Atomic radii, lanthanides

R, R Rare-earth metal, Y, Sc, and the lanthanides Atomic radius of the R component T Tetragonal structure... 

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. 

The atomic radii of the second row of d-metals (Period 5) are typically greater than those in the first row (Period 4). The atomic radii in the third row (Period 6), however, are about the same as those in the second row and smaller than expected. This effect is due to the lanthanide contraction, the decrease in radius along the first row of the / block (Fig. 16.4). This decrease is due to the increasing nuclear charge along the period coupled with the poor shielding ability of /-electrons. When the d block resumes (at lutetium), the atomic radius has fallen from 217 pm for barium to 173 pm for lutetium. 

Hg is much more dense than Cd, because the decrease in atomic radius that occurs between Z = 58 and Z = 71 (the lanthanide contraction) causes the atoms following the rare earths to he smaller than might have been expected for their atomic masses and atomic numbers. Zn and Cd have densities that are not too dissimilar because the radius of Cd is subject only to a smaller d-block contraction. 

As one traverses through the lanthanide series, there is a reduction in the cation size as the atomic number increases. This results in small differences in the strength of interactions of the ligand with the lanthanide ions. These trends are reflected in the IR spectra of these complexes in a few cases. Cousins and Hart (203) have observed an increase in Pp Q with decreasing lanthanide ion radius for the complexes of TPPO with lanthanide nitrates. This observation has been attributed to an increase in the Ln—O bond strength with an increase in the atomic number of the lanthanide ion. 

Symbol Dy atomic number 66 atomic weight 162.50 a lanthanide series, inner transition, rare earth metal electron configuration [Xe]4 5di6s2 atomic volume 19.032 cm /g. atom atomic radius 1.773A ionic radius 0.908A most common valence state +3. 

Symbol La atomic number 57 atomic weight 138.91 a rare-earth transition metal, precursor to a series of 14 inner-transition elements known as the lanthanide series electron configuration [XejSdiGs oxidation state -i-3 atomic radius 1.879A ionic radius (LaS+) 1.061A electronegativity 1.17 two natural isotopes are La-139 (99.911%) and La-138 (0.089%). 

Symbol Lu atomic number 71 atomic weight 174.97 a lanthanide series element an /-block inner-transition metal electron configuration [Xe]4/i45di6s2 valence -1-3 atomic radius (coordination number 12) 1.7349A ionic radius (Lu3+) 0.85A two naturally-occurring isotopes Lu-176 (97.1%) and Lu-175(2.59%) Lu-172 is radioactive with a half-life of 4xl0i° years (beta-emission) several artificial isotopes known, that have mass numbers 155, 156, 167—174, 177—180. 

Symbol Nd atomic number 60 atomic weight 144.24 a rare earth lanthanide element a hght rare earth metal of cerium group an inner transition metal characterized by partially filled 4/ subshell electron configuration [Xe]4/35di6s2 most common valence state -i-3 other oxidation state +2 standard electrode potential, Nd + -i- 3e -2.323 V atomic radius 1.821 A (for CN 12) ionic radius, Nd + 0.995A atomic volume 20.60 cc/mol ionization potential 6.31 eV seven stable isotopes Nd-142 (27.13%), Nd-143 (12.20%), Nd-144 (23.87%), Nd-145 (8.29%), Nd-146 (17.18%), Nd-148 (5.72%), Nd-150 (5.60%) twenty-three radioisotopes are known in the mass range 127-141, 147, 149, 151-156. 

Symbol Tb atomic number 65 atomic weight 158.925 a lanthanide series element an inner-transition rare earth metal electron configuration fXe]4/96s2 valence states -i-3, +4 mean atomic radius 1.782A ionic radii, Tb3+... 

Symbol Tm atomic number 69 atomic weight 168.93 a lanthanide series element a rare earth metal electron configuration iXe]4/i36s2 valence +2, -i-3 atomic radius 1.73 A ionic radius, Tm " " 1.09 A for coordination number 7 one stable, natural isotope Tm-169 (100%) thirty radioisotopes in the mass range 146-168, 170-176 ty, 1.92 years. 

The ionic radius of the lanthanide atom is an important factor governing the formation and successful synthesis of well-defined di- or trisubstituted complexes. This can be nicely illustrated by the example of the alkoxysilylamide ligand [Me2Si(OtBuXNrBu)j This anionic ligand has frequently been employed by Veith and Rosier in main group chemistry [45, 77]. 

The properties of the elements of the sixth period are influenced by lanthanide contraction a gradual decrease of the atomic radius with increasing atomic number from La to Lu. The elements of groups 5 to 11 for the fifth and sixth periods have comparable stmctural parameters. For instance, Nb and Ta, as well as the pair Mo and W, have very similar ionic radii, when they have the same oxidation number. As a result, it is very difficult to separate Nb and Ta, and it is also not easy to separate Mo and W. Similarly, Ag and Au have nearly the same atomic radius, 144 pm. Recent studies of the coordination compounds of Ag(I) and Au(I) indicate that the covalent radius of Au is even shorter than that of Ag by about 8 pm. In elementary textbooks the phenomenon of lanthanide contraction is attributed to incomplete shielding of the nucleus by the diffuse 4f inner subshell. Recent theoretical calculations conclude that lanthanide contraction is the result of both the shielding effect of the 4f electrons and relativistic effects, with the latter making about 30% contribution. 

The rare-earth metals are of rapidly growing importance, and their availability at quite inexpensive prices facilitates their use in chemistry and other applications. Much recent progress has been achieved in the coordination chemistry of rare-earth metals, in the use of lanthanide-based reagents or catalysts, and in the preparation and study of new materials. Some of the important properties of rare-earth metals are summarized in Table 18.1.1. In this table, tm is the atomic radius in the metallic state and rM3+ is the radius of the lanthanide(III) ion in an eight-coordinate environment. 

The decrease in radius in moving from La3+ to Lu3+ is 117.2 to 100.1 pm which is less than 114-88 pm for elements Ca2+ to Zn2+. In the case of Sc3+ to Ga3+, the radius decreases from 88.5 to 76 pm. This comparison shows that the percent contraction is greater in the case of Sc3+ to Ga3+ and Ca2+ to Zn2+ series than lanthanides series. The fact is that the magnitude of the lanthanide contraction is small and the usual interpretation of magnetic and spectroscopic properties of the lanthanides are inconsistent with the idea of considerable shielding of 4/ electrons from the chemical environment of the ion by the 5s25p6 configuration. Thus the implication that the size of lanthanide atoms or ions is determined by the 4 fn subshell must be incorrect. 

These are listed in Table 2.3 and shown in Figure 2.4. It will be seen that the atomic radii exhibit a smooth trend across the series with the exception of the elements europium and ytterbium. Otherwise the lanthanides have atomic radii intermediate between those of barium in Group 2A and hafnium in Group 4A, as expected if they are represented as Ln + (e )3. Because the screening ability of the f electrons is poor, the effective nuclear charge experienced by the outer electrons increases with increasing atomic number, so that the atomic radius would be expected to decrease, as is observed. Eu and Yb are exceptions to this because of the tendency of these elements to adopt the (+2) state, they have the structure [Ln +(e )2] with consequently greater radii, rather similar to barium. In contrast, the ionic radii of the Ln + ions exhibit a smooth decrease as the series is crossed. 

Filling of the inner 4f electron shell across the lanthanide series results in decreases of ionic radii by as much as 15% from lanthanum to lutetimn, referred to as the lanthanide contraction (28). While atomic radius contraction is not rmique across a series (i.e., the actinides and the first two rows of the d-block), the fact that all lanthanides primarily adopt the tripositive oxidation state means that this particular row of elements exhibits a traceable change in properties in a way that is not observed elsewhere in the periodic table. Lanthanides behave similarly in reactions as long as the mnnber of 4f electrons is conserved (29). Thus, lanthanide substitution can be used as a tool to tune the ionic radius in a lanthanide complex to better elucidate physical properties. 

One effect of lanthanide contraction is that the radius of trivalent yttrium ion (Y +) is measured to be between that of Ho + and Er +, and the atomic radius of yttrium is between neodymium and samarium. This results in the chemical properties of yttrium being very similar to those of lanthanide elements. Yttrium is often found with lanthanide elements in natural minerals. The chemical properties of yttrium may be similar to the lighter or the heavier lanthanide elements in different systems and this depends on the level of covalent character of the chemical bonds in those systems. 

Figure 1.5 The relationship between atomic radius and atomic number of lanthanide atoms [1, 5].

The radii of actinide ions and decrease with increasing atomic number in much the same way as do those of the lanthanides, the radius of being about 0-10 A greater than that of M " (e.g. L03 A, 0-93 A). The... 

The structures are formed only by the transition, lanthanide and actinide elements, and not by other metals of comparable atomic radius and electronegativity. 

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