Electroneutrality, Pauling

One classical example that apphes the electroneutrality principle is the electronic structure of carbon monoxide, a diatomic molecule with a very small dipole moment of 0.110 debye. The only electronic structure that satisfies the octet rule for CO is C=0 , a structure that corresponds to C and O, if the shared electron pairs are equally devided by the two atoms. Pauling showed that the electronegativity difference of 1.0 would correspond to about 22% partial ionic character for each bond, and to charges of and 0° +. A second possible electronic structure, C=O , does not complete the octet for carbon. The partial ionic character of the bonds corresponds to C0.44+ If these two structures contribute... 

A critical problem related to this is predicting the relative charge displacements which arise on coordination to a series of metal ions or of one metal ion in a series of different coordination environments. In this latter case there is a guide given by Nyholm 26) which is a corollary of Pauling s principle of electroneutrality. 

The course of modern organometallic chemistry has been greatly influenced by three simple generalizations the Dewar-Chatt-Duncanson synergic bonding model for metal-olefin complexes (40, 72) Pauling s electroneutrality principle (174), and the 18-electron or inert gas rule (202). In this section the impact of recent theoretical calculations on these important generalizations will be evaluated. 

This is, of course, an aspect of the question that Pauling addressed and that led to the definition of the electroneutrality principle. If we consider the interaction of six water molecules with a Cr3+ ion, we may write two limiting valence bond structures. One of these forms localises the positive charge on the metal centre and depicts a Cr3+ ion surrounded by six water molecules. The covalent representation places a single positive charge on each of the water oxygen atoms and a —3 charge on the chromium centre (Fig. 2-13). 

The truth lies somewhere between these two extremes, and the assessment of covalent and ionic contributions to bonding has attracted considerable attention. Pauling devised the electroneutrality principle and suggested that the relative importance of ionic and covalent components was such that the overall charge on any one atom did not exceed 1. In the context of the [Cr(H20)6]3+ ion, this leads to a description in which the metal centre is neutral and each oxygen bears a charge of + 2. The Cr-0 bonds may be described as 50 % covalent. 

In order for unsynchronous resonance to occur, the atoms M+ and M° must have an unoccupied orbital available so that they can accept an additional bond. M does not require such an unoccupied orbital because the electroneutrality principle rules out its accepting an additional bond, which would convert it to M2. Accordingly, the structural requirement for a system to possess metallic character is that the fraction of the atoms M+ and M° have available an unoccupied orbital, called the metallic orbital. The average value of 0.72 orbital per atom for the metallic orbital, as deduced from the Slater-Pauling curve, implies that, with unsynchronous resonance of the covalent bonds, the metal consists of 28% M+, 44% M°, and 28% M. ... 

Pauling s electroneutrality principle is an approximate method of estimating the charge distribution in molecules and complex ions. It states that the distribution of charge in a molecule or ion is such that the charge on any single atom is within the range +1 to —1 (ideally close to zero). 

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