4d transition series

Fig. 2. The energies of the most closed-packed surfaces of the metals in the 4d transition series obtained from first-principles [27].

The atomic structure of the transition metals is such that the J shell is only partly filled. The first transition series (3d) comprises Sc, Ti, V, Cr, Mn, Fe, Co, and Ni the second (4d), Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, and Ag the third (5d), Hf, Ta, W, Re, Os, Ir, Pt, and Au. Carbonyl derivatives of at least one type are found for all these metals. Although only a few are presently used in CVD, many are being investigated as they constitute an interesting and potentially valuable group of precursor materials. 

Looking at the trends in dissociation probability across the transition metal series, dissociation is favored towards the left, and associative chemisorption towards the right. This is nicely illustrated for CO on the 4d transition metals in Fig. 6.36, which shows how, for Pd and Ag, molecular adsorption of CO is more stable than adsorption of the dissociation products. Rhodium is a borderline case and to the left of rhodium dissociation is favored. Note that the heat of adsorption of the C and O atoms changes much more steeply across the periodic table than that for the CO molecule. A similar situation occurs with NO, which, however, is more reactive than CO, and hence barriers for dissociation are considerably lower for NO. 

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. 

Fig. 2.17 The valence s and d energy levels across the 3d and 4d transition metal series (after Herman and Skillman (1963)).

Figure 2.17 shows the valence s and d energy levels across the 3d and 4d transition metal series. The energy levels correspond to the atomic con> figuration dJV-1s, where N is the total number of valence electrons, because this is the configuration closest to that of the bulk metal. Again there are several important features. 

As a first example of the use of the d band model, consider the trends in dissociative chemisorption energies for atomic oxygen on a series of 4d transition metals (Figure 4.6). Both experiment and DFT calculations show that the bonding becomes... 

Figure 4.6. Variations in the adsorption energy along the 4d transition metal series. The results of full DFT calculations are compared to those from the simple d band model and to experiments. Below the same data are plotted as a function of the d band center. Adapted from Ref. [4].

Molybdenum is a metal of the second transition series, one of the few heavy elements known to be essential to life. Its most stable oxidation state, Mo(VI), has 4d orbitals available for coordination with anionic ligands. Coordination numbers of 4 and 6 are preferred, but molybdenum can accommodate up to eight ligands. Most of the complexes are formed from the oxycation Mo(VI)022+. If two molecules of water are coordinated with this ion, the protons are so acidic that they dissociate completely to give Mo(VI)042, the molybdate ion. Other oxidation states vary from Mo(III) to Mo(V). 

The ionic model is of limited applicability for the heavier transition series (4d and 5d). Halides and oxides in the lower oxidation states tend to disproportionate, chiefly because of the very high atomisation enthalpies of the elemental substances. Many of the lower halides turn out to be cluster compounds, containing metal-metal bonds (see Section 8.5). However, the ionic model does help to rationalise the tendency for high oxidation states to dominate in the 4d and 5d series. As an example, we look at the fluorides MF3 and MF4 of the triad Ti, Zr and Hf. As might be expected, the reaction between fluorine gas and the elemental substances leads to the formation of the tetrafluorides MF4. We now investigate the stabilities of the trifluorides MF3 with respect to the disproportionation ... 

Transition metal atoms are distinguished from other atoms by their having partially filled 3d, 4d or 5d orbitals. Here we consider only metals of the first transition series, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn, in which the 3d orbital is involved. 

We have already discussed the microwave/optical double resonance spectrum of the MoN molecule in its X4 groundstate. We now deal with studies of YF, YOand YS, yttrium being the first member of the 4d transition elements analogous to scandium in the 3d series. 

The lanthanide contraction has a knock-on effect in the elements in the 5d transition series. It would naturally be expected that the 5d elements would show a similar increase in size over the 4d transition elements to that which the 4d elements demonstrate over the 3d metals. However, it transpires that the lanthanide contraction cancels this out, almost exactly, and this has pronounced effects on the chemistry, e.g. Pd resembling Pt rather than Ni, Hf is extremely similar to Zr. 

The four transition metal series arise because, for each of these elements, an electron has been added to the next-to-outermost shell. Addition of 10 electrons to the 3t subshell after the completion of the 4s subshell causes 10 elements to occur after calcium to be the first elements in their periodic groups. The second, third, and fourth transition series occnr becanse the 4d, 5d, and 6d snbshells add electrons after the start of the fifth, sixth, and seventh shells. 

In comparing the 3d, 4d, and 5d transition series, it is instructive to consider the atomic radii of these elements (Fig. 20.3). Note that there is a general,... 

The analogous reaction with PdFs , however, does not occur. Evidently as with Ag(III) the tighter binding of the 4d electrons relative to 5d electrons causes Pd(V) to be inaccessible, at least by this route. The decrease in formula-unit volume with increase in atomic number for the set of LiMFs salts, given in Table 2, is much more marked in the second transition series than in the third, and clearly indicates that the effective nuclear charge builds up more, with atomic number, across the second than across the third transition series. 

The transition elements have between 0 and 10 d electrons. The three transition series of the Periodic Table, in which the M, 4d, and 5d electronic orbitals are being successively filled, occupy rows 4, 5, and 6 of the Periodic Table. The first transition series mns from Sc (21) through Zn (30) the second from Y (39) through Cd (48) the third from La (57) to Hg (80). The orbital energies and configuration d electrons of the first and subsequent transition series result in a wide variety of oxidation states (except for the Group Illb metals, which have only the III oxidation state). 

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