The d transition elements

For the d electrons, the situation is similar, but not as dramatic the centrifugal term in this case is smaller, and the potential, instead of a positive barrier between two wells, exhibits only a knee, as shown in fig. 5.5. In spite of this, orbital collapse occurs, but over intervals of more than one atomic unit. 

The exact point at which collapse occurs depends rather critically on the presence and strength of the knee at the edge of the inner well. Being less dramatic, the collapse of d orbitals is also more easily disturbed. Thus, it was found [209] that the radius of the 4s electron in the 3d transition elements is very close to the position of the knee. When the 4s orbital is occupied, the potential experienced by the 3d electrons is 

A good way to see the breakdown of the aufbau principle is to draw up a table of configurations not only for the ground states of the atoms, but also for the corresponding ions. From table 5.1, one can see that, usually, it is easier to ionise the outermost s than the outermost d electron. The exceptions are  

Ti 3d24s2 3d24s Zr 4cP5s2 4d25s Hf 5d26s2 5d6s2 

Mn 3d54s2 3d54s Tc 4d55s2 4d55s Re 5d55s2 5d56s 

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The redox behaviour of Th, Pa and U is of the kind expected for d-transition elements which is why, prior to the 1940s, these elements were commonly placed respectively in groups 4, 5 and 6 of the periodic table. Behaviour obviously like that of the lanthanides is not evident until the second half of the series. However, even the early actinides resemble the lanthanides in showing close similarities with each other and gradual variations in properties, providing comparisons are restricted to those properties which do not entail a change in oxidation state. The smooth variation with atomic number found for stability constants, for instance, is like that of the lanthanides rather than the d-transition elements, as is the smooth variation in ionic radii noted in Fig. 31.4. This last factor is responsible for the close similarity in the structures of many actinide and lanthanide compounds especially noticeable in the 4-3 oxidation state for which... 

In Fig. 1, the valences of the transition 3d-, 4d-, 5d-series are also plotted. As for what regards the spread of valences, an interesting observation is that, at least for the hght actinides (if not for the whole series), there is more similarity between the actinides transition elements than the actinides and lanthanides. 

The exact position of the crossover depends however, on the type of compound that is formed from an actinide. It appears usually for the radioactive heavier actinides. In order to study it, therefore, compoimds of Pu, Am, Cm have to be investigated. Compounds of elements preceding them in the actinide series present properties due to the itineracy of the 5 f electrons, which are somewhat similar to the d-transition elements compounds (especially 3d). Heavier actinides are lanthanide-like although their properties may depart from a true lanthanide behaviour unfortunately, their rarity and the difficulty of their handling is such that very few photoemission results are available for them. 

The relationships between bond enthalpy, bond length and bond order which appear relatively simple in the case of a main group element such as carbon and its compounds, are more difficult to establish when the d-transition metal elements and their compounds are considered. Progress in establishing these relationships for metals is severely hindered by a lack of relevant thermochemical data. This paper reviews some of the more useful information that is available for diatomic molecules, for polynuclear binary carbonyls and for binuclear complexes of the d-transition elements. 

The results of the calculations by Fricke and Waber (56) and Mann (35), which are listed in Table 2, show that formally elements 155 to 164 are the d transition elements of the 8 th period. 

Table 5.1. Ground state configurations of the d transition elements.

The structural chemistry of six-coordinated fluoro complexes of the d-transition elements is the subject of an entire review paper by Babel (7). In contrast, six coordination is rare in actinide fluoro-complexes. The relatively large sizes of the actinide ions (Fig. 3) suggest that low coordination numbers should be found only in the fluoride complexes of the higher oxidation states. The radius ratio, r+/r, predicted for the lower hmit of stabihty of octahedral coordination is ]/2 — 1, which corresponds to a positive ion radius of 0.55 A in coordination with fluoride (1.33 A). 

The eight-fold coordination type with the ligands situated at the corners of a cube maximizes non-bonded repulsions (20, 58, 59,64), and it, therefore, is not an appealing coordination t 7pe. The cube is, indeed, unknown as a structure type among the molecular coordination compounds of the d-transition elements although it is prevalent among inonic compounds with the CaF2 and CsCl lattice types. 

This peculiarity of scandium is also observed in ternary systems. It is a linking element between the rare earth and the d transition elements. Zr, Hf and the heavy rare earths are the elements with which scandium forms statistical mixtures of atoms, up to a complete mutual substitution in the structures. With other elements scandium occupies regular atomic positions in the structures and forms, as usual, ternary compounds of constant composition. 

The discovery of the next two transuranium elements, americium (Z = 95) and curium (Z = 96), depended on an understanding of the correct positions in the periodic table of the elements beyond actinium (Z = 89). It had been thought that these elements should be placed after actinium under the d-transition elements. So uranium was placed in Group VIB under tungsten. However, Glenn T. Seaborg, then at the University of California, Berkeley, postulated a second series of elements to be placed at the bottom of the periodic table, under the lanthanides, as shown in modem tables (see inside front cover). These elements, the actinides, would be expected to have chemical properties similar to those of the lanthanides. Once they understood this, Seaborg and others were able to use the predicted chemical behaviors of the actinides to separate americium and curium. 

Structural characterization of the d° transition-element oxides. Cation coordination numbers and coordination polyhedra. 

The rare earth ions belong to class (a) in the Ahrland, Chatt and Davies (1958) classification or to the hard acid class in the Pearson (1963) designation. Ions in this class bond to hard bases, primarily those which contain oxygen and nitrogen as the donor atoms, and bond only weakly to the soft bases which contain, for example, sulfur or phosphorous as the donor atoms. This means that by far the majority of rare earth complexes contain ligands which utilize at least one oxygen atom. As a result, the variety of types of complexes which can be formed easily with the rare earth ions is more restricted than that which is formed with the d-transition elements. 

The factors considered above demonstrate that in many respects the rare earth ions resemble the alkaline earth ions in their complex-forming tendencies more closely than they do the d-transition elements. Indeed one of the more recent uses of the rare earth ions is as a substitute for calcium in biological systems. 

This order means that the crystal field in the lanthanides is acting to remove some of the degeneracy contained in the individual values of the / quantum number. This additional splitting is generally only on the order of two hundred wave numbers or so whereas in the d-transition elements it is on the order of 10 000-30 000 wave numbers. 

The third class of ligands includes the Schiff bases derived, in general, from salicylaldehyde or its derivatives. In many of these complexes the phenolic group is not ionized which leads to complexes that are less stable and more difficult to characterize than for the d-transition elements. 

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