The Lanthanides

The lanthanides constitute a series of 15 inner-transition elements with very similar physico-chemical properties, some of which are summarized in Table 14.1. These Group III metals usually exhibit an 

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Element Symbol Atomic number Atomic mass Ionic radius Melting point j°C] Boiling point /°Q  

Hirano and Suzuki (1996) have summarized detection limits of the various ions in the lanthanide series for four analytical methods. The most sensitive method for analysis of lanthanides is inductively coupled plasma-mass spectrometry (ICP-MS), where detection limits for the series of ions range from 0.002 to 0.009 Jg L with the exception of Sm (limit of 1.5 Detection limits for lanthanides using ICP-atomic emission spectrometry (ICP-AES) range from 0.02 to 30 jg L h The sensitivity 

As expected based on the 3d10 4s2 configuration, zinc routinely forms +2 compounds, and all of the halides having the formula ZnX2 are known. Anhydrous ZnCl2, which has applications in the textile industry, can be prepared as shown by the following equation  

Evaporation of a solution produced by dissolving Zn in aqueous hydrochloric acid gives ZnCl2.2H20 as the solid product. Heating this compound does not result in the formation of anhydrous zinc chloride because of the reaction 

This is similar to the behavior of aluminum halides discussed earlier in this chapter, and it illustrates the fact that dehydration of a hydrated solid cannot be used as a way to prepare anhydrous halide compounds in some instances. 

The brief discussion of the chemistry of the first-row transition metals presented here shows only a small portion of this vast subject. However, it illustrates some of the differences between the metals and how their chemistry varies throughout the series. For additional details, the reference text by Greenwood and Eamshaw or that by Cotton et al. should be consulted. In Chapters 16 through 22, many other aspects of the organometallic and coordination chemistry of these metals will be presented. 

Electrons generally fill orbitals in atoms as the sum (n + /) increases. Therefore, after the 6 s orbital (for which n + l = 6) is filled at Ba, it would be expected that the next orbitals populated would be those for which the sum (n I 2) 7 but having the lowest n giving that sum. The corresponding orbitals would be 

Although the lanthanides are not usually considered to be transition metals, they are sometimes referred to as inner transition metals because the third transition series begins with La and the next elements are the lanthanides. Accordingly, a brief introduction to their properties and chemistry is included here. 

One of the interesting trends shown by the data in Table 18.4 concerns the gradual decrease in the sizes of the +3 ions in progressing across the series. This effect is the 

Metal Structure m.p. °C Ionic Radius (+3) r, pm -AHhyd First Three Ionization Potential Sum, k mol 1 

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

Another interesting consequence of the lanthanide contraction is that the ionic radii of Eu3+ and Ho3+ are approximately the same as the radius of Y3+ (88 pm). As a result, much of the chemistry of Y3+ is similar to that of some of the lanthanides. 

The nuclear properties are unfavorable for most of the lanthanides, with low transition frequencies and receptivities, or large quadrupole moments. Exceptions are Pr (j = 0.258, 7=5/2), Eu (0.175,5/2), Tb (0.03,3/2) and Ho (0.102,7/2). The detectability of Eu and Ho is again hampered by Q values between 1 and 3 X 10 m, and that of Tb by the low receptivity. There are two spin-1/2 nuclei Tm (N= 100%, s = 5.5 X 10 ) and Yb (14.31, 5.5 x 10 ). Cerium has no stable magnetic nucleus. Yb and Lu + are diamagnetic and these ions at least should have been probed by direct NMR, but no solution NMR on lanthanides has been reported. In contrast, solid state investigations of paramagnetic samples are quite common and a list of selected recent studies is given below  

The Pr resonance in PrV04 at 4.2 K, e.g., shows a nice five-line pattern due to first-order quadrupole splitting by the 7=5/2 Pr nucleus, if the magnetic field is applied parallel to the [110] plane of the crystal.  

The F NMR of (CgF5)2Yb(THF)4 exhibits a satellite system due to Yb- F coupling of 120 Hz.The coupling constant is tentatively assigned to the fluorine atoms in ortho position. The more common use of lanthanide ions as shift reagents has recently been reviewed by Williams.  

The aqueous chemistry of the two rows of f-block elements, the lanthanides (lanthanum to lutetium) and the actinides (actinium to lawrencium), are sufficiently different from each other to be dealt with in separate sections. Similarities between the two sets of elements are described in the actinide section. 

The lanthanide elements are the 15 elements from lanthanum to lutetium. Both La and Lu have been included to allow for the different versions of the Periodic Table, some of which position La in Group 3 as the first member of the third transition series and others that place Lu in that position. If Lu is considered to be the first element in the third transition series, all members of that series possess a filled shell 4f14 configuration. The outer electronic configurations of the lanthanide elements are given in Table 8.1. 

The oxidation states of the lanthanide elements are given in Table 8.2. 

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. ,,  

The very negative Ln t + /Ln potentials are consistent with the electropositive nature of the lanthanide elements their Allred-Rochow electronegativity coefficients are all 1.1 except for europium, which has a value of 1.0. The lighter elements of Group 3, Sc and Y, both have electronegativity coefficients of 1.3. The nearest p-block element to the lanthanides in these properties is magnesium (Mg2+/Mg) = -2.37 V, and its electronegativity coefficient is 1.2. 

PrcifTielhium does not occur naturally. I was iirsi isolated from the liSSion products of uranium. 

The abbreviaSions Ln - lan. hanide and An aclinide are used for general purposes in this lexl 

In KLa2(NH2)7, obtained by reaction of K and La with NH3 under pressure and at 350 °C, the lanthanum atom is eight-co-ordinate.25 The co-ordination polyhedron may be described as a deformed trigonal prism which is bicapped on two four-fold faces. 

In the complex GdL(N03)3, [L = (2)], the metal atom is ten-co-ordinate,27 the co-ordination polyhedron being described as a distorted pentagonal bipyramid. Four of the five equatorial positions are occupied by the N atoms of L, and the remaining three positions by the three bidentate nitrate groups. 

Caughlan, Mazhar-ul-Haque, and F. A. Hart, Inorg. Chem., 1973,12, 2654. 

In [Pr2(H20)4(C4H6N04)2(C4H5N04)]Cl2,3H20, each hydrogen-iminodiacetate and iminodiacetate group was co-ordinated34 to four Pr3 + ions, thereby giving a three-dimensional network structure. The co-ordination polyhedron around each Pr atom is described as a distorted monocapped square antiprism made up of seven carboxylato O atoms and two water molecules. 

In La202S, the lanthanum atom is located35 on a three-fold axis with a triangle of sulphide ions above and a triangle of O atoms, and one axial O atom below. 

Solubility studies of yttrium chloride hexahydrate showed it to be increasingly soluble as the temperature of ethanol was raised from 20 to 60 °C. 

The densities of molten KCI-YCI3 mixtures of various compositions were reported to show a linear dependence upon temperature, and the positive departure of molar volume from additive values was interpreted in terms of a more open structure in the mixture than in the pure compound. The nature of the interaction between yttrium and potassium chlorides was discussed. 

Methods of preparation of YVO4-YPO4 solid solutions have been investigated and deviations from Vegard s Law shown to be the result of side reactions which occur during the preparation of the solid solution. 

Structural Studies.—Y-Ray diffraction studies of rare earth metal sesquisulphides, Ln2S3 (Ln = La—Dy), suggested that the structures are based on a rigid framework involving possible fluctuations in the sulphur atom positions. Lanthanide and actinide phases with UCI3- and PuBr3-type M X3 structures have been described in terms of alternating layers of [MX2] and ions. The existence of different types of 

The tetragonal crystal structures of R3Rh2 compounds (R = Gd, Tb, Dy, Ho, and Er) have been found to be similar to those reported for Y3R2. Four different types of Rh-centred rare earth polyhedra were established and identified as trigonal prisms, cubes, Archimedian antiprisms, and truncated Archimedian antiprisms. 

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Europium(TTI) salts are typical lanthanide derivatives. Europium(ll) salts are pale yellow in colour and are strong reducing agents but stable in water. EuX2 are prepared from EuX -hEu (X=C1, Br, I) or EuFa + Ca EuCl2 forms a dihydrale. EUSO4 is prepared by electrolytic reduction of Eu(III) in sulphuric acid. Eu(II) is probably the most stable +2 stale of the lanthanides... 

As regards the transition elements, the first row in particular show some common characteristics which define a substantial part of their chemistry the elements of the lanthanide and actinide series show an even closer resemblance to each other. 

The reason why lanthanides of high atomic number emerge first is that the stability of a lanthanide ion-citrate ion complex increases with the atomic number. Since these complexes are formed by ions, this must mean that the ion-ligand attraction also increases with atomic number, i.e. that the ionic radius decreases (inverse square law). It is a characteristic of the lanthanides that the ionic radius... 

Another characteristic change across the lanthanide series is that of the paramagnetism of the ions this rises to a maximum at neodymium, then falls to samarium, then rises to a second maximum at gadolinium before falling finally to zero at the end of the series. 

Evidence other than that of ion-exchange favours the view of the new elements as an inner transition series. The magnetic properties of their ions are very similar to those of the lanthanides whatever range of oxidation states the actinides display, they always have -1-3 as one of them. Moreover, in the lanthanides, the element gado-... 

Ytterby, a village in Sweden) Discovered by Mosander in 1843. Terbium is a member of the lanthanide or "rare earth" group of elements. It is found in cerite, gadolinite, and other minerals along with other rare earths. It is recovered commercially from monazite in which it is present to the extent of 0.03%, from xenotime, and from euxenite, a complex oxide containing 1% or more of terbia. 

The atom radius of an element is the shortest distance between like atoms. It is the distance of the centers of the atoms from one another in metallic crystals and for these materials the atom radius is often called the metal radius. Except for the lanthanides (CN = 6), CN = 12 for the elements. The atom radii listed in Table 4.6 are taken mostly from A. Kelly and G. W. Groves, Crystallography and Crystal Defects, Addison-Wesley, Reading, Mass., 1970. 

Filling up the 4/ orbital is a feature of the lanthanides. The 4/ and 5d orbitals are of similar energy so that occasionally, as in La, Ce and Gd, one electron goes into 5d rather than 4f. Similarly, in the actinides, Ac to No, the 5/ subshell is filled in competition with 6d. 

Ion-exchange separations can also be made by the use of a polymer with exchangeable anions in this case, the lanthanide or actinide elements must be initially present as complex ions (11,12). The anion-exchange resins Dowex-1 (a copolymer of styrene and divinylben2ene with quaternary ammonium groups) and Amherlite IRA-400 (a quaternary ammonium polystyrene) have been used successfully. The order of elution is often the reverse of that from cationic-exchange resins. 

Crystal Structure and Ionic Radii. Crystal stmcture data have provided the basis for the ionic radii (coordination number = CN = 6), which are summarized in Table 9 (13,14,17). For both and ions there is an actinide contraction, analogous to the lanthanide contraction, with increasing positive charge on the nucleus. 

In general, the absorption bands of the actinide ions are some ten times more intense than those of the lanthanide ions. Fluorescence, for example, is observed in the trichlorides of uranium, neptunium, americium, and curium, diluted with lanthanum chloride (15). 

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