Pnictides structure

The pyrites structure is exhibited by several pnictides MAs2 and MSb2 (M = Pd, Pt) and PtP2 (Figure 3.10). 

A closer examination of Table 6-3 reveals a rather startling conclusion. It can be seen that most of these compounds are subject to structure-vacancy defects. Certainly all of the pnictides fall into this class. We are even given the defect concentration. Thus in GaP, we have ... 

B. Electronic Features and Structural Principles of Molecular Main Group Metal Pnictides... 

Fig. 1. Structural motifs of metal pnictides two- and three-dimensional (ladderlike aggregation).

A cluster framework such as that seen in U has also been observed for its heavier homologues (14). Apparently, the latter topology is the result of a close packing of anionic pnictide moieties with alkali metal ions that maximizes ionic interactions. The topology of the rhombodo-decahedral Li6PeSi2 framework in U has been simply described as a Pe octahedron representing the anionic partial structure, of which the triangular faces are capped by two p.3-RSi moieties and six /t3-Li ions as counterion partial structure (Fig. 3). 

It has been expected that reactivity of metalated pnictides is strongly dependent on the degree of association. One consequence is that the basicity of amides has been correlated with the grade of association by means of extensive 7Li and 16N NMR investigations (16, 17). In contrast, knowledge of structure-reactivity relationships for... 

The electrostatically favored cation (Li) and anion (RE) arrangement implies the presence of two different E-, Si- and Li sorts, which has been established by solution and solid-state NMR spectroscopy. The electronic structures of the mixed-valent pnictides 10 and 11 have been simply described as electron-deficient clusters with delocalized framework electrons. Formally the latter consist of two low-valent anediyl moieties RE and eight andiides (RE)2- (E = P, As). The relatively large E-E distances of >4 A exclude the occurrence of localized E-E bonds. However, delocalization of the cluster valence electrons is achieved without Li-Li bonds via Li-mediated multiple bonding. Evidence for this has been seen in the NMR spectra (31P, 7Li, 29Si), which are in accordance with the electron delocalization model (see later discussion). 

Notes on the crystal chemistry of selected alloys and compounds of the 15th group elements. Among the pnictides, several series of numerous isostructural compounds are found. The 1 1 compounds with Sc, Y, lanthanides and actinides, having the NaCl-type structure, generally correspond to very stable phases, strongly... 

Fig. 10 Molecular structures of functionalized pnictide cages and pnictide complexes, a) P4SiR2,...

The impact on actinide solid state physics of Zachariasen s ideas has been great. It has triggered a very fruitful effort in structure determination. In the meantime, it has underlined the importance, in the theory of actinides, of knowing exactly atomic volumes as an indicator of the complexities of the metallic bond. The same picture of the variation of atomic radii appears for compounds having partial metallic character. In Fig. 5, for instance, the variation vs. Z of the lattice parameter ao of actinide pnictides is compared with the atomic radii of metals. Actinide pnictides have an NaCl type structure. Their... 

The general shape of these curves at least for pnictides up to AnAs indicates clearly that the metallie bond is dominated by the electronic structure of the actinide atom. In the further discussion of this chapter (and in Chaps. C-F) it will be made clear that the shape is explained, contrarily to Zachariasen s hypothesis, by means of the f participation in the bond. 

In actinide binary compounds an equation of state can also be developed on the same lines. The difference in electronegativity of the actinide and the non-actinide element plays an important role, determining the degree of mixing between the actinide orbitals (5 f and 6 d) and the orbitals of the ligand. A mixture of metallic, ionic and covalent bond is then encountered. In the chapter, two classes of actinide compounds are reviewed NaCl-structure pnictides or chalcogenides, and oxides. 

Relations between the structure and properties have been investigated in a variety of solids such as metal oxides, chalcogenides, pnictides and halides. In addition to studying model systems for testing theoretical predictions, solid state chemists have been preparing new classes of solids as well as novel members of known types of solids. In this section, we have chosen three classes of solids, viz. metal oxides, metal sulphides and metal fluorides, to discuss structure-property relations we shall concentrate especially on their electrical and magnetic properties. 

The 8-N rule states that the number of bonds (or local coordination, x) equals 8 minus the number of the periodic group. This rule is illustrated in Fig. 1.2 where we see that for N — 7 the halogens take dimeric structure types with x = 1, for N = 6 the chalcogenides selenium and tellurium take helical chain structures with x = 2, for N = 5 the pnictides arsenic, antimony, and bismuth take a puckered layer structure with x = 3, and for N = 4 the semiconductors... 

Pnictide-transition metal bonds are essentially covalent coordinate, in which the pnictide provides the electrons. However, this simple picture does not account for all the structural data now available. The consensus view is that three factors are involved in the ligand contribution to the M—E bond (a) a bonding (Section 14.4.2), (b) n bonding (Section 14.4.3), and (c) steric factors (Section 14.3). The effect of the metal will not be discussed here, except insofar as individual complexes are used as examples. 

The interplay between a and n bonding between pnictide ligand and metal has subtle structural consequences. In the trigonal bipyramidal series of complexes M(CO)4(ER3) (M = Fe, Ru, Os E = P, As, Sb R = alkyl, aryl), in which the ER3 ligand may occupy an axial or an equatorial site,... 

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