Lattice beryllium compounds

The bis(propynyl)beryllium compound [Be(C=CMe)2NMe3]2 is unusual in crystallizing with two types of dimeric molecule in the lattice, one of which has a diamond-shaped (Be-C)2 ring very similar to that of 31. The other, structure 32, has a nearly rectangular (Be-C)2 ring, explicable in terms of donation of charge from the alkynyl triple bond into the available metal orbital. 

Fig. lr.1-132 The Brillouin zone of the wurtzite lattice Table ir.1-101 Band structures of beryllium compounds... 

The small lithium Li" and beryllium Be ions have high charge-radius ratios and consequently exert particularly strong attractions on other ions and on polar molecules. These attractions result in both high lattice and hydration energies and it is these high energies which account for many of the abnormal properties of the ionic compounds of lithium and beryllium. 

Staeking faults and sometimes proper polytypism are found in many inorganic compounds - to pick out just a few, zinc sulphide, zinc oxide, beryllium oxide. Interest in these faults arises from the present-day focus on electron theory of phase stability, and on eomputer simulation of lattice faults of all kinds investigators are attempting to relate staeking-fault concentration on various measurable character-isties of the compounds in question, such as ionicity , and thereby to cast light on the eleetronic strueture and phase stability of the two rival structures that give rise to the faults. 

The valence electron configuration of the atoms of the Group 2 elements is ns1. The second ionization energy is low enough to be recovered from the lattice enthalpy (Fig. 14.18). Flence, the Group 2 elements occur with an oxidation number of +2, as the cation M2+, in all their compounds. Apart from a tendency toward nonmetallic character in beryllium, the elements have all the chemical characteristics of metals, such as forming basic oxides and hydroxides. 

Although the differences between the first and second ionization enthalpies, especially for beryllium, might suggest the possibility of a stable +1 state, there is no evidence to support this. Calculations using Born-Haber cycles show that owing to the much greater lattice energies of MX2 compounds, MX compounds would be unstable and disproportionate ... 

A beryllosilicate with the analcime structure can be prepared by direct synthesis. The lattice constant, refractive index, and DTA data are presented for the compound. These results indicate that beryllium may occupy the tetrahedral sites, replacing aluminum in the zeolite structure. 

Group 2 metal peroxides, MO2, are known for M = Mg, Ca, Sr and Ba. Attempts to prepare Be02 have so far failed, and there is no experimental evidence for any beryllium peroxide compound. As for the group 1 metal peroxides, the stability with respect to the decomposition reaction 12.22 increases with the size of the ion. This trend arises from the difference between the lattice energies of MO and MO2 (for a given M) which becomes smaller as increases. Aiattice (MO, s) is always more negative than Aiajtiee/f°(M02, s) (see worked example 12.3). 

The reaction of beryllium metal with aqueous acids yields hydrogen and ionic compounds, such as BeCl2 4 H2O. In BeCl2 4 H2O, water molecules are covalently bonded to Be ions, producing the complex cations [Be(OH2)4] that, together with the anions d , form the crystal lattice. In covalent compounds, Be atoms appear to use hybrid orbitals—sp orbitals in BeCl2(g) and sp orbitals in BeCl2(s) (Fig. 21-12). 

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