Alkaline-earths trimers

Chemical Properties. In addition to the reactions Hsted in Table 3, boron trifluoride reacts with alkali or alkaline-earth metal oxides, as well as other inorganic alkaline materials, at 450°C to yield the trimer trifluoroboroxine [13703-95-2] (BOF), MBF, and MF (29) where M is a univalent metal ion. The trimer is stable below — 135°C but disproportionates to B2O2 and BF at higher temperatures (30). 

Much work has been done on the structure of the metal alkoxides (49). The simple alkaU alkoxides have an ionic lattice and a layer stmcture, but alkaline earth alkoxides show more covalent character. The aluminum alkoxides have been thoroughly studied and there is no doubt as to their covalent nature the lower alkoxides are associated, even in solution and in the vapor phase. The degree of association depends on the bulkiness of the alkoxy group and can range from 2 to 4, eg, the freshly distilled isopropylate is trimeric (4) ... 

On the large scale, cyanuric chloride is produced by the trimerization of cyanogen chloride. The cyanogen chloride is produced by chlorination of hydrogen cyanide and is trimerized by passing it over charcoal impregnated with an alkaline-earth metal chloride at a high temperature (250—480°C). 

Another example of the remarkable reactivity of Mg actuated by our procedure is its reaction with nitriles. In this respect, the Mg resembles an alkali metal more than an alkaline earth. Benzonitrile reacts with Mg overnight, in refluxing DME, to give 2,4,6-triphenyl-l,3,5-triazine and 2,4,5-triphenylimidazole in 26 and 27% yield, respectively, based on magnesium. Jhe imidazole was shown to arise, at least in part, from the action of Mg on the triazine. The trimerization of aromatic nitriles to give symmetrical triazines is not unknown, but generally the reactions are... 

A comparison of these compounds with the corresponding zinc derivatives should clarify the influence of the empty d orbitals involved in the bonding situation of the alkaline earth metal bis(phosphanides). Whereas zinc bis[bis(trimethylsilyl)amide] is monomeric due to the steric demand of the bulky amide ligand [6], the trimethylsilyl substituted phosphanide leads to oligomers such as dimers or trimers [7], The influence of the pnicogen atom is small, thus the phosphorus and arsenic derivatives (Fig. 5.) look very similar [8] or even crystallize isotypically. In contrast to the d metals calcium, strontium and barium, zinc derivatives solely build up monocyclic ring systems. 

The dimers of Be, Mg and Ca are very weakly bound by the electron correlation effects, at the self-consistent field (SCF) level they are not stable. The binding energy of alkaline earth dimers is only 2-4 times larger than that in Kr2 and Xe2 dimers. Thus, alkaline dimers can be attributed to the van der Waals molecules. The situation is changed in many-atom clusters, even in trimers (Table II). This is evidently a manifestation of the many-body effects. The crucial role of the 3-body forces in the stabilization of the Be clusters was revealed at the SCF level previously [3-5], and more recently was established at the Mpller-Plesset perturbation theory level up to the fourth order (MP4) [6,7]. The study of binding in the Ben clusters [8-10] reveals that the 3-body exchange forces are attractive and give an important contribution to... 

It is instructive to study the vacant atomic orbital population in dimers and trimers. As mentioned in the Introduction, in the 80 s Bauschlicher et al. [10,11] came to the conclusion that the promotion of ns-electrons to np-orbitals leading to sp-hybridization is the main mechanism responsible for binding in alkaline-earth clusters. This conclusion was based on a study of the SCF Mulliken population analysis for tetramers, which are stable at the SCF level. At present, we can perform more precise analysis using the Natural Bond Orbital Analysis and calculate it at the electron correlation level. 

We have to take into consideration that some atom-atom interactions, which enhance the excited orbital population, do not lead to a bonding state. The last statement is confirmed by the valence orbital population in the alkaline-earth clusters at the SCF level. According to Table VI, at the SCF level there is a non-negligible p-population, especially for trimers. But in the... 

As mentioned in Section 33.2, the many-body expansion cannot be expected to work for metals. One reason is that most atoms forming metals have open-shell ground states of symmetry other than S, therefore it is difficult to determine quantum states of the subsystems needed in the definition of the expansion, cf. Section 33.10. The second reason is that the complete delocalization of the conduction electrons results in the electronic structure of a metal that is very far from that of monomers. The first problem does not occur for alkaline-earth metals or for high-spin alkali-metal clusters, and the many-body expansion can be defined for such clusters. However, this expansion appears to be very slowly convergent [106-108]. For some specific information about the spin-polarized sodium trimer, see Section 33.10.2. 

Consider isocyanate trimerization as an example. This reaction in the presence of alkali and alkaline-earth metals was described in detail earlier. With crown-ether in contact with the concrete surface, it forms a crown-complex with the calcium cation of calcium oxide... 

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