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Solution Chemistry of Element 105. Pt. 1. Hydrolysis of Group 5 Cations

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Solution Chemistry of Element 105. Pt. 1. Hydrolysis of Group 5 Cations ■1111111111 DE98F3676 SOLUTION CHEMISTRY OF ELEMENT 105. PART I: HYDROLYSIS OF GROUP 5 CATIONS: Nb,Ta, Ha AND Pa V. Pershina 29-13 ■D Gesellschaft fur Schwerionenforschung mbH PlanckstraBe 1 • D-64291 Darmstadt • Germany Postfach 11 05 52 • D-64220 Darmstadt • Germany Solution Chemistry of Element 105. Part I: Hydrolysis of Group 5 Cations: Nb, Ta, Ha and Pa V. Pershina Gesellschaft fur Schwerionenforschung, Planckstr.l, 64291 Darmstadt, Germany Element 105/Hahnium/Transactinide chemistry/Relativistic molecular calculations/Hydrolysis Summary Relativistic molecular orbital calculations of the electronic structure of hydrated and hydrolyzed complexes have been performed for group 5 elements Nb, Ta, Ha and their pseudohomolog, Pa. On their basis, relative values of the free energy changes and constants of hydrolysis reactions were defined. These results show that hydrolysis decreases in the order Nb > Ta > Ha » Pa, which for Nb, Ta and Pa is in agreement with experiment. A decisive factor in the process turned out to be a predominant electrostatic metal-ligand interaction. 1. Introduction A. Importance of hydrolysis The importance of hydrolysis in chemistry of transition elements is known since long. For Nb and Ta, differences in their complex formation in aqueous acidic solutions and extraction by various organic media have often been explained by differences in their hydrolysis [1], Thus, e.g., in the absence of water, Nb(V) and Ta(V) form chlorocomplexes of the same type, MCle". In the presence of water, even in concentrated hydrochloric acid, Nb and Ta hydrolyze, but in different ways: Nb forms complexes containing Nb03+, like NbCOH^CU", NbOCU" or NbOCls2", while hydrolysis of Ta goes further with the formation of the polynuclear products (TaO^Cl^nCl)*, hindering its extraction [2], 1 For metals in trace concentrations, mononuclear hydrolysis products existing in wide range of pH can also profoundly affect the chemical behaviour of the metal. Generally, the composition and charge of these systems can control such important aspects of chemical behaviour as adsorption of the dissolved metals on surfaces of mineral particles, the solubility of hydroxides or oxides, the extent to which the metal can be complexed in solution or extracted from solutions by various agents, and transition to another valence state [3]. In trace concentrations, Nb and Ta show also differences toward the complex formation in HC1 solutions and extraction by various organic media as those found for macro concentrations of these elements. Thus, Nb is better extracted which means less hydrolyzed, while Ta is more hydrolyzed and hence less extracted [4]. How strong hydrolysis of their homolog, Ha (produced as single atoms in the “one-atom-at-a-time- experiments ” [5]), would be and how it would influence its extraction is an important question, especially if one takes into account that trends in properties observed within the lighter elements might not continue when going over to the 6d transition row where relativistic effects are so strong. Until now, hydrolysis of the transactinides has not been investigated. The only publication was that on hydrolysis of rutherfordium, Rf [6]. Studying sorption of the metal cations on glass surfaces coated with cobalt ferrocyanide, the authors of [6] have found rutherfordium to be much less adsorbed than Zr and Hf, which was interpreted as Rf being more hydrolyzed. This conclusion is, however, in contradiction with expectations based on the electrostatic model of the hydrolysis mechanism, which predicts that among ions with the same formal charge, the one with the larger ionic radius (like Rf in comparison with Zr and Hf) should be less hydrolyzed. Thus, to understand hydrolysis of transactinides and its influence on the complex formation, the present theoretical study has been undertaken in the particular case of the group 5 elements, Nb, Ta, Ha, and its pseudohomolog, Pa. Any theoretical consideration on this subject should be helpful, especially in the case of the very heavy elements, where the experimental study [5] is connected with difficulties caused by low production rates and short half-lives of their isotopes. Since the experiments on extraction of Nb, Ta, Ha 2 and Pa are performed with single atoms, all the further considerations will be related to mononuclear species. When discussing hydrolysis, one should distinguish between hydrolysis of cations and hydrolysis of salts (or compounds). In the second case, hydrolysis involves either the cation, the anion, or both and is a reversed process of the complex formation. In general, it can be expressed by the following equilibrium xM(H20)w„z+ + yOH +aL <=> Mx0u(0H)y.2u(H20)wLa(xzya)+ + (xw° + u-w)H20 (i) Another process known to most transition elements is the hydrolysis of cations, which takes place in the non-complexing media. In this case, each step in the formation of a series of mononuclear species can be described as a loss of successive protons [3] M(H20)„z+ « MOH(H20)n .,<z"1)+ + H+ (2) In this way, species containing O2", OH and H20 ligands can be generally produced. In the present publication, we will be concerned primarily with the hydrolysis of cations. The hydrolysis of salts will be considered in a forthcoming paper. B. Hydrolysis products of Nb, Ta and Pa Though the class of mononuclear species has recently received growing attention, these simpler hydrolysis products have not been well characterized. In most cases the stepwise formation of hydroxide complexes is poorly established by available data. The reason for that is the fact that hydrolysis is normally studied at the higher metal concentrations where polynuclear complexes are predominant. As a result, our knowledge of the hydrolysis products of many cations is poor. Potentiometric measurements of the hydrogen ion concentration for Nb and Ta in aqueous media at small concentrations (10"6 M) indicate the dominance of pentavalent 3 species which are limited in concentration by the solubility of the +5 oxides. Nb is found to be more hydrolyzed than Ta (Fig. 1). From relatively uncertain data on solubilities, the following equilibria have been derived for mononuclear species of Nb and Ta in 1M KNO3 at 19° [3] Nb(OH)5 (aq) + H+ <=> Nb(OH)/ + H20 log Q = -0.6 Ta(OH)5 (aq) + H+ o Ta(OH)/ + H20 log Q = 1.0 Nb(OH)5 (aq) + H20 Nb(OH)6 " + H+ log Q = -7.4 Ta(OH)s (aq) + H20 <=> Ta(OH)6 " + H+ log Q = -9.6 confirming, in addition to the data of the potentiometric measurements, a stronger hydrolysis of Nb in comparison with Ta. The data of Fig. 1 also show that hydrolysis of Nb and Ta is strong and proceeds fast to the formation of the ultimate products M(OH)5(aq) and M(OH)6 ". Since almost no intermediate hydrolysis compounds are known, the equlibria that have to be studied here are: Nb(H20)6 5+ Nb(OH)s <=> Nb(OH)6 " (3) Ta(H20)6 5+ o Ta(H20)2(0H)4+ <=> Ta(OH)5 o Ta(OH)6 " (4) Hydrolysis of Pa has been relatively well investigated and the species (in perchloric acid solutions) have been characterized [7] (Fig. 1). In comparison with Nb and Ta, hydrolysis of Pa is much weaker. The presence of small amounts of weakly complexing anions is not changing the results. The following hydrolysis products were shown to exist also in HC1 solutions [8]: Pa(OH)/ <=> PaO(OH)2+ (< 1M HC1) Pa(OH)32+ o PaO(OH)2+ (1-2 M HC1) 4 Pa(OH)23+ <=> Pa03+ (4-6 M HC1) The equilibria describing the hydrolysis of Pa are the following: Pa(H20)6 5+ <=> PaO(OH)2+ <=> PaO(OH)2+ <=> Pa(OH)5 <=> Pa(OH)6 " (5) Since the present study is comparative (hydrolysis of Ha as compared to that of Nb, Ta and Pa under the same conditions), we will concentrate here on a comparative prediction of the stability of the hydrolysis products. Luckily enough, all the known reactions for Nb, Ta and Pa belong to the same type where the utmost hydrolysis product M(OH)& is formed: M(H20)6 5+ M(OH)6 > 6H + (M = Nb, Ta and Pa) (6) (7) <2/6 = [ M(OH)6 -][ H+]6 /[ M(H2o)6 5+] The same type of reaction (6) can be supposed for hahnium. For Pa, the following step M(H20)6 5+ <=> M(OH)2(H20)43+ + 2H+ (M = Pa) (8) showing stability of the intermediate hydrolysis product might be also of interest. Reactions (6) will be considered in the following sections. (M(OH)s complexes known as precipitants will not be analyzed here, since their real coordination number, CN, as soluble mononuclear species is not known). 5 2. Models for mononuclear species A. The simple electrostatic model Generally, the stability of a hydrolysis product Mx0u(0H)v(H20)w<z-2uv)+ can be predicted on the basis of the knowledge of bond distances, charge distribution and covalent bonding energies - the kind of information that can be obtained from a quantum chemical treatment. At the time where such calculations were not available, the following model has been proposed [3]. In a fashion analogous to that of Kassiakoff and Barker [9], the following expression for the free energy of formation of the MxOu(OH)v(H20)w(z"2u v)+ species from the elements was adopted -AGf (u,v,w)l23RT=^ai+'£ja ij +logQ-log(«!v!vv!2w)+(2M+v+l)log555 (9) and log K = -AGr/2.3RT, (10) where AGr is the free energy change of a hydrolysis reaction. The first term on the right hand side of Eq. (9), J/t,, is the nonelectrostatic contribution from M, O, OH, and H20. The next term, , is a sum of each pairwise electrostatic interactions: a tJ = -Bq,q/d,j, 01) 6 where d/j is the distance between moieties i and j, and qj are their formal charges; and B is an independent constant.
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