Second transition series

The total lanthanide contraction is of a similar magnitude to the expansion found in passing from the first to the second transition series, and which might therefore have been expected to occur also in passing from second to third. The interpolation of the lanthanides in fact almost exactly cancels this anticipated increase with the result, noted in preceding chapters, that in each group of transition elements the second and third members have very similar sizes and properties. 

The platinum-group metals comprise ruthenium (Ru), rhodium (Rh) and palladium (Pd) from the second transition series and osmium (Os), iridium(Ir), and platinum (Pt) from thethird transition series. Little or no C VD investigation of palladium and osmium have been reported and these metalsarenotincludedhere. The properties of the other platinum-group metals are summarized in Table 6.9. 

Although STs have been observed for the first (3d) transition series and also partially for the second transition series, by far the greatest number has been reported for iron(II). The earlier developments in iron(II) ST systems were reviewed by Konig [1], Goodwin [2] and Giitlich [3]. Since then, numerous reviews have appeared on specific aspects of the phenomenon [4,5 and references therein]. 

Taking into consideration the deuterium isotope effect (k Yfy/kxfD) = 2.3), they concluded that fca< k a< /cb and that the rate-determining step was the first substitution. Ligand substitution was thought to proceed by the Ia mechanism, on the basis of the negative AS and the independence of the rate on the concentration of acetylacetone. This feature is compatible with the results with tris(acetylacetonato)metal(III) previously obtained [21]. Furthermore, in the second transition series the kt value decreases in the order... 

The substitution-inert character of the metal(III) ion in the second transition series has already been discussed in 2.3. However, interesting behavior has been reported by Kasahara et al. [23], who found that a p-diketone coordinated to the central Ru(III) could easily be replaced by an acetonitrile with the aid of a strong acid. When the reaction was conducted in acetonitrile, its stoichiometry was confirmed by means of spectrophotometrie titration as follows ... 

Kubicka further reported that the specific activities of the metals for benzene hydrogenation fell in the sequence Ru > Pt > Tc Pd > Re. We note that, for the elements of the second transition series, the maximum activity was observed for the element of group VIIIi (group VIII2 was not studied). This should be compared with the results in Fig. 4 which show that the activities for the exchange reaction pass through a maximum at... 

An important advantage of ECP basis sets is their ability to incorporate approximately the physical effects of relativistic core contraction and associated changes in screening on valence orbitals, by suitable adjustments of the radius of the effective core potential. Thus, the ECP valence atomic orbitals can approximately mimic those of a fully relativistic (spinor) atomic calculation, rather than the non-relativistic all-electron orbitals they are nominally serving to replace. The partial inclusion of relativistic effects is an important physical correction for heavier atoms, particularly of the second transition series and beyond. Thus, an ECP-like treatment of heavy atoms is necessary in the non-relativistic framework of standard electronic-structure packages, even if the reduction in number of... 

Osmium is found in group 8 (VIII) of the periodic table and has some of the same chemical, physical, and historical characteristics as several other elements. This group of similar elements is classed as the platinum group, which includes Ru, Rh, and Pd of the second transition series (period 5) and Os, Ir, and Pt of the third series of transition metals (period 6). 

Finally, a word of caution should be offered against making a straightforward comparison of the M-Si bonds in complexes of different metals, since this parameter is strongly affected by the size of the metal. The latter tends to decrease from left to right in a given row of the Periodic Table due to the d-contraction, but increases down the Group (particularly between the first and second transition series). 

For elements of the second transition series and beyond, a choice of lines is often available, with, for example, K and L lines in the second transition series and M and L lines in the third. The high-energy lines were normally used for the analysis because of their lower absorption cross sections (33), and LocKa or M La ratios were used to monitor crystal thickness (see below). The most straightforward analyses are those involving elements with approximately the same atomic numbers, when the K K or L L ratios may be used and the absorption cross sections are normally of similar magnitude (although difficulties can arise if an emission from one element leads to fluorescence from another). For elements that are far removed from each other in the periodic table, mixed ratios... 

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 lanthanide contraction is due to the increase in effective nuclear charge with increasing atomic number as the 4/ subshell is filled. By the end of the lanthanides, the size decrease due to a larger Zeff almost exactly compensates for the expected size increase due to an added quantum shell of electrons. Consequently, atoms of the third transition series have radii very similar to those of the second transition series. 

This sequence is particularly well characterized for fluoride complexes of high-spin cations of the first-series transition elements (Allen and Warren, 1971). Moreover, between successive transition metal series, values of Ac increase by about thirty to fifty per cent. For example, in hydrated cations of the first and second transition series, Ac for [CftHjO) 3 and [Mo(H20)6]3+ are 17,400 cm-1 and 26,110 cm-1, respectively. 

For metal ions in the second transition series, the splitting is usually about 30% larger than for the first-row metal ion having the same number of electrons in the d orbitals and the same charge. A similar difference exists between the third and second transition series. 

The Ru2 Molecule. In addition to the very thoroughly studied Mo2 molecule, there is only one other second transition series molecule that has been shown from theoretical studies to have strong d-d bonding, although it has yet been observed experimentally. An SCF-HF-CI calculation on the Ru2 molecule137 predicts a 7AU ground state based on the... 

Radii. The filling of the 4f orbitals (as well as relativistic effects) through the lanthanide elements cause a steady contraction, called the lanthanide contraction (Section 19-1), in atomic and ionic sizes. Thus the expected size increases of elements of the third transition series relative to those of the second transition series, due to an increased number of electrons and the higher principal quantum numbers of the outer ones, are almost exactly offset, and there is in general little difference in atomic and ionic sizes between the two heavy atoms of a group, whereas the corresponding atoms... 

Molybdenum is very important in the biochemistry of animals, plants, and microorganisms. It is the only element in the second transition series known to have natural biological functions. It occurs in more than 30 enzymes, in some of which it may be replaced by tungsten or vanadium. Tungsten is the only element in the third transition series known to have natural biological functions. Not only does it sometimes occur in enzymes that usually contain molybdenum, but there are some enzymes that are known only with tungsten. 

The analytical chemistry of the transition elements see Transition Metals), that is, those with partly filled shells of d (see (f Configuration) or f electrons see f-Block Metals), should include that of the first transition period (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, and Cu) and that of the second transition series (Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, and Ag). The third transition series embraces Hf, Ta, W, Re, Os, Ir, Pt, and An, and although it formally begins with lanthanum, for historical reasons this element is usually included with the lanthanoids (rare-earth elements) see Scandium, Yttrium the Lanthanides Inorganic Coordination Chemistry Rare Earth Elements). The actinoid elements see Actinides Inorganic Coordination Chemistry) are all radioactive see Radioactive Decay) and those with atomic number see Atomic Number) greater than uranium (Z = 92) are artificial the analytical chemistry of these elements is too specialized to consider here. 

It will also be noticed that all the above catalysts contain second transition series metals. Generally, the slower reactions of the third transition series elements are not normally conducive to catalytic efficiency, although some very active iridium catalysts are now known. First transition series metals seldom form stable, lower oxidation state tertiary phosphine complexes. 

Figure 9 Bond distance versus force constant data for intra-row bonding in the row of the periodic table containing the second transition series (n = 5, Rb to Xe) (a) data fit to empirical force constant-bond distance functions (see text) (b) same data fit to the exponential decay function that we have proposed. Identity of the points is given in Ref. 42...

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. 

As is seen, the most noticeable differences between the all-electron and pseudopotential eigenvalues are observed for the molecular orbitals containing the s-type AOs of Pd by symmetry. It appears to be related to the non-core character of the 4s states in the second transition series atoms therefore, one could take into account for the subvalence shells when constructing pseudopotential [17] or to use some extended basis in such cases. 

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