Ahrland classification

The concept of soft and hard acids and bases (7), which is in effect an extension of the Chatt-Ahrland classification (2) of A and B metals, is applied in the 1963 paper by Pearson (7) particularly to equilibria involving mainly inorganic systems. This paper follows an earlier discussion by Edwards and Pearson (3) of the Swain-Edwards equation (4) (1) for nucleophilic reactivity. 

Analogous to the classification of Lewis acids and bases in hard and soft species, Ahrland et al. have su ested a division of donors and acceptors into classes a and 6. See Ahrland, S. Chatt, J. Davies, N.R. Quart. Rev. 1958, 77, 265... 

Table 2.4. Classification of acceptor atoms in their normal valent states (Ahrland, Chatt Davies, 1958)... 

They indicated that the softness parameter may reasonably be considered as a quantitative measure of the softness of metal ions and is consistent with the HSAB principle by Pearson (1963, 1968). Wood et al. (1987) have shown experimentally that the relative solubilities of the metals in H20-NaCl-C02 solutions from 200°C to 350°C are consistent with the HSAB principle in chloride-poor solutions, the soft ions Au" " and Ag+ prefer to combine with the soft bisulfide ligand the borderline ions Fe +, Zn +, Pb +, Sb + and Bi- + prefer water, hydroxyl, carbonate or bicarbonate ligands, and the extremely hard Mo + bonds only to the hard anions OH and. Tables 1.23 and 1.24 show the classification of metals and ligands according to the HSAB principle of Ahrland et al. (1958), Pearson (1963, 1968) (Table 1.23) and softness parameter of Yamada and Tanaka (1975) (Table 1.24). Compari.son of Table 1.22 with Tables 1.23 and 1.24 makes it evident that the metals associated with the gold-silver deposits have a relatively soft character, whereas those associated with the base-metal deposits have a relatively hard (or borderline) character. For example, metals that tend to form hard acids (Mn +, Ga +, In- +, Fe +, Sn " ", MoO +, WO " ", CO2) and borderline acids (Fe +, Zn +, Pb +, Sb +) are enriched in the base-metal deposits, whereas metals that tend to form soft acids... 

Present understanding of Lewis acidity and basicity is based mainly on the A and B type classification of metal ions of Ahrland, Chatt,... 

Many observations concerning these trends had been made over the years, and in the 1950s S. Ahrland, J. J. Chatt, and M. Davies presented a classification of metals based on their preferred interaction with donor atoms. Class A metals are those that interact preferentially when the donor atom is in the first row of the periodic table. For example, they prefer to bond to N rather than P donor atoms. Class B metals are those which interact better when the donor atom is in the second row of the periodic table. For example, a class B metal would bond better to P than to N. The following table summarizes the behavior of metal atoms according to this classification. 

Ahrland s (1973) classification distinguishes incompletely hydrolyzed ions into types a and b. Cations of type a form stable complexes with electronic donors of the first row of the periodic chart through electrostatic interactions those of... 

Bare earths belong to class (a) of the Ahrland, Chatt and Davies classification [256], but complexes where oxygen acts as donor atoms... 

This treatment thus leads, in simple cases, to a quantitative representation of the Chatt-Ahrland principle (2), the classification into A and B... 

From the study of complexes, Ahrland Chatt and Davies arrived at a classification scheme comprising A-type and B-type central atoms corresponding to lithophilic and chalcophilic elements. 

Later workers have correlated the classification of elements in class (a) and class (b) with Pearson s principle of hard and soft acids and bases (HSAB principle) (see Hard Soft Acids and Bases) on the basis that class (a) metal ions are hard acids and class (b) are soft acids. Borderline elements in the Ahrland-Chart-Davies classification tend to be harder in the higher oxidation states and softer in their lower oxidation states. 

Another helpful scheme is the classification of ions with hard and soft Lewis acids by Ahrland (82) and Pearson (83). While a specific definition or scale of softness is not universally accepted, the general principles are clear. They can be used to give somewhat more information for predictions of what compounds or complex ions might be expected for the superheavy elements. 

Chatt appreciated early on that phosphine ligands are particularly effective in forming stable complexes with the platinum metals. Indeed his determination to put this characteristic on a more quantitative basis led to the important Class (a) and Class (b) classification scheme of metal-ligand interactions proposed jointly with Ahrland and Davies.This work was subsequently subsumed into Pearson s general Hard and Soft acid and base classification scheme. ... 

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. 

The fact that type b/soft metal ions (for our purposes those that form stronger coordinate bonds to S than to 0) have their own distinctive set of coordination preferences has clear consequences for both the distribution of these metals in the mammalian body and the most effective reagents for use in facilitating their removal. Their toxicity is presumably due to the combined action of their coordination preferences and their stereochemistry in the determination of those biological binding sites which are preferentially attacked and subsequently inactivated. For purposes of the present discussion we will use a combination of the classification schemes of Ahrland, Chatt and Davies (T) and Pearson ( 2). 

Their tendency to complex with type B metals is the opposite sequence. This led to a classification of ligands into type A ligands (e.g. F"), which tend to complex with type A metals, and type B ligands (e.g. I ), which tend to complex with type B metals. The classification was introduced by Ahrland in 1958. See also HSAB principle. 

The main criterion adopted by Pearson in his classification of Lewis acids is more or less the same as that employed by Ahrland et al. (4). Hard acids will form complexes whose stability sequences are... 

TABLE 14.6 Ahrland s classification of metal ions and ligands. ... 

The order Cl" > Br > T is in harmony with the inclusion of Ga in group 1 of AHRLAND s classification (3), for which this is the order of stability of halide complexes. 

For the special case of metal ions as acids, AHRLAND,CHATT and DAVIES (2) made a very important and useful classification. All metal ions were divided into two classes depending on whether they formed their most stable complexes with the first ligand atom of each group, class (a), or whether they formed their most stable complexes with the second or a subsequent member of each group, class (b). 

Based on the classification of cations and ligands proposed by Ahrland, Pearson (Pearson 1963, Pearson 1966) developed the type (a) and type (b) by explaining the differential complexation behavior of cations and ligands in terms of electron pair-donating Lewis bases and electron pair-accepting Lewis acids ... 

Table 5.2 Classification of acceptor species (after Ahrland, Chattand Davies)...

The rare earth ions belong to class (a) in the Ahrland, Chatt and Davies (1958) classification or to the hard acid class in the Pearson (1963) designation. Ions in this class bond to hard bases, primarily those which contain oxygen and nitrogen as the donor atoms, and bond only weakly to the soft bases which contain, for example, sulfur or phosphorous as the donor atoms. This means that by far the majority of rare earth complexes contain ligands which utilize at least one oxygen atom. As a result, the variety of types of complexes which can be formed easily with the rare earth ions is more restricted than that which is formed with the d-transition elements. 

Table VI gives the classification of a large number of Lewis acids into the three categories, hard, soft, and borderline. Hard and soft acids correspond to and are extensions of class a and class b acids in the earlier criterion of Ahrland, Chatt, and Davies.In general, acceptor atoms of hard acids are small in size, of high positive charge, and do not contain unshared electron-pairs in their valence shell (not all of these properties need be possessed by any one acid), leading to high electronegativity and low polarizability. On the contrary, soft acids have...

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