APPLICATIONS OF HSAB PRINCIPLE

APPLICATIONS OF HSAB PRINCIPLE 

In hydrogen bonding: The strong hydrogen bond is possible in cases of  H₂O, NH₃ and HF, since the donor atoms (F, O & N) are hard lewi bases and their interactions with partially positively charged H, which is a hard acid, are stronger.

Linkage of ambidentate ligands to metal atoms: The ambidentate ligand, SCN^- can bind either by S end or N end. The bonding mode can be determined by using HSAB principle. 

It bonds through sulfur atom (soft base) when bonded to Pt²⁺, a soft acid. 

However it bonds through nitrogen atom (a hard base) when linked to Cr³⁺, a hard acid. 

Site preference in organic reactions: RCOX is a hard acid and reacts with the nitrogen end of SCN^- ion to form an acyl isothiocyanate.

Whereas the softer methyl group bonds to the Sulfur atom and forms methyl thiocyanate.

 

Inorganic reactions: HSAB principle is used to predict the outcome of some of the reactions. 

1) The following reaction is possible because As is softer than P and I^- is softer than F^-. 

Remember that both As and P are soft but relatively As is softer. 

2) The following reaction is possible since Mg²⁺ is harder acid than Ba²⁺ and O^2- is harder base than S^2-. 

Precipitation reactions: The softer acids like Ag⁺, Hg⁺, Hg²⁺ etc., and border line acids like Fe²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Pb²⁺ etc., can be precipitated as sulfides from their aqueous solutions since S^2- ion is a softer base.