Class A and Class B metal Ions
Class A and Class B metal Ions
As early as 1950s, Ahrland, Chatt and Davies had classified metals on the basis of stability of complexes as class A and class B. Metals which forms more stable complexes with the first element of each group (N,O,F) and form unstable complexes with second members of the group (P,S,Cl). Metals which forms more stable complexes with second members of the group (P,S,Cl) and forms unstable complexes with the first members (N,O,F).Class A metals are hard metal ions, less polarized, and tend to form hard acids and ionic bonds with hard ligands (e.g., O donors). Alkali, alkaline earth metals, lighter transition metal ions with higher oxidation states (cr+3, Ti+3, Co+3, Fe+3 etc), B, Al, Si ions lighter metal ions with no d electrons or less number of d electrons are of class A metals. These metals, such as iron, aluminium, titanium, sodium, calcium, and the lanthanides, would rather bond with fluorine than iodine. They form stable products with hard bases, which are bases with ionic bonds.
Class B metals are soft metal ions, more polarized, and preferentially form covalent bonds with soft ligands (e.g., S and N electron donors). Lower transition metal cations in low oxidation state and higher transition metal cations in all the oxidation states are of class B metals. These metals, such as lead, gold, palladium, platinum, mercury, and rhodium, would rather bond with iodine than fluorine. They form stable products with soft bases, which are bases with covalent bonds.
Borderline metal ions share a ligand-binding behavior intermediate between the class A and class B metals.
Similarly ligands were also divided into two classes-
1. Class A: Ligands having donor atom of high electronagativity, low polarisibility and having tendency to coordinate with class A metals.
F > Br > Cl > I > O > S > Se > Te > N > P > As > Sb
2. Class B: Ligands having donor atom of low electronegativity, high polarisibility and having tendency to coordinate with class B metals.
F < Br < Cl < I < O < S < Se < Te < N < P < As < Sb
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