General features relating to stability—filled shells of electrons

General features relating to stability filled shells of electrons 

It is well known that much of carbon chemistry is controlled by kinetic factors for example diamond would change spontaneously to graphite, and acetylene to benzene, at room temperature and atmospheric pressure if thermodynamics were controlling. In addition, nearly aU organic compounds are thermodynamically unstable to oxidation, and exist in the presence of air only because no suitable low energy oxidation mechanism is available. 

This kinetic stability of carbon compounds has a variety of causes, notably the/ // use of the four valence orbitals sp ) in carbon (leading to the common maximum co-ordination number of four—exceptions, e.g. Me4Li4, (Me3Al)2 in which the co-ordination number rises to S-7 are discussed in Chapter 3), and the high energy of empty antibonding or nonbonding orbitals into which electrons could either be promoted to initiate thermal decomposition or donated in the case of nucleophilic attack. 

For compounds of the transition elements, however, empty valence shell (e.g. 3, Ad or 5d) orbitals are often available, and can markedly decrease their kinetic stability. For example Me4Ti is unstable at room temperature, decomposing to hydrocarbons and other products, whereas Me4Si may be heated to at least SOOT. In transition metals the closed shell concept remains but can apply in several different ways. The commonest of these is where the closed shell consists of 18 electrons, i.e. n, np, and (n-iy . Transition metal atoms are, like atoms of Group lA and IIA elements, inherently electron-deficient, and this electron-deficiency must be satisfied if thermally stable organometallic compounds are to be formed. The 18-electron rule and related ideas are discussed further in Chapter 5. 

Cotton and G. Wilkinson, Advanced Inorganic Chemistiy, 2nd Edition (Intersdence, New York, 1966). 

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