Molecules multiple bond counts

The simplest descriptors of the degree of unsaturation of a molecule are -> count descriptors based on the presence of double bonds, triple bonds, and aromatic bonds they are the double-bond count, the triple-bond count n= and the aromatic-bond count The MCB index was proposed as the number of multiple C-C bonds in the molecule accounting for double, triple, and aromatic bonds [Bakken and Jurs, 1999b]. Another simple global multiple bond count is defined as ... 

The electrons in a multiple bond (double or triple) are counted as a single pair when working out the shape of a molecule or ion. 

This ID number accounts for multiple bonds in the molecule for saturated molecules each bond weight is equal to one, therefore the ID number coincides with the - total path count. 

Some molecnles have satisfactory electron-dot structures with octets but have more reasonable formal charge distributions in their strnctures with expanded electron counts. In each of the cases in Figure 3.6, the actual molecules and ions are consistent with electron counts greater than 8 on the central atom and with a large contribntion from the resonance structure that uses multiple bonds to minimize formal charges. The multiple bonds may also influence the shapes of the molecules. 

Let s now consider whether the representation chosen—the traditional one with multiple bonds versus the newer one—affects the valence of the central atom. Let us do so with a couple of examples involving higher-valent sulfur, say, SOCI2 and SO2CI2. If we consider the traditional structural formulas with double bonds, the S is clearly tetra- and hexa-valent in the two molecules, respectively. Simply count the electrons, denoted by blue dots, that the sulfur has used to form bonds. 

The relation between delocalized m.o.s and structural formulae can begin by just counting the total number of bonding and anti-bonding electrons, Ab and Aab respectively, and then determine what could be called a global bond order as the sum of the multiplicities of the several bonds in the molecule ... 

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