Pearson’s classification

Table 3.20 Pearson s Classification of Lewis Acids and Bases...

It would be closing the eyes to half of the content of Pearson s classification to consider only systems where complex formation constants have (or can) been measured. If one wants to stabilize unusually low oxidation numbers, it is well-known that the best chances are to select the ligands among the class H-, 1 ,... 

For the chemist, there is no doubt whatsoever that the softness of the central atoms do not follow the a values, though this is valid for the halides. Thus, Cs(I) is more polarizable than Ag(I), Ba(II) more than Cd(II) and La(III) more than In(III) in disagreement with Pearson s classification. Said in other words, the chemical bonding involves far stronger perturbations than the linear electric fields inducing the a-polarizability. 

Originally, it was argued (75, 75) that 7(K2 3/2) closely follow the order of anions in Pearson s classification (76, 77) of hard and soft bases, with the conspicuous exception of iodide. This statement is slightly less clear-cut when cor-... 

Bismuth occurs in nature as Bi203 and as bismuth sulfide ores. This reflects the fact that Bi(ni) is an intermediate acid in the Pearson s classification of hard and soft acids-bases (see Hard Soft Acids and Bases). Interestingly, over-the-counter dmg formulations involve primarily oxygen-donor ligands snch as salicylate, citrate, and nitrate, while in vivo reactions... 

Pearson s classification is insensitive to both chemical compositions and stoichiometries of metallic alloys. It is quite useful because all known intermetallic crystal structures are grouped according to their structural symbols, which are quite simple. Thus, once the symmetry and the content of the unit cell of a new alloy phase have been established, it only makes sense to search for potentially isostructural compounds among those that have identical Pearson s symbols. 

Pearson s classification was intended to simplify and illuminate the problem, but it did not, and was not intended to, explain it. We are now in a position to point out one of the ways in which molecular orbital theory does explain it.19 First of all let us look at thermodynamic acidity and basicity, namely the way in which the equilibrium... 

Pearson s classification of hard and soft acids comes from a consideration of a series of donor atoms placed in order of electronegativity ... 

Phosphines PX3 (X = alkyl or aryl, RO-, halogen) generally exhibit a distinct preference for coordination with heavy Group VIII metals, i.e., those defined by Chatt, Ahrland, and Davies (51) as having class b character. Alternatively, using Pearson s classification (259), phosphines are considered to be soft bases and thus readily coordinate with soft acids, e.g.. Group VIII metal ions and other transition metals in low- or zero-oxidation states. 

Pearson s classification of ligands and metals as soft acids and bases can readily be applied to metal 71-complexes. Here the term soft means polarizable and hard indicates nonpolarizable. According to the general guideline,... 

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. 

The compositions, space groups and Pearson s classification symbols for the members of this series are listed in table 2. For n = 00 the octahedra are replaced by an infinite three-dimensional framework of centred trigonal prisms that is known as the Alc B -type. This type is found with 67(75) 0( 2581.75)2 (GIB, 65) and many other R(T, Si)2 or R(T, Ge)2 compounds. Since, in the known examples of R-Ni-Si alloys, no regularity of the Ni/Si ratio at the prism centre sites is shown, and since the homogeneity ranges of these phases have not been investigated, the last two digits of the composition code have not been specified in table 2. Thus the compositions of the different members of this series can be correlated only by their content of rare earth (or non-rare-earth) element. 

LC Code Ternary compound Pearson s classif. symbol Space group Relation to binary types Class m n number of slabs of type Hypothetical composition for order a) Hypothetical composition Fig. 

LC Code Hypo- thetical formula Pearson s classif. symbol Space group m n... 

Code Compound Structure type Pearson s classif. symbol Space group a-Mn unit C + TT+T+CO bcc unit C + CB + OH + CO y-brass unit IT-l-OT-l-OH-l-CO CajAgg unit OH + CB + CO Ti2Ni unit OH + T + CO... 

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