Lanthanide ions, hydrated

One of the consequences of the lanthanide contraction is that some of the +3 lanthanide ions are very similar in size to some of the similarly charged ions of the second-row transition metals. For example, the radius of Y3+ is about 88 pm, which is approximately the same as the radius of Ho3+ or Er3 +. As shown in Figure 11.8, the heats of hydration of the +3 ions show clear indication of the effect of the lanthanide contraction. 

FIGU RE 11.8 Heat of hydration of + 3 lanthanide ions as a function of ionic radius. 

It is quite difficult to measure an accurate enthalpy of solution A//( olutioni with a calorimeter, but we can measure it indirectly. Consider the example of sodium chloride, NaCl. The ions in solid NaCl are held together in a tight array by strong ionic bonds. While dissolving in water, the ionic bonds holding the constituent ions of Na+ and Cl- in place break, and new bonds form between the ions and molecules of water to yield hydrated species. Most simple ions are surrounded with six water molecules, like the [Na(H20)6]+ ion (VI). Exceptions include the proton with four water molecules (see p. 235) and lanthanide ions with eight. 

The enthalpies of solution and solubilities reviewed here provide much of the experimental information required in the derivation of single-ion hydration and solvation enthalpies, Gibbs free energies, and entropies for scandium, yttrium, and lanthanide 3+ cations. 

The lanthanide ions, Ln are known to contract with increasing atomic number (Z), from La with a hydrated radius of 103 pm to Lu of 86 pm (lanthanide contraction). Thus one expects that the neutral LnAj complex becomes smaller with increasing atomic number, and consequently that P3 should increase and K c decrease with increasing Z. Figure 4.15d shows that measured... 

The lanthanides all have their +3 states as the stable species in acidic solutions, as indicated by the data given in Table 8.3. The + 3 states are produced by the removal of the 6s" pair of electrons plus either the single 5d electron or one of the 4f electrons. In this respect they behave like the members of Group 3, any additional ionization being normally The jdiv.-.v unsustainable by either lattice production or ion hydration. I,-. 1. ,, ... 

Russian workers have continued to study hydrated salts of lanthanide ions. Some of these studies are of a rather detailed nature and a summary of some of the more recent work will be given here to indicate its general nature. There is considerable emphasis on phase studies, X-ray powder data, thermogravimetric and differential thermal analysis, and IR data. 

Nitrate complexes, including hydrated nitrates, have been well studied. This ion is almost always bidentate towards a lanthanide ion, and discrete complexes or complex anions are usually formed, in contrast to the sulfate complexes mentioned above. The complex anions [M(N03)5]2-, where M = Ce, Eu, Ho or Er, have had their structures reported and are pseudotrigonal bipyramidal. The M—O distances are Ce—O = 2.553-2.591 A and Er—0 = 2.392-2.493 A.385-8 The (Hpy)N03-Yb(N03)3 aqueous system has been studied, aftd (Hpy)2[Yb(N03)5] isolated from it and characterized by X-ray powder pattern.389... 

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... 

Table 14 Thermodynamic and Electrochemical Parameters for Hydrated Dipositive Lanthanide Ions...

It might be considered that the way in which to understand the water complex ion chemistry of the lanthanide ions in aqueous solution would be to start from their hydrates. In fact the hydrates prove to be most intransigent complexes. Their structures are still somewhat uncertain. I shall therefore start from a study of the tris-dipicolinates, i.e. tris 2,2 -carboxy-pyridine complexes, Ln(dipic)3, about which a great deal is known. 

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