Beryllium and Magnesium Compounds

Exclusive hypercoordination of all of the metal-attached carbon atoms is 

Although their hydrogen atoms were not located, their relatively short metal-metal distances and acute M-C-M angles at the hypercoordinated carbon atoms show the metal-carbon bonding to resemble that in A Mce discussed previously. This resemblance to the aluminum system is underlined by the structure of the mixed metal methyl Mg(AlMe4)2 (28), also established by an X-ray study.  

Another example of a bimetallic mixed M-Mg complex is the Ni-Mg system 29 composed of bis(ethylene)-Ni(O) and Me2Mg complexed with tetrameth-ylethylenediamine (Me2N( I I.( I I2NMC2- I MI DA). The bridging Ni-C and Mg-C bond distances differ signihcantly, and a Ni-Mg interaction associated with the NiMeMg bridge is evident from the Ni-Mg distance. 

In all of these systems, the metal-carbon distances involving hypercoordinated carbon atoms are significantly longer than those involving the four-coordinate carbon atoms of the terminal alkyl groups (monomeric BMe2 has a Be-C distance of 1.70 A as shown by an electron diffraction study of the vapor, while two-center Mg-C bonds are typically about 2.16-2.17 A in 

As is the case with alkyl bridges between aluminum atoms, these bridges between beryllium and magnesium atoms are relatively weak, and the metal orbitals are put to better use by addition of Lewis bases (L), which cleave the polymer chains, forming MR2L2 monomeric molecules, in which carbon atoms are no longer hypercoordinated [Eq. (2.8)]. In weakly basic solvents dimers (30) that retain alkyl bridges (and so hypercoordinate carbon atoms) may be formed. 

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Beryllium and magnesium belong to the 2nd group of the Periodic Table together with calcium, strontium, barium and radium. Characteristic differences, however, may be noticed between the chemistry of Be and Mg and that of the alkaline earth s proper. Be has a unique chemical behaviour with a predominantly covalent character. The heavier elements (Ca, Sr, Ba, Ra) have a predominant ionic behaviour in their compounds. Mg has a chemistry in a way intermediate but closer to that of Be. Analogies between the Mg and Zn chemistries may also be underlined. 

Although beryllium and magnesium salts do not form stable mctal-ammines yet they unite with ammonia, forming additive compounds of the hydrate type which are sometimes referred to as ammoniates or ammonio-compounds. These appear to be of the same type as the metal-anunines, and the difference seems to be merely one of stability. The ammonio-compounds are formed by the addition of ammonia gas to dry or fused salt, and most of them decompose with liberation of ammonia when dissolved in water. 

Most of the Group IA and IIA metals react with hydrogen to form metal hydrides. For all of the metals in these two groups except Be and Mg, the hydrides are considered to be ionic or salt-like hydrides containing H ions (see Chapter 6). The hydrides of beryllium and magnesium have considerable covalent character. The molten ionic compounds conduct electricity, as do molten mixtures of the hydrides in alkali halides, and during electrolysis of the hydrides, hydrogen is liberated at the anode as a result of the oxidation of H ... 

Beryllium and magnesium have a formal oxidation state of +2 in their compounds see Oxidation Number) other oxidation states appear to exist only as transient reaction intermediates. Therefore, organometallic compounds of general formula R2M and RMY may exist, in addition to the nonorganometallic MY2. 

The compound [Mn(CH2SiM63)2], for example, is polymeric in the crystal, -with a structure (75) like those of dialkyls of beryllium and magnesium (MR2) (M = Be or Mg R = Me or Et), Each metal atom, tetrahedrally coordinated. 

Beryllium (Be, at. mass 9.012) forms cations Be ". In its chemical properties, beryllium resembles magnesium and aluminium. Beryllium hydroxide is precipitated at pH 6, and dissolves in alkali hydroxides. Freshly precipitated Be(OH)2 dissolves in NaaCOs solution to form a rather unstable carbonate complex. Beryllium also forms weak complexes with citrate, tartrate, and fluoride anions. Beryllium and its compounds are highly toxic. 

The curcumin method (in either the rosocyanin or rubrocurcumin version) has been applied for determining traee amounts of boron in biologieal materials [10], soils and plants [17], waters [51], silicon [52], chlorosilanes [20], uranium [1,53], zirconium and its alloys [53,54], nickel [55,56], copper alloys [56], cast iron and steel [12,57-59], beryllium and magnesium [53], and phosphates [2]. This method was also used for determining boric acid admixtures (about 0.05%) in powdered boron [11]. Some synthetic compounds having the structure similar to that of curcumin, were used in determining boron in water [60]. 

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