18 valence electron related compounds

The location of electrons linking more than three atoms cannot be illustrated as easily. The simple, descriptive models must give way to the theoretical treatment by molecular orbital theory. With its aid, however, certain electron counting rules have been deduced for cluster compounds that set up relations between the structure and the number of valence electrons. A bridge between molecular-orbital theory and vividness is offered by the electron-localization function (cf p. 89). 

It is shown that the stabilities of solids can be related to Parr s physical hardness parameter for solids, and that this is proportional to Pearson s chemical hardness parameter for molecules. For sp-bonded metals, the bulk moduli correlate with the chemical hardness density (CffD), and for covalently bonded crystals, the octahedral shear moduli correlate with CHD. By analogy with molecules, the chemical hardness is related to the gap in the spectrum of bonding energies. This is verified for the Group IV elements and the isoelec-tronic III-V compounds. Since polarization requires excitation of the valence electrons, polarizability is related to band-gaps, and thence to chemical hardness and elastic moduli. Another measure of stability is indentation hardness, and it is shown that this correlates linearly with reciprocal polarizability. Finally, it is shown that theoretical values of critical transformation pressures correlate linearly with indentation hardness numbers, so the latter are a good measure of phase stability. 

To an increasing weight of the chemical bond factor (ionic and/or covalent bonding) will correspond, as an extreme case, the formation of valence compounds. According to Parthe (1980), a compound CmAn can be called a normal valence compound if the number of valence electrons of cations (ec) and anions (eA) correspond to the relation... 

It was pointed out by Hume-Rothcry 4 in 1926 that certain interns etallic compounds with close]y related structures but apparently unrelated stoichiometric composition can be considered to have the same ratio of number of valence electron to number of. atoms,. For example, the j8 phases of the systems Cu—Zti, Cu—-Alv and Ou -Sn are analogous in structure, all being based on the -4.5 arrangement their compositions correspond closely to the formulas CuZn, CusAl, and CtttSn. Considering copper to be univalent, zinc bivalent, aluminum trivalent, and tin quadrivalent, we see that the ratio of valence electrons to atoms has the value f for each of these compounds ... 

IN the past twenty years the electronic structures of many organic molecules, particularly benzene and related compounds, have been discussed in toms of the molecular orbital and valence bond methods.1 During the same period the structures of inorganic ions have been inferred from the bond distances f a bond distance shorter than the sum of the conventional radii has been attributed to the resonance of double bonded structures with the single bonded or Lewis structure. 

M4E4 (38-41) compounds where the total number of valence electrons, Z, is directly related to the degree of metal-metal bonding in the M3 resp. M4 frameworks. Table I summarizes their results. 

In a metallic compound the valence electrons form a collective belonging to the whole crystal. In a non-metallic compound, on the other hand, it is a useful approximation to consider the bonding valence electrons as localized between cation and anion in covalent crystals or on the anion in purely ionic crystals. Moreover, the electron balance is not influenced by the degree of covalency of the bonds, so that formally we can treat all cation-anion bonds as if they were ionic. For the valence electrons of a normal ionic compound Mm X the following relation holds ... 

A general review by Cundy et al. (76) on silicon-transition metal complexes has appeared, and again, we select only a few molecules to highlight different properties and chemistry within an isoelectronic group. In the four examples (Cp)Mn(CO)2(/u-H)Si(F)(Ph)2 (30), (Cp)Fe(CO)2Si-(F)(Ph)2 (31), (Cp)Mn(CO)2(M-H)Si(Cl)3 (32), and (Cp)Fe(CO)2Si(Cl)3 (33), compounds 30-31 and 32-33 are isoelectronic pairs based on the total number of valence electrons. Compounds 30 and 32 also form a related pair with respect to the three-center, two-electron Si—H—Mn interactions. 

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