Heteroatoms electron counting

Clearly, in view of a diversity in the types of heterocyclic compounds, one may hardly expect that all the manifestations of their aromaticity (antiaromaticity) could be rationalized in terms of some simple regularities. We shall therefore attempt to trace certain characteristic trends in the dependence of the aromaticity on the type of heteroatoms, their number and positions in the molecular structure. Our reasoning will be based on the nature of the aromaticity criteria and of the electron count rules. Then turning to individual compounds, we shall add details to the picture. 

The porphyrin ring contains 22 electrons in its n orbitals. As explained above, the Hiickel rule cannot be applied to this electron count, because the molecule is not monocyclic. However, the porphyrin ring can be formally derived from neutral [18]annulene, by introduction of appropriate heteroatoms and bridges (Fig. 3). The macrocycle is thus shown to be aromatic in the Hiickel sense, and is denoted [18]porphyrin. As we will show in subsequent sections, this approach is readily generalized to other porphyrinoids, whose aromatic character can be predicted by defining a neutral annulenoid pathway in the macrocycle. These pathways will be... 

A simplified model may be used to rationalize the bonding in the heteroatomic species [Zn ZnsBi4 Bi7]5. According to the electron counting theory proposed by Wade, the formation of a closo deltahedra of 12 vertices is stabilized by 13 skeletal electron pairs. The total of 26 electrons required for skeletal bonding may be considered to be provided as follows 2 from the interstitial Zn atom, (8x0 + 4x3 = 12) from the ZngBi4 icosahedral unit (each vertex atom carries an exo lone pair or bond pair), 7 from the capping Bi atoms, and 5 from... 

The discussion above suggests endo-H atoms are not just contributors to the cluster electron count. They have a role in determining which of the possible cluster core structures will be observed. Another characteristic of these skeletal H atoms is evident from NMR studies. Movement of endo-H atoms on a cluster framework can be quite facile relative to the timescales on which one does bench chemistry. Hence, in contrast to the movement of heteroatoms in many cluster frameworks, endo-H atoms are found in their most stable positions. 

Pi-bond nucleophiles use the pair of electrons in a it bond, usually a C=C bond, to form a cr bond between one of the atoms in the it bond and the electrophilic atom. The formal charge and total electron count of the nucleophilic atom of the 77 bond do not change, but the other atom of the 7r bond is made electron-deficient, and its formal charge increases by 1. The 7r bonds of simple alkenes and arenes are weakly nucleophilic 7r bonds that are directly attached to heteroatoms, such as in enolates (C=C-0 ), enols (C=C-OH), enol ethers (C=C-OR), and enamines (C=C-NR2), are much better nucleophiles. 

SCHEME 11.40 Impact of d-electron count on metal-heteroatom multiple bonding (X = OR, NHR, etc.). 

Symmetries given in Table 2 are idealized (and refer to the symmetry of the cluster without taking into account heteroatoms) and many of the clusters show distortions from these ideal symmetries. For example, the group 14 clusters Geg, Sng and Pbg have 2n + 4 cluster electrons and exist as the expected nido clusters with C4V monocapped square antiprismatic geometry. On the other hand, the bismuth cluster Big +, which also has a 2n + 4 electron count, instead exists in the closo Z>3h form with a tricapped trigonal prismatic structure. In fact, all these 9-atom clusters show some distortions from the idealised symmetry, and the energy differences between different geometries are small.The arachno clusters Sb4, Bi4 , Se4 + and Te4 + (square planar), and Sbg + and Big + (square antiprismatic) have... 

Worked example 15.5 Electron counting in heteroatomic Zintl ions... 

Ralph Rudolph made major contributions to our understanding of the structure and bonding of polyhedral cluster compounds and he had an abiding interest in developing a rationale which would enable the structure of individual compounds to be systematized and related to each other. He independently arrived at a method of counting skeletal electrons which is now generally referred to as Wade s Rules, and this has had a decisive influence on our general perceptions of polyhedral cluster compounds. Related to this was his preoccupation with the problem of heteroatoms such as sulfur, and the number of electrons which such atoms contribute to the heteroborane clusters. 

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