Pearson's HSAB Theory

HSAB concept is an initialism for "hard and soft (Lewis) acids and bases". Also known as the Pearson acid-base concept, HSAB is widely used in chemistry for explaining stability of compounds, reaction mechanisms and pathways.

1. TOPIC- PEARSON’S HSAB CONCEPT SUBMITTED BY – LARA SAHU ROLL NO. – 19 BSC BED 6 TH SEMESTER RIE Bhopal CHEMISTRY ASSIGNMENT

2. Pearson’s HSAB concept. In 1963 R. G. Pearson extended and generalized the qualitative correlation between Lewis acids and Lewis bases by classifying them into two categories Hard and Soft. This qualitative concept explains the stability of metal complexes and the mechanisms of their reactions This concept is also known as Hard and Soft Acids and Bases (HSAB) Theory.

3. Lewis acids and bases can be classified by designating them as hard or soft. Acids and bases are not strictly hard or soft, since many ions and compounds are classified as intermediate. The categorizations are based on three factors : (1) charge density, or charge-to-size ratio ( 2) polarizability (3) covalent vs. ionic nature of interactions.

4. Hard Acids/Bases: In general, "Hard" acids and bases have a  high charge density , are  not very  polarizabl e , and form bonding interactions that are more  ionic  in nature. These atoms and ions tend to have high charge and/or small radius . Typical Hard Acids:  metal ions with high positive charges and smaller ionic sizes tend to be hard acids. Early transition metal ions in the 3d series tend to be hard Lewis acids . Typical Hard Bases : Small anions and neutral molecules; heteroatoms of the second row of the periodic table are typically hard (N,O,F). Some examples of hard acids and bases include: H + , O 2- , OH - , F - , Fe 3+ , and Al 3+ . Oxygen atoms are always hard, and N atoms are  usually  hard.

5. Soft Acids/Bases: In general, "Soft" acids or bases have a  low charge density , are  more polarizable , and form bonds that are more  covalent  in nature. These atoms/ions tend to have low charge and/or large radius . Typical Soft Acids : Transition metals with (+1) charge (such as Cu + ) or that are in the late 4d and 5d series (like Cd 2+  and Hg 2+ ), are classified as soft. Soft acids often include transition metals in the second and third row of the periodic table that have a +1 or +2 charge, as well as late transition metals (especially those in the 4d and 5d series) with filled or almost completely filled  d   orbitals . Typical Soft Bases:  Larger anions and neutral molecules. For example, I -  and S 2-  are soft bases.

6. Intermediate Acids/Bases: These are acids and bases with intermediate character, between hard and soft. For example, trimethylborane , Fe 2+ , and Pb 2+   cations are intermediate acids, and pyridine and aniline are examples of intermediate bases.

7. HSAB Principle   According to HSAB concept, hard acids prefer binding to the hard bases to give ionic complexes, whereas the soft acids prefer binding to soft bases to give covalent complexes. It is sometimes referred to as Hard-Soft Interaction Principle (HSIP).

8. The large electronegativity differences between hard acids and hard bases give rise to strong ionic interactions. The electronegativities of soft acids and soft bases are almost same and hence have less ionic interactions. i.e., the interactions between them are more covalent. The interactions between hard acid - soft base or soft acid - hard base are mostly polar covalent and tend to be more reactive or less stable. The polar covalent compounds readily form either more ionic or more covalent compounds if they are allowed to react.

9. APPLICATIONS OF HSAB PRINCIPLE  There are numerous applications of the HSAB principle. It helps in understanding organic reaction mechanisms, metal- ligand interactions in metal complexes, ore processing in metallurgy, precipitations in qualitative analysis etc. In hydrogen bonding:  The strong hydrogen bond is possible in cases of  H 2 O, NH 3 , and HF, since the donor atoms (F, O & N) are hard Lewis bases and their interactions with partially positively charged H, which is a hard acid, are stronger.

10. Linkage of ambidentate ligands to metal atoms: It is one of the important applications of the HSAB principle. The SCN -  ligand is an ambidentate ligand and can be S-bound to metal (M-SCN) and referred to as thiocyanate or can be N-bound to metal (M-NCS) and is referred to as isothiocyanate. The choice among S-bound or N-bound is decided by soft or hard acid-base behavior. S is a comparatively soft base than N atom. Hence soft metal ions are S-bound while hard metal ions are N-bound. 1 ) SCN -  bonds through sulfur atom (soft base) when bonded to Pt 2+ , a soft acid.  2 ) It bonds through nitrogen atom (a hard base) when linked to Cr 3+ , a hard acid. 

11. Solubility in water:  The compound formed due to soft acid-soft base combination is more covalent and less soluble in polar solvents like water. For example, Silver iodide, AgI is insoluble in water as it has covalent nature since it is the combination of soft acid, Ag +  and soft base, I - . On the other hand, Lithium iodide, LiI is the result of a combination of Li +  (hard acid) and I -  (soft base). Thus it is polar covalent and thus soluble in water.

12. Site preference: 1) RCOX is a hard acid and reacts with the nitrogen end of SCN -  ion to form an acyl isothiocyanate .  2) Whereas the softer methyl group bonds to the Sulfur atom and forms methyl thiocyanate.

13. LIMITATIONS OF PEARSON'S HSAB MODEL Pearson's HSAB theory is in direct contradiction with Fajan's rules. For example, the later predict the nature of Beryllium salts to be more covalent. But according to the HSAB principle, the Be 2+  ion is hard acid and is expected to show charge controlled bonding that results in more ionic nature for beryllium compounds. But this is not true. Since hydrogen ion, H +  is a hard acid and hydride ion, H -  is a soft base, according to HSAB principle the interactions between them must be polar covalent and H 2  must be unstable. Indeed H 2  is a stable molecule with pure covalent nature.

14. Thank You.