Relationship with other transition elements

Molybdenum is in Sub-Group VI A/B of the Periodic Table, and in the second series of transition elements. Transition elements are those which have an incomplete inner orbit in their atomic structure (see Table 3.1), and such an incomplete orbit is less stable than a filled orbit. The result is that the transition elements, and their compounds, show resemblances to each other and peculiarities in comparison with non-transition elements. It is therefore interesting that a number of compounds of other transition elements have been studied for solid lubricant use, and some of them have been found to be very effective, but no-one has yet shown any particular relationship between transition element structures and lubricating performance. The electron orbital assignments for these various elements are shown in Table 3.1. 

Scheelite oxides are a good example of how structure-property relationships have been used to create a modem catalyst. More detailed descriptions of these and other catalysts are given in Oxide Catalysts. The scheelite stmcture was discussed in Section 3.4.5. Scheelite based oxides are of interest as catalytic materials because of their ability to form with transition elements in high but easily reducible valence states and to form a variety of defects and defect stmctures that provide numerous sites for selective catalytic activity. 

The transition-metal clusters often display highly symmetrical metal frameworks, normally with symmetries derived from the platonic or archimedian solids or variations thereof. The post-transition element clusters, on the other hand, are not necessarily aflfected by such confinements. However, as will be explained in Sec. 1.29.4, relationships between the structure and the electron count do exist for the naked clusters, and regular, closed geometries are found for a rather large group of these species. For instance, the trigonal prismatic symmetry of the Rhf, framework in Fig. lA is also found in the naked tellurium cluster... 

The octahedral voids in the MnsSi3 type structure can be filled not only by a late transition metal. As an example Guloy and Corbett (1994) have tested this possibility for LasPb3. The voids could be filled with P, S, Cl, As, Se, Sb, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ru, and Ag atoms and also partially with boron or carbon. Filling of the voids leads to a small increase of the lattice parameters. On the other hand, lead can also act as the interstitial element on the octahedral site. The structural relationships of various M5X4 structures have recently been discussed by Guloy and Corbett (2005). 

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