Formation of a stable complex between the vacant orbital of an acid (metal ions)and filled unshared orbital of a base ( ligand ) depends upon the nature of the metal ion and the ligand*.

For a long time co-ordination chemist’s had recognized the fact that certain legends* form more stable complexes with metal ions such as Ag⁺ , Hg²⁺, Pd²⁺ and pt²⁺ but other legends show preference for metal ions such as Al³⁺, Ti⁴⁺ and Co³⁺. Therefore, metal ions and legends were classified as belonging to type (a) or type (b) according to their preferences.

Class (a) metal ions include: alkali metal ions, alkaline earth metal ions, B, Al, is ions, lighter transition metal ions (first row 3d metal ions, such as Ti⁴⁺, Cr³⁺Fe³⁺, Co²⁺ etc.) and H⁺ .

Class (b) metal ions include: Heavy transition metal ions(2nd and 3fd row i.e. 4d and 5d metal ions, such as, Hg²⁺,Pd²⁺,Pt²⁺, Cd²⁺ etc.) and those in lower oxidations states such as Cu⁺, Ag⁺, Hg+, etc.

Legends have been classified as belonging to either type (a) or type (b) depending upon their preference for class (a) or (b) metal ions, respectively.

Class (a) legends include those molecules or ions where the donor atom is N, O, F, CI etc . These legends combine preferably with type (a) metal ions or atoms (AI³⁺, Mg2+, Cr³⁺, Fe³⁺, etc.)

Class (b) legends include those molecules or ions where the donor atom is P, As, S, Se, I, etc. These legends combine preferably with type (b) metal atoms or ions (Pd²⁺, Pt²⁺, Hg²⁺, Cd²⁺, Ag⁺, etc.)

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