Electronic Considerations

Li3(BN2) have already demonstrated the decomposition of (BN2) ions into boron nitride. The remaining nitride can lead to the formation of a binary metal nitride or reduce the transition metal ion under the formation of N2. Both mechanisms have been obtained experimentally, depending on the stability of the metal nitride. For instance niobium pentachloride forms NbN, titanium trichloride forms TiN, and nickel dichloride forms Ni, plus BN and nitrogen, respectively, in reactions with Li3(BN)2 (at 300-600°C) [24]. 

We note that the valence orbitals of metal atoms order in energy as AE Ln M. The d-levels of transition elements (M) range the lowest, and are therefore most sensitive for reduction, or to form a stable binary metal nitride. This may also explain the virtual absence of d-element compounds with 16 (valence) electron species, such as [N=N=N] , [N=C=N] , [N=B=N] T [C=C=CfT or [C=B=C] T at least through high-temperature syntheses. 

The same reasoning may hold for the virtual absence of transition metal dicarbides with (C2) anions. On the other hand, stable multinary compounds in A-M-(BNx), AE-M-(BNx), or Ln-M-(BNx) systems can exist with an electron-rich transition metal (M), similar to the known ternary dicarbides AM(C2) with A=alkali and M = Pd, Pt [25]. In these cases the valence electrons provided from A or AE metals fill the bonding (BNx) or (C2) levels, and the transition metal retains an approximate d ° configuration. 

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Steric Selectivity. In addition to the normal regularities that can be rationalized by electronic considerations, steric factors are important in coordination chemistry. To illustrate, 8-hydroxyquinoline, or 8-quinolinol (Hq) [148-24-3J, at 100°C precipitates both Mg " and AE" from aqueous solution as hydrated Mg(q)2 (formulated as Mg(q)2(H20)2 [56531 -18-1]) and as Al(q)3 [2085-33-8] respectively. 2-Meth54-8-hydroxyquinohne [826-81-3] (6),... 

Clusters with Interstitial Atoms Available Internal Volume Can Override Electronic Considerations... 

A general conclusion from these DPT studies on M Eio clusters of various types is that the Wade-Mingos rules [13-16] are most applicable when the Eio unit has as large a cavity as possible, i.e., when E is a relatively heavy element such as bismuth or lead. Plowever, if the Eio cavity is too small, particularly when E is arsenic or germanium, then maximizing the internal volume of the polyhedral cavity can override any electronic considerations from the Wade-Mingos rules. 

While the Sn2 reaction represents an extreme case, it is clear that the solvent is capable of selectively stabilizing (or destabilizing) one product over another in a thermodynamically-controlled reaction, or one transition state over another in a kinetically-controlled reaction. Differentiation might be effected by steric and/or electronic considerations. 

Jurs-PNSA-2 (characterization of positively charged surface area as a function of molecular shape and electronic considerations)... 

As early as 1928, Roginskil and Schul tz (4) stressed the importance of electronic considerations, and Rideal and Wansbrough-Jones (5) related the work function of metals to the activation energy for their oxidation. Brewer, 1928 (6), Schmidt, 1933 (7), and Nyrop, 1935 (8) proposed that the surface must be capable of effecting ionization of the adsorbed species in some catalytic processes. Lennard-Jones (9) in his... 

C6H5—C—Co— and that steric and electronic considerations indicate... 

Azomethine ylides are not typically isolable but must be used in situ. They undergo cycloaddition reactions that produce highly functionalized pyrrolidines, dihydropyiroles and pyrroles. The success of these reactions often depends on a judicious choice of dipole and dipolarophile. Azomethine ylides are reluctant to cycloadd to nonactivated alkenes, in large part owing to electronic considerations. The LUMO of most azomethine ylides is high in energy and there is a large gap with the HOMO of a nonactivated alkene. 

However, 2,2-dimethylcyclopropanone as well as the tetramethyl derivative have reactivities lower than expected from electronic considerations, and the authors suggest that steric effects associated with inward rotation of methyl groups in the transition state must also be considered. More recently, Hoffmann 120> has also suggested certain conformational... 

In accordance with all of these structural and electronic considerations, it is shown in the following sections that it would be wrong to simplify our understanding of the selectivity of sucrose transformations to merely the reactivity order of primary versus secondary hydroxyl groups. 

My own criticism on the theories proposed for superplasticity can be summarized in one word, electrons , or more accurately the lack of electronic consideration. This is similar to the theoretical consideration brought forward in the study of LME (liquid metal embrittlement) described in the previous chapter - no electronic consideration. As shown in Table 2, all superplastic alloys of binary system are found either at eutectic or eutectoid compositions. This is illustrated in Fig. 14 in which a few binary phase diagram involving superplastic alloys are shown. However, the people who made efforts in the formulation of theories did not consider this well-known fact important enough to incorporate into their theory formulation [24], In fact, this observation is so consistent one should ask the question of the special attributes associated with eutectic or eutectoid composition. Or the fact that the intermetallic compounds with superelastic property are all of the peritectic type. It must be emphasized that to this date there is no report of finding superplasticity in congruently-melting compounds. 

Instability of the compound formation brought about through electronic consideration - whether it be eutectic, eutectoid or peritectic compound is the cause for the inherent instability. Subsequently, this inherent instability is the reason for grain size being small,... 

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