Electronegativity Allred-Rochow values

Figure 5. y,-SCS(X) values in 2-substituted adamantanes 25 plotted against Allred-Rochow electronegativities (X). 

The very negative Ln t + /Ln potentials are consistent with the electropositive nature of the lanthanide elements their Allred-Rochow electronegativity coefficients are all 1.1 except for europium, which has a value of 1.0. The lighter elements of Group 3, Sc and Y, both have electronegativity coefficients of 1.3. The nearest p-block element to the lanthanides in these properties is magnesium (Mg2+/Mg) = -2.37 V, and its electronegativity coefficient is 1.2. 

Several other approaches have been proposed for determining electronegativities but only two of these will be mentioned here. Mullikcn proposed an electronegativity scale based on taking x be the average of the E and Z values of the element. Allred and Rochow proposed that eleetronegativity be defined as the force exerted by the nucleus on the valence electrons,. so that the Allred-Rochow electronegativity. can be calculated from the... 

It is very often necessary to estimate values of electrical-effect parameters for groups for which no measured values are available. This is of particular importance with regard to substituents in which arsenic, antimony or bismuth is the central atom. It has long been known the electrical-effect parameters of substituents X whose structure can be written as MZ are a function of the electrical effect of the Z when M is held constant In later work it was shown that when Z is held constant and M is allowed to vary, the substituent constant is a function of the Allred-Rochow electronegativity of M, Xm and the number of Z groups, n. It has been shown that when both Z and M vary, values for all groups of the type X = MZ Z Z are given by an equation of the form ... 

Equations 24, 26 and 27 apply to M Z =M Z and M =M Z groups. They may be used to calculate substituent constants for both frons-vinylene and vinylidene groups. For these groups the value of xm Xm > Allred-Rochow electronegativity of carbon is 2.50 and the value of the number of bonds in the M —bond, is 1. Thus, equations 24, 26 and 27 simplify to ... 

Thus, the electronegativity values are linearly dependent on a parameter similar to the polarizing action of the cation. The dependence of the solubility on the Allred-Rochow electronegativity is divided into two sharply bounded and practically linear plots with close slopes for 3d-elements and for alkaline-earth metal-oxides (see Fig. 3.7.13). Since the slopes of these dependences are close, different positions of these plots may be explained by different relationships between the nuclear charge and Z for metals characterized by different electronic configurations. From the above-said it may be concluded that in high-temperature alkali-metal halide melts a correlation of metal-oxide solubilities with the crystal-lochemical radii of the cations is considerably simpler, i.e. it does not require the introduction of any corrections of the nucleus charge, such as Z. ... 

Allred and Rochow chose as a measure of electronegativity of an atom the electrostatic force exerted by the effective nuclear charge eff (estimated from Slater s rules, see Box 1.6) on the valence electrons. The latter are assumed to reside at a distance from the nucleus equal to the covalent radius, r y, of the atom. Equation 1.36 gives the method of calculating values of the Allred-Rochow electronegativity, X... 

Fig. 7 Correlation between surface/subsurface frequency shifts (with respect to the Pt NMR reference H2PtCl6) and the Allred-Rochow electronegativity. The dashed horizontal line indicates the Knight shift of bulk platinum atoms. The solid straight lines are linear fits to the surface and subsurface shifts as a function of the electronegativity. Both have values of ca. 0.92. (Reproduced with permission from Ref [10], Copyright by American Chemical Society 2000.)...

Slater s rules are used to calculate the values of the Allred-Rochow electronegativity coefficients. In the calculations of the screening constants for the elements, Allred and Rochow applied Slater s rules to all the electrons of the atoms. In normal use of the rules, one of the outermost electrons is chosen to be the one feeling the force . Applying Slater s rules in the normal way leads to a best fit with the older accepted Pauling/Mullikcn values for electronegativity coefficients, given by the equation ... 

Covalent radii and electronegativity values from Ref. 10. AEN is the difference in electronegativity between the two elements using the Allred-Rochow scale. 

Covalent radii and electronegativity values (Allred-Rochow) are abstracted from Huheey, J. E. Keiter, E. A. Keiter R. L. Inorganic Chemistry, 4th ed. Harper Row New York, 1993 pp. 187 and 292. 

Table 1.2 Electronegativity Values According to Allred and Rochow...

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