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Hard and Soft Acids and Bases

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Hard and Soft Acids and Bases Hard and soft acids and bases Introduction: Metal complexes stability depends upon both the type of metal ion and ligand. Arland Chatt and Devis devided the metal ions into two classes a & b according to their preference for various ligands. They are 1. Class (a) metal ions: This group includes alkali, alkaline earth metals, lighter transition metal ions with higher oxidation states (cr3+, Ti3+, Co3+, Fe3+ etc), B, Al, Si ions lighter metal ions with no d electrons or less number of d electrons. These metals have small size, high polarizing power, high oxidation state and form stable complexes with first member of each group (N,O,F) and form unstable complexes with second members of the group (P,S,Cl). 2. Class (b) metal ions: This group of metal ions include heavier transition metal ions in the lower oxidation state + + 2+ 2+ 2+ (Cu , Ag , Hg2 , Pt ,Pb etc), metal ions with low valancy. They have large size, low positive or zero oxidation state and low polarizing power. They form stable complexes with second members of the group (P,S,Cl) and unstable complexes with the first members (N,O,F). 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) metal ions. F<Br<Cl<I<O<S<Se<Te<N<P<As<Sb Pearson’s HSAB principle: As metal ions are electron acceptors & ligands are electron donors they can be considered as Lewis acid and bases. Pearson named class (a) & class(b) metal ions as Hard and soft acids, and ligands as hard and soft bases. To explain the stability of the complexes he proposed HSAB principle as “A hard acid prefers to form bond with hard base while soft acid prefers to form bond with soft base”. Bonding in hard-hard and soft -soft interactions Various theories are available to explain the bonding in these interactions. 1. Electrostatic and Ionic interactions (hard-hard): Hard acids have vacant ‘d’ orbital to accept the electrons from hard bases. This results in development of positive and negative charges with ionic interactions. 2. Covalent character (soft-soft): Most of the soft acids have 6 to 10 ‘d’electrons in the outermost shell. These‘d’ electrons can be easily polarized. Soft bases can also be easily polarized, so the interactions between them are covalent. 3. 휋- bonding interactions (soft-soft): According to this theory soft acids have ‘d’electrons which can be donated to empty ‘d’ orbitals of ligand 푡표 푓표푟푚 휋 bond. 4. Pitzer’s theory: The forces between the soft-soft interactions are London, Vander Waals forces. Applications of HSAB theory: • Predicts the existence of the complex - Ex: [AgF2] does not exist as it is made of soft acid - and hard base. [AgI2] exists as it made of soft acid and soft base. • Predicts the feasibility of the reaction The feasibility of the following reaction can be predicted as follows LiI + CsF → LiF + CsI h-s s-h h-h s-s from the above equation we can say that the reaction is feasible in forward direction because the products formed are hard-hard, soft-soft combination and are very stable according to Pearson’s theory. • Predicts coordination in complexes of ambidentate ligands The ligand [SCN]- coordinates through N-atom to form complex [Co(NCS)]2- as it results in hard-hard combination. The same ligand coordinates through s- atom in the complex [Pd (SCN)]2- as it forms a stable soft-soft complex. • Predicts solubility It is generally observed that hard solutes dissolve in hard solvents and soft solutes dissolve in soft solvents. • Predicts the mechanism of reactions This rule is used to understand the course of many reactions. For example in the following reaction + + H + CH3HgOH → H2O + CH3Hg (1) + + H + CH3HgSH → H2S + CH3Hg (2) the reaction(1) proceeds to right as hard acid H+ combines with hard base OH- to form water. The reaction (2) proceeds to the left as soft acid + - CH3H combines with soft base HS . • Predicts the occurrence of minerals in their forms It is found that hard metals ions of Ca+2, Mg+2, Al+3 3- 2- combine with hard bases like CO3 , O to form carbonates and oxides, while soft metal ions like Ag+, Cu2+, Hg2+ combine with soft bases like S2- to form sulphides. • Poisoning of catalysts Soft metals like Pd & Pt act as catalysts in many reactions. To poison these catalysts soft bases like P,As, CO and olefins are used which block the active sites by adsorbing on the surface of the calayst. • Predicts the behavior of BF3 and BH3 Although both are tri halides of boran BF3 is acidic while BH3 is nuetral, this can be explained as follows. - The presence of F in BF3 makes it easy to add to a hard base NH3 making it an acid. - In BH3, H donates the negative charge to boran atom making the molecule neutral. • Effect of variable valancy It is found that with increase of oxidation number of metal ion hardness increases. • Relative strengths of halogen acids The strengths of various halogen acids in aqueous solutions are HF<HCl<HBr<HI. This can be explained as follows HF is formed by combination of hard acid and hard base, so it is highly stable. The reaction of halides with water is + - HX + H2O → H3O + X As the stability of hydrogen halides decreases from HF to HI their acidic properties increases in the reverse order. .
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