Coordination numbers of carbon

Carbon is known with all coordination numbers from 0 to 8. Some typical examples are given in Table 14.3.2, and their structures are shown in Fig. 14.3.3. In these examples, the compounds with the high coordination numbers, such as 5, do not belong to the class of hypervalent compounds, but rather to electron-deficient systems. Hypervalent molecules usually have a central atom which requires the presence of more than an octet of electrons to form more than four 2c-2e bonds, such as the S atom in SF6. 

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Graphite forms extended two-dimensional layers (see Fig. 5.22). (a) Draw the smallest possible rectangular unit cell for a layer of graphite, (b) How many carbon atoms are in your unit cell (c) What is the coordination number of carbon in a single layer of graphite ... 

Coordination number of carbon. Molecules in which the carbon is four-coordinated generally have longer CO bonds than those in which carbon is three-coordinated, which in turn are longer than the bonds in CO2 and CO, consistent with the LCP model. 

Pikunic et al. [27] have implemented a set offlexible constraints in the RMC procedure. Assuming that most carbon atoms have sp hybridization, they constrain the coordination number of carbon atoms and the bond angles using a set of simple expressions ... 

In inorganic terms, the coordination number of carbon in CO is one. In view of the possibility for carbon to acquire higher coordination numbers, corresponding to different hybridizations (from sp to sp, CO inserts into organic and inorganic bonds by reducing its C—O unsaturation ... 

To describe the structure of this compounds it has long been accepted to use two-electron three- and multicentre bonds, while the coordination number of carbon may vary from 5 to 8. 

Hence it is essential to know whether a new bond arises before the old one is freed, or whether the old bond is liberated and the new one arises simultaneously. The maximum coordination number of carbon is equal to four, and as a result there is no bond capacity, on the basis of which a complex with five groups at the carbon atom might have an appreciable duration of existence. Hence the old bond is liberated and the new one arises simultaneously in hydrocarbons. However, in silicon such a complex is possible, since it possesses a greater "bonding capacity, " i.e., as is frequently stated, it possesses the ability to expand its "valence sphere" to a maximum coordination number equal to six [1]. The possibility of the existence of a pentavalent intermediate complex in organosilicon compoxmds has been demonstrated ... 

Crystalline methyllithium consists of tetrameric aggregates (Fig. 3.4). Each carbon atom is bound to three hydrogen atoms and is equidistant from the three lithium atoms which are at the corners of a triangular face of the Li4 tetrahedron. The formal coordination number of carbon is therefore seven. The structure consists of two interpenetrating and unequal tetrahedra, one Li and one C. The Li C skeleton can thus be represented as a distorted cube. The methyl carbon of one tetramer is only 2.36 A from a lithium atom of a neighbouring tetramer. This interaction makes methyllithium involatile and sparingly soluble in hydrocarbons. v... 

The coordination number of carbon with oxygen is three (cf., Table 1), which is also the coordination number found in elemental carbon (graphite). The coordination number of four is found only in compounds in which carbon is bonded to hydrogen or a halogen, or in compounds such as diamond that are formed under pressure. See Sect. 8.1.1 for a fuller discussion of the coordination number of hydrogen. 

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