Pearson electronegativity

Mendes et al. (2010) QCARs for Predicting Cation Toxicity Using Standard Reduction-Oxidation Potential, Electronegativity, Pearson and Mawby (1967) softness parameter and in combination with the Logarithm of the First Hydrolysis Constant, Covalent Index, and Atomic Radius... 

Log metal hydroxide solubility Ionization potential Allred-Rochow electronegativity Pauling s electronegativity Mulliken Electronegativity Pearson and Mawby softness parameter The heat of formation of inorganic oxides The heat of formation of aqueous ion Atomic number... 

Pearson (1966) defines a soft base as one in which the donor atom is of high polarizability and low electronegativity and is easily oxidized or associated with empty, low-lying orbitals . A hard base has opposite properties. The donor atom is of low polarizability and high electronegativity, is hard to reduce, and is associated with empty orbitals of high energy. ... 

Parr RG, Pearson RG (1983) Absolute hardness companion parameter to absolute electronegativity. J Am Chem Soc 105(26) 7512-7516... 

These descriptors have been widely used for the past 25 years to study chemical reactivity, i.e., the propensity of atoms, molecules, surfaces to interact with one or more reaction partners with formation or rupture of one or more covalent bonds. Kinetic and/or thermodynamic aspects, depending on the (not always obvious and even not univoque) choice of the descriptors were hereby considered. In these studies, the reactivity descriptors were used as such or within the context of some principles of which Sanderson s electronegativity equalization principle [16], Pearson s hard and soft acids and bases (HSAB) principle [17], and the maximum hardness principle [17,18] are the three best known and popular examples. 

As mentioned in the Introduction, no structural information on these species was available for more than 40 years after the discovery of the first Zintl metal cluster anions, since no pure crystalline phases could be isolated and characterized structurally. Nevertheless, early efforts to rationalize the observed formulas and chemical bonding of these intermetallics and related molecules utilized the Zintl-Klemm concept [75, 76] and the Mooser-Pearson [77] extended (8 — N) rule. In this rule N refers to the number of valence electrons of the more electronegative metal (and thus anionic metal) in the intermetallic phases. 

Notice that the aforementioned compositional scheme is a necessary condition for building the tetrahedral structures, but not every compound that fulfils this condition is a tetrahedral compound. The influence of other parameters, such as the electronegativity difference, has been pointed out. By means of diagrams, such as that reported by Mooser and Pearson (1959) (average principal quantum number vs. electronegativity difference) the separation of tetrahedral structures from other structures can be evidenced. 

On the conceptual side, the powers of DFT have been shown to be considerable. Without going into detail, I mention only that the Coulson work referred to above anticipated in large part the formal manner in which DFT describes molecular changes, and that the ideas of electronegativity and hardness fell into place, as do Ralph Pearson s HSAB and Maximum Hardness Principles. 

Later on, Pearson [75] introduced the concept of hard and soft acid and bases (HSABs) hard acids (defined as small-sized, highly positively charged, and not easily polarizable electron acceptor) prefer to associate with hard bases (i.e., substances that hold their electrons tightly as a consequence of large electronegativities, low polarizabilities, and difficnlty of oxidation of their donor atoms) and soft acids prefer to associate with soft bases, giving thermodynamically more stable complexes. According to this theory, the proton is a hard acid, whereas metal cations may have different hardnesses. 

Since Pearson (7, 2) introduced the words soft and "hard in their new sense, there has been a persistent argument that it has something to do with electric dipole polarizabilities. Certainly, in most cases, the softness and high polarizabihty a run roughly parallel, in the same way as in many, but not all, cases, the soft ligands have low electronegativities. However, though a connection undoubtedly exists, a closer analysis of recent results (3) makes a a far more physical than chemical quantity. 

Pearson, R. G. (1986). Absolute electronegativity and hardness correlated with molecular orbital theory. Proc. Natl. Acad. Sci. U.S.A. 83, 8440-8441. 

Figure 1.9 Molecular orbital diagram for the general case of a diatomic molecule AB, where B is more electronegative than A. Reprinted, by permission, from R. E. Dickerson, H. B. Gray, and G. P. Haight, Jr., Chemical Principles, 3rd ed., p. 464. Copyright 1979 by Pearson Education, Inc.

The Lewis acid/base complex is formed via an overlap between a doubly occupied orbital of the donor D and vacant orbital of the acceptor A. This acid/base approach was extended by Pearson who divided Lewis acids and bases into two groups, hard and soft, according to their electronegativity and polarizability (principle of hard and soft acids and bases (HSAB concept). Hard acids (e.g., H, Lf, Na, BF3, AICI3, hydrogen-bond donors HX) and hard bases (e.g., F", CL, HO, RO, H2O, ROH, R2O,... 

Pearson, R. G. Absolute electronegativity, hardness, and bond energies, Bonding Energetics in Organometallic Compounds , Ed. Marks, T. J. American Chemical Society Washington, 1990, pp. 251-262. 

R. G. Pearson, Hard and soft acids and bases—the evolution of a concept. Coord. Chem. Rev. 100, 403-425 (1990) R. G. Pearson, Absolute electronegativity and hardness. Application to inorganic chemistry. Chem. Br. 31, 444-447 (1991) R. G. Pearson, Recent advances in the HSAB concept. J. Chem. Educ. 64, 561 (1987). 

A plot by Mooser and Pearson of average quantum number vs. difference in electronegativities of the M and X atoms was reasonably successful in separating CaF2, MX2 layer structures, Ti02 and Si02 (E, Moser and W.B. Pearson, Acta Crystallogy. 1959 12 1015.)... 

Pearson has observed that the reactivity of various organic substrates, including radicals, can be correlated with their absolute electronegativies [56]. It can be seen that although trifluoromethyl and pentafluoroethyl radicals are much more electronegative than the more nucleophilic alkyl radicals, such as ferf-butyl, methyl itself should not be much more nucleophilic than trifluoromethyl. Nucleophilicities of alkyl radicals increase CH3 < 1° < 2° < 3°. Although there are not sufficient IP or EA data available to substantiate the issue, the reactivity studies which are described in Sect. 5 demonstrate that the electrophilicities of perfluoroalkyl radicals increase CF3 1° < 2° < 3°. 

Chemical hardness and softness are much newer ideas than electronegativity, and they were quantified only fairly recently. Parr and Pearson (1983) proposed to identify the curvature (i.e. the second derivative) of the E versus N graph (e.g. Fig. 7.10) with hardness, rj [151]. This accords with the qualitative idea of hardness as resistance to deformation, which itself accommodates the concept of a hard molecule as resisting polarization - not being readily deformed in an electric field if we choose to define hardness as the curvature of the E versus N graph, then... 

Parr and Pearson 1301 defined a parameter 17, which they called "absolute hardness" (17 V4[IP-EA]), and calculated 17 for a variety of neutral and ionic Lewis acids and bases possessing from one to four atoms. These authors showed that the qualitative predictions of the HSAB model regarding the relative reactivities of these species toward one another may be obtained using the results from simple calculations of stabilization energies using 17 and electronegativity values. 

Just like Sanderson s electronegativity equalization principle, the Hard and Soft Acids and Bases principle was originally introduced without strong theoretical basis. Nevertheless, it was used widely from its formulation on. The principle states that hard acids prefer to coordinate with hard bases and soft acids with soft bases [82], In 1983, Parr and Pearson provided a definition for the chemical hardness [25]... 

Another way of characterizing the readiness of molecules to gain or lose electrons upon interaction is based on the concepts of molecular electronegativity and hardness (Berkowitz and Parr, 1988 Parr et al., 1978 Pearson, 1986 1991). The starting point is the consideration that both the extent and ease of electronic deformation will affect the reactivity of a chemical compound (cf. Schiiiirmann, 1998a). The electronegativity (EN) characterizes the tendency of atoms and molecules... 

---