Complexes of Beryllium

To our knowledge there is no experimental evidence for n bonding between a BeR2 unit and a simple alkene or alkyne ligand. However a n interaction has been found in the X-ray crystal structure of the dimeric dipropynylberyllium trimethylamine adduct (48). The unit cell of this molecule contains two independent centrosymmetric dimers in which the alkynyl groups exhibit different types of interaction with the beryllium atoms, one of them forming an effectively electron-precise dimer unit by n interaction as portrayed in XX. 

The synthesis of the first cyclopentadienylberyllium compound was reported by Fischer and Hofmann in 1959 (49). Reaction of cyclopentadienyl alkali-metal compounds with BeCl2 in diethyl ether or benzene leads to dicyclopentadienylberyllium (beryllocene), a rather volatile, colorless, diamagnetic complex, easily soluble in benzene, diethyl ether, and hydrocarbons, and very sensitive to air and moisture. 

The structure of this interesting n complex has stimulated controversial discussion and is still a subject of debate. From the large dipole moment (p25°c= 2.46 0.06 D in benzene, 2.24 0.09 D in cyclohexane), it is evident that the structure must be unsymmetrical. An X-ray structural investigation at low temperature (— 120°C) shows that the molecule assumes a sandwich structure 

In several theoretical papers at different degrees of sophistication attempts have been made to determine the most favorable structure for beryllocene (59-66). Ab initio molecular orbital calculations indicate that in the lowest energy form one pentahapto- and one monohapto-bonded cyclopentadienyl ring are present, as portrayed in structure H. Neither the slip sandwich F nor the off-center double well potential structure G suggested experimentally are indicated to be favorable. Even the symmetrical D5 ) sandwich structure 

All the complexes XXIa-i are air-sensitive colorless solids or liquids. Structural parameters are available mainly from X-ray structure data (76,266) and from electron diffraction and microwave studies (67-69,71,79,80). In nearly all cases the molecules are monomeric in the solid state, in solution, and 

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F NMR Data for Fluoro Complexes of Beryllium(II) in Aqueous Solution ... 

Complexes of beryllium with 1,2-dihydroxybenzene (catechol) and its more soluble sulfonate derivatives (188, 198, 251-256) have been investigated with particular reference to their possible use as antidotes for beryllium poisoning. 

Complexes of beryllium with phosphonate ligands have been extensively researched, mainly by potentiometry. The ligands investigated are as follows methylphosphonate (259, 260) chloromethyl-phosphonate (259) 2-aminopropane-l-phosphonate (259) methylene-diphosphonate (130, 260, 261) hexamethylenediphosphonate (259) l-hydroxyethane-l,l-diphosphonate (261) 1,1-dimethyl-1-amino-... 

Numerous other complexes of beryllium with organic ligands such as alloxides (276-280), /6-diketonates (90, 281-297), SchifF bases (64, 298-301), thiols (302), pyridines (303), bipyridyl (304), phthalocya-nine (305), hydroxyquinolines (306, 307), tropolones (308, 309), pyra-zolylborates, (94, 310), phosphinates (311), hydrazides (312), phenyl-hydrazonocarboxylates (313), dinaphthofuchsonedicarboxylates (314),... 

A new area of research concerns exposure assessment for beryllium in the production of nuclear weapons at nuclear defense industries. A safe level of exposure to beryllium is still unknown. Potential explanations include (1) the current exposure standard may not be protective enough to prevent sensitization, or (2) past exposure surveillance may have underestimated the actual exposure level because of a lack of understanding of the complexity of beryllium exposures. Task-based exposure assessment provides information not directly available through conventional sampling. It directly links exposure to specific activity associated with contaminant generation and provides in-depth evaluation of the worker s role in a specific task. In-depth task analysis is being used to examine physical, postural, and cognitive demands of various tasks. 

The coordination process may either stabilize or destabilize aromatic Schiff bases. If nickel (II) salts are added to ammoniacal solutions of salicylaldehyde, the precipitate obtained is the inner complex salt of nickel (II) and salicylaldimine (61). If beryllium chloride is added to the Schiff base derived from 2-hydroxy-l-naphthaldehyde and ethylamine, however, the Schiff base is decomposed and the inner complex of beryllium (II) and 2-hydroxy-1-naphthaldehyde is obtained (59). Here the strength of the coordinate bonds formed with the metal seems to determine which complex will be formed. 

Other anionic complexes of beryllium that have been detected are (NH4)2Be(N03)4,141 (NH4)2Be(NCS)4-MeCN and (NH4)2Be(NCS)3-MeCN.142 ESCA studies on beryllium and magnesium complexes of the type [M(NCS)4]2 and [M(CNS)3L] (L = DMF, py, MeCN) are consistent with N-bonding thiocyanate anions being present.142 The structure of K[Be(NH2)3] shows the beryllium to be in a trigonal planar unit.143... 

Assay of beryllium metal and beryllium compounds is usually accomplished by titration. The sample is dissolved in sulfuric acid. Solution pH is adjusted to 8.5 using sodium hydroxide. The beryllium hydroxide precipitate is redissolved by addition of excess sodium fluoride. Liberated hydroxide is titrated with sulfuric acid. The beryllium content of the sample is calculated from the titration volume. Standards containing known beryllium concentrations must be analyzed along with the samples, as complexation of beryllium by fluoride is not quantitative. Titration rate and hold times are critical therefore use of an automatic titrator is recommended. Other fluoride-complexing elements such as aluminum, silicon, zirconium, hafnium, uranium, thorium, and rare earth elements must be absent, or must be corrected for if present in small amounts. Copper—beryllium and nickel—beryllium alloys can be analyzed by titration if the beryllium is first separated from copper, nickel, and cobalt by ammonium hydroxide precipitation (15,16). 

In the above examples, the nucleophilic role of the metal complex only comes after the formation of a suitable complex as a consequence of the electron-withdrawing effect of the metal. Perhaps the most impressive series of examples of nucleophilic behaviour of complexes is demonstrated by the p-diketone metal complexes. Such complexes undergo many reactions typical of the electrophilic substitution reactions of aromatic compounds. As a result of the lability of these complexes towards acids, care is required when selecting reaction conditions. Despite this restriction, a wide variety of reactions has been shown to occur with numerous p-diketone complexes, especially of chromium(III), cobalt(III) and rhodium(III), but also in certain cases with complexes of beryllium(II), copper(II), iron(III), aluminum(III) and europium(III). Most work has been carried out by Collman and his coworkers and the results have been reviewed.4-29 A brief summary of results is relevant here and the essential reaction is shown in equation (13). It has been clearly demonstrated that reaction does not involve any dissociation, by bromination of the chromium(III) complex in the presence of radioactive acetylacetone. Furthermore, reactions of optically active... 

Some typical complexes of beryllium have already been considered, and it was seen that complex formation is favored if the complexing atoms become incorporated into a cyclic structure. Indeed, in the case of the heavier alkaline-earth metals (whose radii are far greater and... 

Chlorophyll (Fig. 6-3) and the calcium complex of yersene (Fig. 6-2) are, like the complexes of beryllium shown on page 110, chelate structures. 

Table 2 Stability constants for aliphatic bidentate complexes of beryllium...

It has been noted that although the linear gemnetry is consistently predicted for cationic Lewis acid carbonyl ctxnplexes in ab initio calculations, extrapolation of these results to neutral Lewis acid c< n-plexes may not be justified. Semiempirical MNDO calculations predicted the bent conformation as the lowest energy structure for neutral Lewis acidic derivatives of beryllium, boron and aluminum cmn-plexed with rranr-2,3-dimethylcyclopropanone, whereas linear structures were predicted for the cationic complexes of beryllium and aluminum (Table 1). ... 

Maximal and stable absorbance of the triple complex is obtained with a large excess of surfactant in relation to CAS. In these systems, ternary complexes are formed in which the ratio CAS Be is greater than in binary systems (without surfactant). Figure 9.1 shows the absorption spectra of Chrome Azurol S and the binary and ternary (with CTA) complexes of beryllium... 

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