Metallic bonding alkaline earth metals

Although the lUPAC has recommended the names tetrahydroborate, tetrahydroaluminate, etc, this nomenclature is not yet ia general use. Borohydrides. The alkaU metal borohydrides are the most important complex hydrides. They are ionic, white, crystalline, high melting soHds that are sensitive to moisture but not to oxygen. Group 13 (IIIA) and transition-metal borohydrides, on the other hand, are covalendy bonded and are either Hquids or sublimable soHds. The alkaline-earth borohydrides are iatermediate between these two extremes, and display some covalent character. 

Bismuthides. Many intermetaUic compounds of bismuth with alkafl metals and alkaline earth metals have the expected formulas M Bi and M Bi, respectively. These compounds ate not saltlike but have high coordination numbers, interatomic distances similar to those found in metals, and metallic electrical conductivities. They dissolve to some extent in molten salts (eg, NaCl—Nal) to form solutions that have been interpreted from cryoscopic data as containing some Bi . Both the alkafl and alkaline earth metals form another series of alloylike bismuth compounds that become superconducting at low temperatures (Table 1). The MBi compounds are particularly noteworthy as having extremely short bond distances between the alkafl metal atoms. 

Electrons are not only charged, they also have a characteristic physicists call spin. Pairing two electrons by spin, which has two possible values, up or down, confers additional stability. Bei yllium (Be, atomic number 4) has two spin-paired electrons in its second shell that are easily given up in chemical reactions. Beryllium shares this characteristic with other elements in column two, the alkaline earth metals. These atoms also generally form ionic bonds. Boron... 

Selectivity to primary metathesis products is usually less than 100%, as a consequence of side reactions, such as double-bond migration, dimerization, oligomerization, and polymerization. The selectivity can be improved by adding small amounts of alkali or alkaline earth metal ions, or, as has recently been shown, thallium 40), copper, or silver ions (41)-... 

Another marked physical difference between sulphides and sulphoxides (or sulphones) is that sulphoxides (and lower alkyl sulphones) are hygroscopic and dissolve quite readily in water or protic solvents such as alcohols, and even more so lower alkyl or alkyl aryl sulphoxides are almost freely miscible with water. This can be accounted for by the formation of the strong hydrogen bond between the S—O bond in the sulphoxides and water molecules. Moreover, lower alkyl sulphoxides and sulphones such as dimethyl sulphoxide (DMSO) or sulpholene can dissolve a number of metallic salts, especially those of alkali and alkaline earth metals, and hence these compounds have been widely utilized as versatile and convenient solvents in modern organic chemistry26 (Table 3). 

The alkaline earth metals, by assuming the configuration nsnp, are able to form twice as many bonds as the alkalis. Similarly the succeeding elements in the periodic table can form bonds in increasing number. 

It is interesting that the bond energy relative to the bond-forming state of the atoms shows the same monotonic trend for the alkaline-earth metals as for the alkali metals. The irregularity in the heats of sublimation at magnesium is due to the high... 

Table 4. Energies of resonating bonds in alkaline-earth metals...

Hexaborides of a CaBg type are formed by K, the alkaline earths, Y and the larger lanthanides, as well as Th and some actinides ". The crystal structure of these compounds with cubic symmetry (Pm3m, O, ) (see Fig. 1) is characterized by a three-dimensional skeleton of Bg boron octahedra, the interstices of which are filled by metal atoms. The connection between two octahedra is by a B—B bond of length 1.66 X 10 pm, whereas the B—B bond lengths in one octahedron are 1.76 X 10 pm. ... 

Of course, the chemistry of zirconium cluster phases has been well described and reviewed in the literature [1-4]. Apart from a very few examples, mostly in the binary halides, almost all reduced zirconium halides contain octahedra of zirconium atoms centred on an interstitial atom Z. Several possible and experimentally realized Z include H, Be-N, K, Al-P, and the transition metals Mn-Ni. All these compounds have the general formula Ax"[(Zr6Z)Xi2X[J], with a " = alkali or alkaline earth metal cation, X=C1 Br or I, X =inner edge-bridging halide [5], X =outer exo-bonded halide, and 0[Pg.61]

Fig. 8.7 Superposition of density-of-states for B-N bonding BN>. states with corresponding metal states, reflected by their valence states from alkaline-earth (as Ca), lanthanide (as La), and 3d-metal (as Ni), and corresponding block schemes...

The large amounts of natural gas (mainly methane) found worldwide have led to extentive research programs in the area of the direct conversion of methane [1-3]. Ihe oxidative transformation of methane (OTM) is an important route for the effective utilization of the abundant natural gas resources. How to increase catalyst activity is a common problem on the activation of methane. The oxidation of methane over transition m al oxides is always high active, but its main product is CO2, namely the product of deep oxidation. It is because transition metal oxides have high oxidative activity. So, they were usually used as the main corrqtonent of catalysts for the conqilete oxidation of alkane[4]. The strong oxidative activity of CH4 over tran on metal oxides such as NiO indicates that the activation of C-H bond over transition metal oxides is much easier than that over alkaline earth metal oxides and rare earth metal oxides. Furthermore, the activation of C-H bond is the key step of OTM reaction. It is the reason that we use transition metal oxides as the mam conq>onent of the OTM catalysts. However, we have to reahze that the selectivity of OTM over transition metal oxides is poor. 

The most fundamental issues of the structures of heavier group 14 element-centered anionic derivatives R3EM (R = alkyl, aryl, silyl E = Si, Ge, Sn, Pb M = alkali or alkaline earth metals) turned out to be the questions of their aggregation states (monomeric, dimeric, or oligomeric), nature of the E-M bond (covalent or ionic), and configuration of the anionic centers E (tetrahedral, pyramidal, or planar). The most important experimental techniques that are widely used to clarify these questions are NMR spectroscopy and X-ray diffraction analysis. 

The structural information on the alkaline earth metal derivatives of the heavier group 14 elements is markedly limited. The first silyhnagnesium compound to be structurally characterized was (Me3Si)2Mg (DME), which is monomeric with distorted tetrahedral geometry of the Mg atom and Si-Mg bond length of 2.630(2)... 

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