Thermal decomposition of polynuclear

The metal-carbon cluster systems we have considered so far in the present chapter, like the carboranes considered in the previous chapter, have contained one or more skeletal carbon atoms occupying vertex sites on the cluster deltahedron or deltahedral fragment. We now turn to some molecular cluster systems in which hypercoordinated carbon atoms occupy core sites in the middle of metal polyhedra. Most are metal carbonyl carbide clusters of typical formulae Mj (CO)yC. Their carbide carbon atoms are incorporated within polyhedra, which in turn are surrounded by y carbonyl ligands. Such compounds, for which few controlled syntheses are available, have been found primarily among the products of thermal decomposition of polynuclear metal carbonyls Mj (CO)j, their carbide carbon atoms result from disproportionation reactions of carbonyl ligands (2 CO CO2 + C). 

The most important feature that has been noted within the temperature range has been the progressive aromatization of the solid leading to the production of carbon lamellae of increasing diameter and such behavior has been confirmed by the thermal decomposition of polynuclear aromatic compounds to coke or carbon (Lewis and Singer, 1988). The results of such studies have been projected by others to assumptions about the structural nature of coal. 

In summary, it has been assumed, on the basis of the behavior of the thermal decomposition of polynuclear aromatic systems, that coal must also consist of large polynuclear aromatic systems (Chapter 10). Be that as it may, such assumptions are highly speculative and, to say the least, somewhat lacking in caution. As an example, similar lines of thought have been applied to structural assumptions about petroleum asphaltene constituents when it is known from other pyrolysis studies that smaller, but polar, systems can produce as much thermal coke as the larger nonpolar highly condensed systems (Speight, 2007). 

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