Beryllium Covalent Compounds

J W 7 Chem Soc Faraday Trans I 1982, 78, 289, and references quoted therein 

Lindman, B, Forsen, S NMR and the Periodic Table, Harris, R K, and Mann, B E, Eds, Academic New York, 1978, Chap 6, The Alkali Metals  

A I Proc Symp Spectrosc Electrochem Charact Solute Species in non Aqueous Solvents 1976, 2, 47, 271 

Farmer, R M, Popov, A I Inorg Nucl Chem Lett 1981, 17, 51 

---

Beryllium compounds are very toxic and must be handled with great caution. Their properties are dominated by the highly polarizing character of the Be2+ ion and its small size. The strong polarizing power results in moderately covalent compounds, and its small size limits to four the number of groups that can attach to the ion. These two features together are responsible for the prominence of the... 

Some atoms are able to form compounds even though the resulting structure doesn t provide eight valence electrons. For example beryllium and boron do not complete their octet in their covalent compounds because these atoms have less than four valence electrons. For example, in BeF2 (F - Be - F) beryllium shares its two valance electrons but it doesn t complete its octet, it is only surrounded by four electrons. In BF3, the boron atom shares its three valence electrons but does not complete its octet as it has just three electron pairs (six electrons) surrounding it. 

All group 2 elements are metals, but an abrupt change in properties between Be and Mg occurs as Be shows anomalous behavior in forming mainly covalent compounds. Beryllium most frequently displays a coordination number of four, usually tetrahedral, in which the radius of Be2+ is 27 pm. The chemical behavior of magnesium is intermediate between that of Be and the heavier elements, and it also has some tendency for covalent bond formation. 

I herc is also an interesting series of basic beryllium complexes with the carboxylic acids which are covalent compounds of remarkable stability. Basic beryllium acetate, Be40(00C.CH3)g melts at 283°, boils at 330° without decomposition and dissolves in CHCI3 as the monomer it lacks ionic properties. Its structure has a central O atom surrounded tetrahedrally by tour... 

Although magnesium and the alkaline-earth metals situated below it in the periodic table form ionic chlorides, beryllium chloride (BeCl2) is a covalent compound. 

A. Most covalent compounds of beryllium. Be. Because Be contains only two valence shell electrons, it usually forms only two covalent bonds when it bonds to two other atoms. We therefore vise four electrons as the number needed by Be in step 2, Section 7-5. In steps 3 and 4 we use only two pairs of electrons for Be. 

Write the Lewis formula for gaseous beryllium chloride, BeCl2, a covalent compound. 

Most of the halides can be classified into two categories. The flnorides and chlorides of many metallic elements, especially those belonging to the alkali metal and alkaline earth metal (except beryllium) families, are ionic componnds. Most of the halides of nonmetals snch as snlfnr and phosphorus are covalent compounds. As Fignre 4.10 shows, the oxidation nnmbers of the halogens can vary from -1 to +7. The only exception is flnorine. Because it is the most electronegative element, flnorine can have only two oxidation nnmbers, 0 (as in F2) and -1, in its compounds. 

Beryllium chloride is a substance of low melting point, is non-conducting when in the molten state, and is soluble in many organic solvents. All these characteristics point to a covalent compound, but it is difficult to see how this is to result from a beryllium atom with a fully filled outer s orbital. X-ray studies have established that the molecule contains two linear Be—Cl bonds, of equal strength. The problem is solved with the introduction of a concept of hybrid orbitals. 

The octet rule is remarkable in its ability to realistically model bonding and structure in covalent compounds. But, like any model, it does not adequately describe all systems. Beryllium, boron, and aluminum, in particular, tend to form compounds in which they are surrounded by fewer than eight electrons. This situation is termed an incomplete octet. Other molecules, such as nitric oxide ... 

LiH (lithium hydride) ionic compound BeH2 (beryllium hydride) covalent compound B2H6 (diborane, you aren t expected to know that name) molecular compound CH4 (methane, do you know that one ) molecular compound NH3 (ammonia, you should know that one) molecular compound H2O (water, if you didn t know that one, you should be ashamed) molecular compound HF (hydrogen fluoride) molecular compound. LiH and BeH2 are solids, B2H6, CH4, NH3, and HF are gases, and H2O is a liquid. 

The result is that the first element of the main groups (lithium, beryllium as well as boron, carbon, nitrogen, oxygen, and fluorine described in Chapter 12) are different from the rest of their group members. Usually, the first element is much smaller than the rest of the elements in that group, and it tends to form covalent compounds and complexes. This tendency is called Fajans rule after Kazimierz Fajans, a physical chemist who in 1923 postulated it. 

Structure. Older concepts picturing the basic beryllium compounds as derivatives of condensed acids or as structural analogs of the true basic salts of the other elements have been discarded in favor of unitary structures comparable with those ascribed to other strictly covalent compounds. 

The most electFonegative elements are the nonmetals (( oups 5A-7A and the least electronegative elements are the alkali and alloiline earth metals (( oups 1A-2A) and aluminum (t oip 3/. Beryllium, the first member of < oip 2A, fornis mostly covalent compounds. 

Beryllium, unlike the other Group 2A elements, forms mostly covalent compounds of which BeH2 is an example. 

---