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1.2.4 THE“CLASS (A)” AND “CLASS (B)” CLASSIFICATION
OF THE METAL IONS AND LIGANDS BY AHRLAND, CHATT,
AND DAVIES
The classification of acceptors and donors in chemical reactions accord-
ing to their ability to accept/or donate, especially in inorganic reactions,
is a very old and intriguing problem [13-16]. Aquated metal ions tend to
group into two categories in their reactions with halides ions. One class
favors reactions with small, unpolarizable bases such as fluoride ion and
other favors reactions with lag, polarizable bases such as iodide ion. Swar-
zenbeach [15] referred to these classes of metal ions as class A acceptors
and class B acceptors, respectively. In 1958, Ahrland et al. [19(a), (b), (c)]
extended the categories of metal ions through consideration of reactions
with ligands other than halides. They [19(a)] correlated the preference of
certain metal ions to react with certain ligands with the electron acceptance
tendencies of the metal ions from the ligands and classified electron pair
acceptors into two types—class (a) acceptor and class (b) acceptor—and
as a consequence electron pair donors are also classified into two types:
class (a) and class (b).
Class (a) acceptors or class (a) acids are those that can form the most
stable complexes with the fi rst ligand atom of each group. Class (a) accep-
tors include the following:
1. Alkali metal cations: Li
+
to Cs
+
2. Alkaline earth metal cations: Be
2+
to Ba
2+
3. Lighter transition-metal cations in higher oxidation states: Ti
4+
,
Cr
3+
, Fe
3+
, Co
3+
4. The proton, H
+
Proton and most metals in their common valence states belong to class
(a) and, therefore, the affi nities of ligand for class (a) acceptors tend to
run nearly parallel to their basicities, except when steric and other factors
intervene.
The affi nity toward the donor ligands of a class (a) acceptor follows the
following trend:
(i) For group 15: N > P > As > Sb > Bi
(ii) For group 16: O > S > Se > Te
(iii) For group 17: F > Cl > Br > I
The class (b) character appears to depend on the availability of elec-
trons from the lower d-orbitals of the metal for dative pi-bonding. The
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