Bond, covalent multicentre

Acetylenes can also form covalent o-bonds to metals so that mixed pi-a-com-plexes are possible. A good example of such a compound is [Co4(CO)10(EtC=CEt) shown in D.XXV 9>. The acetylene should now be regarded as an olefin with u-bonds from each of the doubly-bonded carbon atoms to different cobalt atoms and a multicentred pi-bond linking the remaining two cobalt atoms and the C=C bond. The C=C bond length is 1.44 A. 

Silvi and Gatti were interested in a direct space representation of the metallic bond and for that piupose performed periodic HF calculations on the bee lattices of Li, Na, K, V, and on the fee ones of Al, Ca, Sc, Cu. A topological analysis of p and of ELF reveals that, except for Al, the valence basins have a synaptic order larger than 2, and that the metallic bond appears to be a partial covalent bond, which is often multicentric and characterized by a low population of the valence basins (less than l.Oe ) and by synaptic orders as large as 6. 

If we skip even that, we speak of integral chemistry (1C). This is the realm of model building presented in this book. At this levelofchemistry, all multiplicities of covalent bonds are integers. However, there are phenomena in chemistry not compatible with this restriction (see Section 2.4). Mesomerism and multicentric bonds need a further generalization. We collect all these more general situations underthe notion of multicenter chemistry (NIC). 

The obvious way to classify substances containing M-M links is in terms of the nature of the bond present, i.e. multicentre, ionic or covalent. However, the distinction between these is not always clear and the following division into four types of substances is more convenient. 

Boron.— The remarkable structural behaviour of boron, which tends to confound the newcomer, continues to receive considerable attention, although the extent of the coverage of boron compounds is perhaps reduced by the difficulties of synthesis which often exist. Since it is immediately to the left of carbon in the periodic table and only sli tly less electronegative, covalent behaviour predominates, but the electron-deficiency of simple tervalent-boron compounds leads to an extensive variety of molecules in which boron acquires electrons in various ways, particularly from nearby bonds or lone pairs. Interpretation of the structures observed frequently involves molecular-orbital calculations, which emphasize the multicentred bonding so often found. 

We are now exploring a different approach to the synthesis of multicentre redox systems, in which the redox fragments are held together not by covalent bonds, but through coordinative interactions. Assembled systems in which more subunits are linked together through non-covalent interactions have been defined as supramolecular systems [9]. In this sense, coordina-tively linked multicentre systems belong to the realm of supramolecular chemistry and, in particular, they shovdd be defined as supramolecular coordination compounds. 

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