Structure multi-centre bonding

Polymeric hydrides (white solids) are formed by Be and Al. In BeH2 (Figure 9.13), each Be centre is tetrahedral, giving a chain structure in which multi-centre bonding of the type described for B2H5 is present. The structure of AIH3 consists... 

Some authors have proposed more radical departures from the conventional organisation of the connection table, especially with regard to the representation of bonds, putting emphasis on the electronic structure of the atoms involved. These approaches have distinct advantages in the handling of certain chemical species, such as carbenes and those involving multi-centre bonds. 

The main challenge, after having defined the Kohn-Sham equations, is to solve them in an accurate way where the main problem is to find an accurate description of the multi-centre Coulomb potential. The method used is, to a large extent, determined by the character of the atoms involved in the bonding. The electronic structure of alkali and noble metals is for example rather simple... 

If we consider the various structure types adopted by metals and then try to provide a model for localized metal-metal bonding, we run into a problem there are not enough valence shell orbitals or electrons for each metal atom to form two-centre two-electron bonds with all its neighbours. For example, an alkali metal has eight near-neighbours (Table 5.2), but only one valence electron. We must therefore use a bonding model with multi-centre orbitals (see Sections... 

A very unusual by-product (0-5% yield) of the reaction between Fe3(CO)i2 with certain acetylenes such as 1-pentyne is the black crystalline complex Fe5(CO)i5C, 8.24. The X-ray structure of this complex shows that there is a formally penta-co-orAmzXs carbon atom which is located slightly below the basal plane of four iron atoms. The carbon atom is approximately equidistant from the five iron atoms. The average Fe-Fe distance is 2-64 A [79]. It is not possible to give a simple conventional description of the bonding in this complex, and a multi-centred molecular orbital approach is necessary. In this molecule the environment of the carbon must be similar to that of carbon atoms in some metal carbides (average Fe-C distance = 1 -75 A). 

Supramolecular architectures in which transition metal cationic centres are linked via hydrogen-bonded supramolecular synthons [1,2] comprise an increasingly important class of inorganic co-ordination polymers [3], owing to their multi-dimensional, multi-functional network structures. 

Trisubstituted carbon-centred radicals chemically appear planar as depicted in the TT-type structure 1. However, spectroscopic studies have shown that planarity holds only for methyl, which has a very shallow well for inversion with a planar energy minimum, and for delocalized radical centres like allyl or benzyl. Ethyl, isopropyl, tert-butyl and all the like have double minima for inversion but the barrier is only about 300-500 cal, so that inversion is very fast even at low temperatures. Moreover, carbon-centred radicals with electronegative substituents like alkoxyl or fluorine reinforce the non-planarity, the effect being accumulative for multi-substitutions. This is ascribed to no bonds between n electrons on the heteroatom and the bond to another substituent. The degree of bending is also increased by ring strain like in cyclopropyl and oxiranyl radicals, whereas the disubstituted carbon-centred species like vinyl or acyl are bent a radicals [21]. 

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