Lanthanides hydration numbers

It has always been assumed that the hydration numbers for the lanthanides are higher than six, probably between 8 and 10, in analogy with the presence of enneaaquo ion [M(OH2)9 +] in neodymium bromate and ethylsulphate (see later, p. 121). Lanthanide hydration numbers have not been rigorously established, but some attempts have been made to study the problem by NMR technique (13—15). It is rather unfortunate that only low value for the hydration numbers ( 6) have been obtained, except for Er(III) and Yb(III) ions (756), where the hydration number is seven. 

The coordination chemistry of the large, electropositive Ln ions is complicated, especially in solution, by ill-defined stereochemistries and uncertain coordination numbers. This is well illustrated by the aquo ions themselves.These are known for all the lanthanides, providing the solutions are moderately acidic to prevent hydrolysis, with hydration numbers probably about 8 or 9 but with reported values depending on the methods used to measure them. It is likely that the primary hydration number decreases as the cationic radius falls across the series. However, confusion arises because the polarization of the H2O molecules attached directly to the cation facilitates hydrogen bonding to other H2O molecules. As this tendency will be the greater, the smaller the cation, it is quite reasonable that the secondary hydration number increases across the series. 

Fig. 4.15 The system La(III) acetylacetone (HA) - IM NaC104/benzene at 25°C as a function of lanthanide atomic number Z. (a) The distribution ratio Hl (stars, right axis) at [A ] = 10 and [HA] rg = 0.1 M, and extraction constants (crosses, left axis) for the reaction Ln + 4HA(org) LnA3HA(org) + 3FE. (b) The formation constants, K , for formation of LnA " lanthanide acetylacetonate complexes (a break at 64Gd is indicated) circles n = 1 crosses n = 2 triangles w = 3 squares w = 4. (c) The self-adduct formation constants, for the reaction of LnA3(org) + HA(org) LnA3HA(org) for org = benzene. (A second adduct, LnA3(HA)2, also seems to form for the lightest Ln ions.) (d) The distribution constant Ajc for hydrated lanthanum triacetylacetonates, LnAs (H20)2 3, between benzene and IM NaC104. (From Ref. 28.)...

A further application of relaxation rate measurements is that similar 1/71 ratios in a series of lanthanide complexes may be taken to indicate an isostructural series. However, this approach has the limitation that if only part of the complex is studied, perhaps an organic ligand, its 71 ratios would be independent of changes, for example changes in the extent of hydration in the remainder of the complex, provided that the conformation of the ligand relative to the lanthanide ion were preserved. An excellent example of the use of 71 data in a quite different way is its use to determine hydration numbers of lanthanide dipicolinate complexes.562... 

In recent years, X-ray diffraction studies of aqueous solutions have established primary hydration numbers for several fast-exchange cations 45,187-190 the timescale of X-ray diffraction is very much shorter than that of NMR spectroscopy. Octahedral hydration shells have been indicated for Tl3+,191 Cd2+, Ca2+, Na and K+, for example. For the lanthanides, [Ln(OH2)9]3+ is indicated for La, Pr and Nd, but [Ln(OH2)8]3 for the smaller Tb to Lu.192,193 Sometimes there are difficulties and uncertainties in extracting primary hydration numbers from X-ray data. Thus hydration numbers of eight and of six have been suggested for Na+ and for K+,194 and for Ca2+,195 and 8 and 9 for La3+, 196 In some cases rates of water exchange between primary and secondary hydration shells are so fast as to raise philosophical questions in relation to specific definitions of hydration numbers.197... 

Ln3+ induced water 170 shifts of [Ln(DOTA)] solutions show that the hydration number of the complexes is one across the lanthanide series [59]. The substantial pseudocontact contribution to its LIS indicated that this water ligand has a preferred location in the complex. Two sets of peaks have been observed in H and 13C NMR spectra of [Ln(DOTA)] complexes at room temperature showing the presence of two slowly interconverting structural isomers [60-63]. In the spectra of the paramagnetic complexes, one isomer has larger LIS values than the other. These structural features have been confirmed by luminescence studies [51, 64]. The temperature dependence of the H and 13C NMR spectral features of both the dia- and paramagnetic Ln3+ complexes indicates that the... 

Information on the hydration state of the Gd(III) chelate in solution is indispensable for the analysis of its proton relaxivity Several methods exist to determine q, though they are mostly applicable for other lanthanides than Gd(III). In the case of Eu(III) and Tb(III) complexes, the difference of the luminescence lifetimes measured in D20 and H20 can be related to the hydration number [15, 16]. For Dy(III) chelates, the lanthanide induced 170 chemical shift of the bulk water is proportional to the hydration number [17]. Different hydration states of the same chelate may also coexist in solution giving rise to a hydration equilibrium. Such an equilibrium can be assessed by UV-Vis measurements on the Eu(III) complex [18-20]. These techniques have been recently discussed [21]. 

An electrostatic hydration model has been applied to the trivalent lanthanide and actinide ions in order to predict the standard free energy (AG°) and enthalpy (AHt) of hydration for these series. Assuming crystallographic and gas-phase radii for Bk(III) to be 0.096 and 0.1534 nm, respectively, and using 6.1 as the primary hydration number, AG298 was calculated to be -3357 kJ/mol, and A/Z298 was calculated to be -3503 kJ/mol (187). 

Hydration and Hydrolysis. The various oxidation states of plutonium form strong ion-dipole bonds with water to become strongly hydrated in aqueous solution. To a first approximation, we can expect the hydration numbers of the first coordination sphere to be the same as the most probable coordination numbers suggested in the preceeding section. This means values of 8 or 9 for Pu(lll), 7 or 8 for Pu(Vl), and, perhaps, 4 for PuOj and 6 for PuOj. However, the polarization of the water dipoles of the primary hydration layer leads to attraction of additional waters of hydration. Estimates of the total number of waters of hydration for trivalent lanthanides and actinides have been given as 12 - 15 model of a small number of... 

Bergstrom and Lindgren s Determination of Primary Hydration Number from IR Measurements (Transition-MetaMons and Lanthanides)... 

Metals that form relatively stable aqua ions in solntion have generally been well characterized. The hydration nnmbers of the transition metals, for example, have commonly been found to be six, consistent with the results of solid-state studies of crystalline hydrates. Exceptions inclnde Ag+, which has a hydration number of four, and Cn +, which has been found to have four short M-0 interactions and two longer ones, consistent with a Jahn-Teller distortion (see Jahn - Teller Effect) of the complex. The hydration numbers of the lanthanide ions in solution have been the subject of many investigations and some controversy recent resnlts snggest a decrease from nine to eight across the series from Ta + to... 

The hydration munber, or the nimiber of bound water molecules in the lanthanide coordination sphere, can be calculated using a method introduced by Horrocks and Sudnick for terbimn and europium complexes (50). The relationship between Tb or Eu excited state lifetimes (t), which are experimentally determined in H2O and D2O, and the hydration number (q) is given in Eq. (1)... 

Chauvin, A.S., Gumy, R, Matsubayashi, I., etal. (2006) Fluorinated fl-diketones for the extraction of lanthanide ions photophysical properties and hydration numbers of their Eu " complexes. European Journal of Inorganic... 

Lanthanide complexes [Ln(D02A)(H20) ]+ of the potentially hexadentate D02A have been studied by measuring the lanthanide-induced shifts in the O-NMR spectra. Analysis of the contact contribution indicates a decrease in the hydration number from = 3 (Ln = Ce-Eu) to n = 2 (Ln = Tb-Lu). Study of the Dq transition in the UV-visible spectra of the Eu complex... 

An electrostatic hydration model, previously developed for ions of the noble gas structure, has been applied to the tervalent lanthanide and actinide ions. For lanthanides the application of a single primary hydration number resulted in a satisfactory fit of the model to the experimentally determined free energy and enthalpy data. The atomization enthalpies of lanthanide trihalide molecules have been calculated in terms of a covalent model of a polarized ion. Comparison with values obtained from a hard sphere modeP showed that a satisfactory description of the bonding in these molecules must ultimately be formulated from the covalent perspective. 

Semiempirical calculations of free energies and enthalpies of hydration derived from an electrostatic model of ions with a noble gas structure have been applied to the ter-valent actinide ions. A primary hydration number for the actinides was determined by correlating the experimental enthalpy data for plutonium(iii) with the model. The thermodynamic data for actinide metals and their oxides from thorium to curium has been assessed. The thermodynamic data for the substoicheiometric dioxides at high temperatures has been used to consider the relative stabilities of valence states lower than four and subsequently examine the stability requirements for the sesquioxides and monoxides. Sequential thermodynamic trends in the gaseous metals, monoxides, and dioxides were examined and compared with those of the lanthanides. A study of the rates of actinide oxidation-reduction reactions showed that, contrary to previous reports, the Marcus equation ... 

The free energy of cocrystallization decreases from La to Pm, then increases slowly from Pm to Gd and rather sharply from Gd to Lu. Because of the constancy of the coordination number in the solid phase with respect to water molecules such a trend suggests the change in the hydration number of lanthanides, occurring between Pm and Gd. 

Choppin and Friedman (27) tried to rationalize the trends in the case of simple carboxylates and hydroxycarboxylates on the basis of the difference in hydration numbers between the lighter and heavier lanthanides. They supported their argument on the ground that the hydration numbers for the lanthanide ions changes through the series (26) ... 

These hydration numbers seem to be rather high, but this may partly be due to the conductometric method used for their measurements. Mioduski and Siekierski (34) as well as Spedding et al. (35) prefer a change in coordination number of the aquoion from nine to eight in the heavier lanthanide region, although Geier and Karlen (36) have provided evidence for a constant hydration number of nine for the whole series. 

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