Lanthanide contraction examples

Relativistic effects are cited for changes in energy levels, resulting in the yellow color of gold and the fact that mercury is a liquid. Relativistic effects are also cited as being responsible for about 10% of lanthanide contraction. Many more specific examples of relativistic effects are reviewed by Pyykko (1988). 

A technologically important effect of the lanthanide contraction is the high density of the Period 6 elements (Fig. 16.5). The atomic radii of these elements are comparable to those of the Period 5 elements, but their atomic masses are about twice as large so more mass is packed into the same volume. A block of iridium, for example, contains about as many atoms as a block of rhodium of the same volume. However, each iridium atom is nearly twice as heavy as a rhodium atom, and so the density of the sample is nearly twice as great. In fact, iridium is one of the two densest elements its neighbor osmium is the other. Another effect of the contraction is the low reactivity—the nobility —of gold and platinum. Because their valence electrons are relatively close to the nucleus, they are tightly bound and not readily available for chemical reactions. 

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. 

Although in many respects relativistic effects and the lanthanide contraction serve to make the postlanlhanide elements less reactive than would otherwise be the case, in other respects their bonding ability is increased. For example, bis(phosphine)plati-num(0) complexes react with molecular hydrogen, but the analogous palladium complexes do not 3... 

Explain what is meant by the lanthanide contraction and give examples of its effect. 

It will be noted in Table 4.1 that the metallic radii along the 5d series are very close to the values for the corresponding atoms in the 4d series. This observation is related to the remarkable chemical similarities between Zr and Hf. The effect of the lanthanide contraction on metallic radii persists to the end of the 5d series, but its chemical consequences become less marked as we pass from left to right. It is important not to make too much of similarities in metallic radii in chemical arguments, however. For example, the triad Mn, Tc and Re all have practically equal metallic radii, but their chemical behaviour shows dramatic differences. The near-equivalence of their metallic radii is less marked when we look at bond lengths in molecules. For example, the M-M distances in (CO)5M—M(CO)5 are respectively 293, 304 and 302 pm for M = Mn, Tc and Re. 

Scheme I and, in more detail, Table 4 represent the trend of ionic radii of these large cations which prefer formal coordination numbers in the range of 8-12 [77]. For example, considering the effective Ln(III) radii for 9-co-ordination, a discrepancy of 0.164 A allows the steric fine-tuning of the metal center [60]. The structural implications of the lanthanide contraction can be visually illustrated by the well-examined homoleptic cyclopentadienyl derivatives (Fig. 2) [78], Three structure types are observed, depending on the size of the central metal atom A, [( j5—Cp)2Ln(ji— 5 rf — Cp)] x, 1 < % < 2 B Ln(fj5 —Cp)3 C, [fo -CpJjLnCi- 1 ff1—Cp)], these exhibit coordination numbers of 11 (10), 9, and 8, respectively. Also a small change in ligand substitution leads to a change in coordination behavior and number (10), as...

Another example of the lanthanide contraction at work, verified by crystallographers, lies in the complexes of the lanthanide nitrates with dimethyl sulfoxide (dmso). The early lanthanides form 10-coordinate Ln(dmso)4(N03)3 (Ln = La-Sm), whilst the heavier metals form Ln(dmso)3(N03)3(Ln = Eu-Lu, Y). 

However, in spite of the similarity of the physical properties (hke size, charge, and lipophilicity, which can determine the biodistribution) expected for analogous complexes of Re and Tc, on account of, for example, the lanthanide contraction , the former are usually easier to oxidize (harder to reduce) and less kinetically labile than the latter. The easier oxidation in vivo of the Re complexes to [Re04] can be advantageous in terms of excretion from the body (via the kidneys) of the radionuclide. 

Another effect of lanthanide contraction is that the third row of the d-block elements have only marginally larger atomic radii than the second transition series. For example, zirconium and hafnium, niobium and tantalum, or tungsten and molybdenum have similar ionic radii and chemical properties (Zr + 80 pm, Hf + 81 pm Nb + 70 pm, Ta + 73 pm Mo + 62 pm, W + 65 pm). These elements are also found in the same natural minerals and are difficult to separate. 

Because of lanthanide contraction, the radius of lanthanide ions decreases gradually as the atomic number increases, resulting in regular changes in the properties of lanthanide elements as the atomic number increases. For example, the stability constant of lanthanide complexes usually increases as the atomic number increases the alkalinity of lanthanide ions decreases as the atomic number increases the pH at which hydrates start to precipitate from an aqueous solution decreases gradually as the atomic number increases. 

Statement (c) is incorrect the atomic volumes of the transition metals decrease steadily from left to right the lanthanide contraction also affects the atomic volumes of third-row transition elements so that their atomic volumes are almost identical to those found for the second-row transition metals directly above. As a consequence, transition metals are very dense. For example, osmium with density of 22.6 g/cm is one of the densest materials known. 

Lanthanide contraction effects the properties of post lanthanide elements (elements of third transition series). For example,... 

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