Electron bookkeeping

So far we have emphasized structure in terms of electron bookkeeping We now turn our attention to molecular geometry and will see how we can begin to connect the three dimensional shape of a molecule to its Lewis formula Table 1 6 lists some simple com pounds illustrating the geometries that will be seen most often m our study of organic chemistry... 

The concept of oxidation number is used to simplify the electron bookkeeping in redox reactions. For a monatomic ion (e.g., Na+, S2 ), the oxidation number is, quite simply, the charge of the ion (+1, —2). In a molecule or polyatomic ion, the oxidation number of an element is a pseudo-charge obtained in a rather arbitrary way, assigning bonding electrons to the atom with the greater attraction for electrons. 

The anows can be written in either directional sense since these reactions are concerted rearrangements with all bond making-bond breaking taking place at die same time. This example emphasizes the fact that curved-arrow notation is merely an electron bookkeeping method. 

Structure (i) could be misleading it is doubtful whether the Be2+ ion has any independent existence under ordinary conditions and the bonds between Be and Cl can be rendered simply as ordinary rather than coordinate bonds. Structure (iii) is objectionable because the lack of arrows could imply that the bonds are all ordinary covalent bonds to which each atom contributes equally, leading to untidy electronic bookkeeping it is not immediately apparent that the Be and N atoms obey the octet rule which is almost invariably followed by atoms in that Period (see Section 6.1). Structure (ii) is to be preferred. 

Lewis structures are partieulariy usc-rul because they make electron bookkeeping possible and act as reminders of the number of valence electrons present. Simpler, hn-wever, 1 the use of Kckule structures, or Hn bond structures, in which two-electron covalent bond is indicated as a line drawn between atoms. Lone pairs of nonbonding valence electrons are often not shown when drawing lane-bond structures, though it s still necessary to keep track of them mentally. Some examples ure shown inT ble 1.2. 

To manage electron bookkeeping in a Lewis structure, organic chemists use formal charge. 

The reduced porphycene 3.90 may be considered as an analog of chlorin 3.91 (Figure 3.1.18), a species which, in turn, may be considered as a dihydro-analog of porphyrin. In the specific case of 3.90, confirmation of the chlorin-like nature of the system in terms of formal 7i-electron bookkeeping, was obtained from H NMR experiments. These same experiments provided insights into the specific site of reduction. For instance, it was found that the P-pyrrolic CHCH protons resonated at lower field than analogous protons in pyrrole (i.e., at 5 = 8.2-9.3 ppm). On the other hand, the CH2CH2 protons of the reduced pyrrolic subunit were found at a relatively upheld location (i.e., 8 = 4.6-4.9 ppm) this is as one would expect for aliphatic protons of this type. 

To arrive at this divalent beryllium atom, let us do a little electronic bookkeeping. First, we promote one of the 2s electrons to an empty p orbital ... 

Simple errors can lead to some rather absurd predictions. Arrow pushing is a form of electron bookkeeping therefore the arrows and Lewis structures must correlate and be accurately drawn for the results to make sense. The total charge on both sides of the transformation arrow should be the same. Pay attention to detail. 

The writing of Lewis formulas is an electron bookkeeping method that is useful as a first approximation to suggest bonding schemes. It is important to remember that Lewis dot formulas only show the number of valence electrons, the number and kinds of bonds, and the order in which the atoms are connected. They are not intended to show the three-dimensional shapes of molecules and polyatomic ions. We will see in Chapter 8, however, that the three-dimensional geometry of a molecule can be predicted from its Lewis formula. 

Theoretical justification for the electron bookkeeping device of treating CH and Co(CO)3 or Ni(n -Cd 1) units as similar sources of three AOs and two electrons for cluster forming use followed from analyses of the frontier orbitals [HOMOs and lowest unoccupied molecular orbitals (LUMOs)] of such conical transition metal units by Hoffmann,Mingos, " and others who coined the term isolobal to describe their relationship to a CH unit. Although the transition metal units use pd hybrid AOs where carbon uses p or sp hybrids, the numbers, energies, extensions in space, and lobal characteristics of these orbitals are very similar, justifying the description of these units as isolobal, written as follows (cf. Fig. 1.19 in Chapter 1) ... 

So far our concern has emphasized electron bookkeeping. We now turn our attention to the shapes of molecules. 

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