Pearson s rule

In principle, this constitutes a strong point for the HSAB treatment. In practice, it is its Achille s heel. The reason is that Pearson s rule (a hard reagent attacks preferentially a hard site) should apply only to the reactive species in the transition state. 

The writer36) analyzed the behaviour of manganese in the various oxidation states from Mn(-I) to Mn(VII) and suggested the modification of Pearson s rule that... 

PCS = photon correlation spectroscopy = QELS see electromagnetic radiation Pearson s rule 3.185 Peclet number 1.7.97 pendant drop see drop, pendant penetration depth (evanescent waves) [1.7.10.121 period (of a wave) 1.7.4 permeability see porous plugs perpetutd motion 1.2.8 of second kind 1.2.23... 

Whereas the hard-hard interactions in Pearson s Dual Rule essentially are Coulombic, the soft-soft interactions frequently invite the comment that they represent polarizability. This word has connotations for the chemists which are not very different... 

In addition to the reinterpretation of Pauling s rules developed by Burdett and McLarnan (1984), there have been a number of other studies related to various aspects of these standard rules. Due to the substantial number of errors in classifying AB compounds in terms of ionic radius ratio (e.g., Phillips, 1970 Tossell, 1980b), there have been numerous attempts to create structure maps that have two atomic quantities as coordinates and that can provide a unique separation of the different structure types. Such atom quantities may be related primarily to size or energy or to some combination of the two. Some of the most important such approaches are those of Mooser and Pearson (1959), Phillips (1970), and Simons and Bloch (1973). 

J0rgensen proposed the principle of symbiosis with respect to hard and soft acid-ba.se behavior. This rule of thumb states that hard species will tend to increase the hardness of the atom to which they are bound and thus increase its tendency to attract more hard species. Conversely, the presence of some soft ligands enhances the ability of the central atom to accept other soft ligands. In terms of the electrostatic versus covalent picture of Pearson s hard and soft or Drago s and... 

The coordination is depending on the nature of the monomer and on the metal of the catalyst and follows generally the hard-soft acid-base rule of the Pearson s classification. 

For example, the well known Walsh rules 52 for molecular geometries were first based on simple molecular orbital arguments depending on orbital overlap and the difference between s and p atomic energies. Pearson 51 re-interpreted them from a symmetry point of view as the following example will show. 

Most of the (b)-acceptors so far discussed are more or less typical metal ions. For these, a large number of -electrons seems to be indispensible in order to bring about (b) -behaviour, as has already been stated. This evidently also applies to the few non-metal (6)-acceptors entered in Table 1, which are iso-electronic with metal (ft)-acceptors. For many non-metal acceptors, however, which have lately been classified as (b) by Pearson (2) the -electrons are either non-existant (e. g. HO+, RO+ and various benzene derivatives), or too well shielded to be of any consequence for the bonding (e. g. RSe+, RTe+, Br+, I+). Evidently all these acceptors are able to form essentially covalent bonds using only the electrons available in the s- and -shells. It is therefore not surprising that they often constitute exceptions from the rule that a lowering of the oxidation state means more of class (a)-character for elements beyond the transition series. This rule evidently ceases to be valid as soon no d-electrons participate in the formation of the covalent bond. 

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