants. Compounds which have contributed to the development of the aqueous solution chemistry, including the aqua ions themselves, are considered under Section 36.1.2. It is only since 1971 that the aqua forms of oxidation state II-V ions have been identified, and... 

Molybdenum is at present unique in having aqua ions in five oxidation states. Whereas the complexities of Mo(VI) aqueous solution chemistry have been understood in general terms for some time, it is only in the last 15 years that the aqua ions of the lower oxidation states II through V have been identified, and their structures clearly established. Metal aqua ions are notoriously difficult to crystallize for X-ray diffraction studies, and structures of derivative complexes... 

The aqua ions can be isolated as solids [M3Q4(H20)9]Cl4-nH20 after evaporation of solvent (preferably on a vacuum line). The yields were determined by UV-vis spectrophotometry (see Table 1). 

The U02 - XOs - H2O (X= C or N) systems have been chosen for analysis for two reasons. First, they are well studied experimentally, since carbonate and nitrate uranyl complexes are important from technological standpoints. Second, the isoelectronic anions XOs have the same planar structure (symmetry Dsh) and form bidentate coordination around uranyl ions (the type). However, their electron-donor characteristics are different El = 3.1 and 3.4 for the NOs and COs ions, respectively. From this viewpoint, it would be of interest to understand how the difference in the electron-donor properties influences complex formation in the U02 - XOs " - H2O systems. As in the case of aqua-complexes, we shall use the 18-electron rule to obtain answers to the following questions (a) what is the composition of stable complexes in aqueous solutions containing carbonate or nitrate uranyl complexes (b) what is the coordination number of U(VI) in these complexes. 

Analysis of the data obtained indicates that, under equilibria described by the equations (20) and (22), replacement of H2O molecules in the complexes V by carbonate and nitrate ions is accompanied by the decrease in the Ne parameter (it was already less than 18 for the [U02(H20)5] complex). In other words, the increase of the XOs 02 ratio in the complexes VI, VII, XI, and XII (schemes (20) and (22)) is accompanied by the increase of ANe from 0.7 to 1.5 and 2.1 for X = C and N, respectively. From the viewpoint of the 18-electron mle (the most stable complexes have Ne= 18 0.3 e ), the reactions (20) and (22) are energetically unfavorable. That is, the reactions (20) and (22) will deliberately occur only from the right to the left, since the aqua-complexes V are more stable than their acid derivatives VI, VII, XI, and XII. 

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