Elements post-transition metals

Attempts to classify carbides according to structure or bond type meet the same difficulties as were encountered with hydrides (p. 64) and borides (p. 145) and for the same reasons. The general trends in properties of the three groups of compounds are, however, broadly similar, being most polar (ionic) for the electropositive metals, most covalent (molecular) for the electronegative non-metals and somewhat complex (interstitial) for the elements in the centre of the d block. There are also several elements with poorly characterized, unstable, or non-existent carbides, namely the later transition elements (Groups 11 and 12), the platinum metals, and the post transition-metal elements in Group 13. 

But first the synthesis had to come John was interested in reduced metal halides, particularly for the post-transition metals cadmium, galHum, and bismuth (his Ph.D. dissertation was on anhydrous aluminum halides and mixed halide intermediates, a good start for what was to come ). However, he was not yet actively interested in rare-earth metals and their remarkable solubility in their halides. But these elements lured him one floor below where Adrian Daane headed the metallurgy section of Spedding s empire. He knew how to produce rare-earth metals with high purity and in sufficient quantity and also how to handle tantalum containers. What if one gave it a tr/ and reduced some rare-earth metal halides (John insists that this term is used correctly) from their respective metals at high temperatures under appropriate conditions. 

The main group duster chemistry discussed in this book can be considered to originate from two important, but apparently unrelated developments in inorganic chemistry in the 1930s. The first was the identification of the neutral boron hydrides by Stock [1]. The second was the observation by Zintl and co-workers [2-5] of anionic clusters formed from potentiometric titrations of post-transition metals (i.e., heavy main group elements) with sodium in liquid ammonia. 

The polyhedral boranes and carboranes discussed above may be regarded as boron clusters in which the single external orbital of each vertex atom helps to bind an external hydrogen or other monovalent atom or group. Post-transition main group elements are known to form clusters without external ligands bound to the vertex atoms. Such species are called bare metal clusters for convenience. Anionic bare metal clusters were first observed by Zintl and co-workers in the 1930s [2-5], The first evidence for anionic clusters of post-transition metals such as tin, lead, antimony, and bismuth was obtained by potentiometric titrations with alkali metals in liquid ammonia. Consequently, such anionic post-transition metal clusters are often called Zintl phases. 

Rules for counting the number of skeletal electrons provided by each vertex atom need to be established in order to determine the number of skeletal electrons in polygonal and polyhedral clusters of the post-transition elements. The rules discussed above for polyhedral boranes can be adapted to bare post-transition metal vertices as follows ... 

GaUium is truly an exotic element in that it has so many unusual characteristics. It can form fflonovalent and divalent as weU as trivalent compounds. It is considered a post-transitional metal that is more hke aluminum than the other elements in group 13. It has few similar characteristics to the two elements just below it in group 13 (In and Ti). 

From the trend in acidities of the hydrogen halides in water, it follows that fluoride is the most basic or nucleophilic of the halides and iodide the least basic if the hydrogen ion is considered the reference acid. It should be recalled (p. 169) that this order of halide basicities is the same as that toward small, multicharged ions with rare-gas structures (for example, Be2+, A 3, and Si4+). A different, and sometimes reversed, order of basicities or nucleophilicities is observed toward certain ions of the post-transition metals (for example, Cu+, Hg +). For a number of ions (for example, Be+2, B+3 and Ta+6), fluoride complexes may exist in aqueous solution, whereas the other halo-complexes do not. Only a few of the elements having positive valence states form no halo-complexes the most important of these are carbon, the rare earths, the alkali metals, and the heavier alkaline-earth metals. 

If one compares the E(III)/E(V) (E = P, As) bond energies and the ionization energies of oxidation state -f-III P and As compounds, both the diminished stability of the As(V)-element bond [vs P(V)-element] and the unusual high Ip of As(III) compounds [vs P(III) compounds] can best be explained by the post transition metal effect (see above). Two examples from inorganic chemistry (just due to the better accessibility of data) are given in Table 2 . ... 

Bi complexes are summarized in Table 5. The lack of As(V) complexes of the type R3 AsL2 may be explained by the resistance of As to realize high coordination numbers and oxidation states, which are usually discussed on the basis of the post-transition metal effect. Element(V) species of the type R3EL possessing only one carboxylato chelate ligand which coordinates symmetrically show trigonal bipyramidal structures. 

In this chapter we will discuss some representative metals and some /-transition metals. The representative elements are those in the A groups of the periodic table. They have valence electrons in their outermost s and p atomic orhitals. Metallic character increases from top to bottom within groups and from right to left within periods. All the elements in Groups lA (except H) and TTA are metals. The heavier members of Groups niA, rVA, and VA are called post-transition metals. 

Aluminum is the most reactive of the post-transition metals. It is the most abundant metal in the earth s crust (7.5%) and the third most abundant element. Aluminum is inexpensive compared with most other metals. It is soft and can be readily extruded into wires or rolled, pressed, or cast into shapes. 

Scope Non-transition metals include groups 1 and 2 of the. s-block elements, group 12, and /(-block elements in lower periods. Aluminum and the elements of groups 1 and 2 are classed as pretransition metals, the remaining ones as post-transition metals. 

Non-cationic Many of the elements can form anionic species. Compounds with covalent bonding are also chemistry known these include organometallic compounds and (especially with post-transition metals) compounds containing metal-metal bonds. 

Metallic elements from periods 4-6 in groups following the transition series are post-transition metals. They are less electropositive than the pre-transition metals... 

Although formally part of the d block, the elements of group 12 do not show typical transition metal characteristics, as the d orbitals are too tightly bound to be involved in chemical bonding. These elements are better regarded as post-transition metals, and are dealt with in Section G (Topic G4). 

Norman, Nicolas C. Periodicity and the p-Block Elements. Oxford Oxford University Press, 1994. This book describes group properties of post-transition metals, metalloids, and nonmetals. 

All the cited literature references to the above compounds have described solid-state syntheses at temperatures of 700-1200°. Such synthesis conditions will always lead to pyrochlore structure compounds in which all of the octa-hedrally coordinated sites are occupied by the noble metal cation, thus requiring the post-transition metal to noble metal molar ratio always to be 1.0. This paper focuses on solution medium syntheses at quite low temperatures (<75°), thereby stabilizing a new class of pyrochlore compounds in which a variable fraction of the octahedrally coordinated sites are occupied by post-transition element ca-tions.5,6 The specific example here involves the Pb2[Ru2 Pb4+]06 s series. The synthesis conditions may be simply adapted, however, to accommodate preparation of a wider range of pyrochlores which can be described by the formula A2[B2 xAx]07.3> where A is typically Pb or Bi, B is typically Ru or Ir and 0 < 1, and 0 < 1. 

Pyrochlore A2B2O7 A = rare earth or element with a lone electron pair B = transition or post transition metal 3m g- (1000"C) 2m g- (1200 C) Active phase/support... 

Pyrochlores - The pyrochlores are a group of materials with the general formula A2B2O7. They have been mentioned as a material for catalytic combustion. The structure allows vacancy at the A site and the O sites to some extend. The A position can be a rare earth metal or an element with lone pair of electrons and the B position can be a transition metal or a post-transition metal. This make the structure rather flexible as the oxidation state of the transition metal B can be varied as well as the nature of the A and B metal ions. Subramanian and Castro et al. have prepared several pyrochlores. When studying the thermal stability of different complex oxides, Zwinkels et al. have shown that La2Zr207 pyrochlores have a surface area lower than 5 m g , already after calcination at 1000 °C. Hence, such materials are probably not suitable for high temperature applications unless the preparation method is improved. However, pyrochlore compounds have been patented for catalytic combustion applications, see Section 5.5. 

All of the elements in the first 12 groups of the periodic table are referred to as metals. The first two groups of elements on the left-hand side of the table are the alkali metals and the alkaline earth metals. All of the alkali metals are extremely similar to each other in their chemical and physical properties, as, in turn, are all of the alkaline earths to each other. The 10 groups of elements in the middle of the periodic table are transition metals. The similarities in these groups are not as strong as those in the first two groups, but still satisfy the general trend of similar chemical and physical properties. The transition metals in the last row are not found in nature but have been synthesized artificially. The metals that follow the transition metals are called post-transition metals. 

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