The Electroneutrality Principle

A useful principle in writing electronic structures for substances is the electroneutrality principle. This principle is that stable molecules and ciystals have electronic structures such that the electric charge of each atom is close to zero. Close to zero means between —1 and 4-1. 

That this principle is a reasonable one may be seen by the consideration of values of the ionization energy and electron affinity of atoms. The electron affinity of atoms of nonmetals is about 350 kJ mole for the first electron added, to convert the atom F - into the anion F . or the atom 0- into the anion 0- (Section 6-9). But there is in general no significant affinity for a second electron to convert 0- into —. 

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In principle, the effects of the concentration of ions can be removed by dividing A2.4.31 by the concentration. Taking Avagadro s constant as L and assuming a concentration of solute c mol m, then from the electroneutrality principle we have = A = z cL and clearly... 

According to the electroneutrality principle for this system, Ch = Ca + Cqh and = Ca + Cqh subtracting these equations gives... 

The transfer of an electron to the iron atom is compatible with the electroneutrality principle. The electronegativity of iron is 1.8, leading to 12% ionic character of the iron-carbon bonds and to the satisfactory value +0.04 for the resultant charge on an iron atom that has accepted an electron and is forming nine bonds with carbon atoms. 

The development during the past year of a statistical theory of unsynchronized resonance of covalent bonds in a metal, with atoms restricted by the electroneutrality principle to forming bonds only in number u — 1, u, and v + 1, with u the metallic valence, has led directly to the value 0.70 0.02 for the number of metallic orbitals per atom.39 This theory also has led to the conclusions that stability of a metal or alloy increases with increase in the ligancy and that for a given value of the ligancy, stability is a maxi-... 

Here the phosphorus atom has four shared electron pairs and one unshared pair, using five orbitals. (In PC15, eg, the transargononic phosphorus atom has five shared pairs in its outer shell.) However, because of the electroneutrality principle such a structure is allowed only for structure 1. Transargononic structures do not occur for first-row atoms, so this phenomenon is not found in NF3. These ideas concerning the bonding in NF3 and PF3 are implicit in the discussion by Marynick, Rosen and Liebman61 of the inversion barriers of these molecules. 

In the discussion of hypoelectronic metals in ref. 4, the number of ways of distributing Nv/2 bonds among NL/2 positions in a crystal containing N atoms with valence v and ligancy L was evaluated. The number per atom is the Nth root of this quantity. Structures for which the number of bonds on any atom is other than v-l,v, orv + l were then eliminated with use of the binomial distribution function [only the charge states M+, M°, and M are allowed by the electroneutrality principle (5)]. In this way the following expression for rhypo, the number of resonance structures per atom for a hypoelectronic metal, was obtained ... 

Accordingly, the CO moiety acquires negative charge. The consequent exigencies of the electroneutrality principle are then met by the CO group donating this charge back to the metal via its now expanded <7-donor orbital ... 

Consider the closely related ion [FeCHiO/e] ". The only difference is in the formal oxidation state of the metal ion. If an ionic model is assumed (9.6), the charge on the metal center is +2. A purely covalent model results in the placing of a formal quadruple negative charge upon the iron center (9.7). To satisfy the electroneutrality principle, and establish a near-zero charge on the metal, each oxygen atom is... 

Attempts to rationalize the role of ligands in these reactions and in the even more intriguing case of cooligomerizations have been successful only in part. Steric and electronic effects of the substrate should not be considered without taking in account both the other ligands present and the oxidation state of the metal. Donor substrates are generally best stabilized by acceptor ligands and vice versa (10c). in accordance with the electroneutrality principle. 

Structure (3.226c), for example, depicts a central heptavalent Cl atom (Fa = 7), exceeding the normal valence octet by six electrons (These excess electrons are assumed to be accommodated in chlorine 3d orbitals, whereas d-orbital participation is prevented in first-row compounds.) Hypervalent structures such as (3.226a)-(3.226c) are claimed to be justified by the electroneutrality principle, which stipulates that second-row central atoms have zero formal charge (whereas first-row oxyanion Lewis structures commonly violate this principle).148... 

Note that is a negative number, so that v+ z+ + v = 0.) The electroneutrality principle is equivalent to stating that it is impossible to produce a solution that contains, for example, only cations or an excess of positive charge (i.e., that ions cannot be considered as independent components of solutions). Only entire electrolytes are components that can be added to a solution. 

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... 

Condition 3.1. The stoichiometry must obey the electroneutrality principle, namely that the sum of all the atomic valences (formal ionic charges), having regard to their sign, is zero. 

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