hard and soft acid and bases

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1. DEPARTMENT OF PHARMACEUTICAL SCIENCES Dr. HARISINGH GOUR CENTRAL UNIVERSITY, SAGAR (M. P.) SUPERVISED BY: PROF. ASMITA GAJBHIYE PRESENTED BY: KALYAN GHOSH M.PHARM(2 nd SEM) Y18254022 HARD AND SOFT ACID AND BASES

2. CONTENTS 1.HISTORY 2.DEFINATION 3.POLARIZABILITY 4.PRINCIPLE 5.KEY CHARACTERISTICS 6.FROMPERIODIC TABLE 7.PEARSON’S ABSOLUTE HARDNESS 8.SOME EXAMPLES 9.APPLICATION OF HSAB PRINCIPLE 10.DRAWBACK OF PEARSON’S HSAB MODEL 11.CONCLUSION 12.REFERENCE

3. HISTORY The Lewis concept of acids and bases involves covalent interaction to form a covalent (coordination) bond : An acid = an electron pair acceptor A base = an electron pair donor 1963 : Ralph Pearson introduced the hard-soft-acid-base (HSAB ) principle. “Hard acids prefer to coordinate the hard bases and soft acids to soft bases”. This very simple concept was used to rationalize a variety of chemical information, as an attempt to unify inorganic and organic reactions. 1983 : By Ralph Pearson and Robert Parr, the qualitative definition of HSAB was converted to a quantitative one by using the idea of polarizability . A less easily polarized atom or ion is “ hard” and a more easily polarized atom or ion is “soft”

4. DEFINATION Hard and Soft Acids and Bases (HSAB) Theory  is a qualitative concept introduced by Ralph Pearson to explain the stability of metal complexes and the mechanisms of their reactions. TYPES In short, Hard acids and bases are small and non-polarizable, whereas Soft acids and bases  are larger and more polarizable. 

5. POLARIZABILITY The capacity of a group of atoms in a molecule and/or an ion to polarize its electron. CHEMICAL SPECIES HARD SOFT A MORE EASILY POLARIZED SPECIES ELECTRON SHARING (COVALENT)INTERCTION A LESS EASILY POLARIZED SPECIES ELECTROSTATIC (IONIC) INTERCTION

6. 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) 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.

7. KEY CHARACTERISTICS HARD ACIDS HARD BASES SOFT ACIDS SOFT BASES • Low polarizability • High positive charge • Small size • Not easily oxidized • Low polarizability • Spread donor orbital • High electronegativity • Not easily oxidized • High polarizability • Low positive charge • Large size • Easily oxidized • High polarizability • Diffuse donor orbital • Low electronegativity • Easily oxidized

8. FROM PERIODIC TABLE

9. PEARSON’S ABSOLUTE HARDNESS = η 1) Quantitative method to measure hardness and softness, predict matches 2) Formula uses Ionization energy (I) and Electron Affinity (A) 3)Defines Hardness as a large difference between I and A I = HOMO energy A = LUMO energy 4) Sof tness = s = 1/h 5)Halogens as an example:- a ) Trend in h parallels HOMO energy (LUMO’s are about the same) b ) F = most electronegative, smallest, least polarizable = hardest c ) Cl Br I hardness decreases as HOMO energy increases

10. SOME EXAMPLES A VERY SOFT METAL ION GOLD (Au) The softest metal ion is the Au+( aq ) ion. It is so soft that the compounds AuF and Au2O are unknown. It forms stable compounds with soft ligands such as PPh3 and CN-. The affinity for CN- is so high that it is recovered in mining operations by grinding up the ore and then suspending it in a dilute solution of CN Au C N Typical linear coordination geometry found for Au(i) in [Au(CN)2]- and [Au(CN)(PPh3)] P Au Ph

11. A VERY HARD METAL ION An example of a very hard metal ion is Al(III). It has virtually no affinity in solution for heavier halides such as Cl -. Its solution chemistry is dominated by its affinity for F- and for ligands with negative O-donors. AMBIDENTATE LIGAND Thiocyanate (SCN-) is a particularly interesting ligand . It is ambidentate , and can bind to metal ions either through the S or the N. Obviously, it prefers to bind to soft metal ions through the S, and to hard metal ions through the N. This can be seen in the structures of [Au(SCN)2 ]- and [Fe(NCS)6]3- in FIGURE SHOWING:- Thiocyanate Complexes a ) N-bonding in the [Fe(NCS)6]3- complex with the hard Fe(III ) ion, and b ) S-bonding in the [Au(SCN)2 ]- complex With the soft Au(I) ion

12. 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 lewis bases and their interactions with partially positively charged H, which is a hard acid , are stronger. 2) 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. 3) 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 .

13. 4) 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-. As 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 O2- is harder base than S2-. 5) 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 S2- ion is a softer base.

14. 6)NUCLEOPHILIC ADDITION OF -UNSATURATED CARBONYL COMPOUNDS: α,β The α,β -unsaturated carbonyl compounds have two type of carbon electrophilic centers . 1 ) Carbonyl carbon - Hard electrophilic center. 2 ) β-carbon - Soft electrophilic center. Therefore, the hard nucleophiles like Grignard reagents attack the carbonyl carbon ( hard electrophile ) resulting in 1,2-nucleophilic addition to C=O group. Whereas, the soft nucleophiles like Lithium organocuprates , thiols etc., attack the   β-carbon (soft nucleophile ) resulting in 1,4-conjugate addition.

15. DRAW BACKS OF PEARSON'S HSAB MODEL The Pearson's HSAB theory is in direct contradiction with Fajan's rules. For example, Fajan’s rule predict the nature of Beryllium salts to be more covalent. But according to 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 . Similarly 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 .

16. CONCLUSION Hard electrophiles tend to react with hard nucleophiles and soft electrophiles tend to react with soft nucleophiles . The hard-hard interaction is governed primarily by electrostatic attraction between the positively-charged electrophiles and negatively-charged nucleophiles . The soft-soft interaction is governed primarily by the mixing (overlap) of orbitals between the LUMO of the electrophiles and the HOMO of the nucleophiles . THE HSAB THEORY STATES THAT: We have also seen examples of hard/soft electrophiles and nucleophiles , and the role played by HSAB theory in conjugate addition. Also the HSAB theory is only one of several factors that determines the outcome of a nucleophilic addition to α,β -unsaturated carbonyl compounds.

17. REFERENCE 1. Jolly , W. L. (1984).  Modern Inorganic Chemistry . New York: McGraw- Hill .  ISBN   978-0-07-032760-3 . 2.E .-C. Koch, Acid-Base Interactions in Energetic Materials: I. The Hard and Soft Acids and Bases (HSAB) Principle-Insights to Reactivity and Sensitivity of Energetic Materials,  Prop.,Expl.,Pyrotech . 30   2005 3.Pearson , Ralph G. (1963). "Hard and Soft Acids and Bases".  J. Am. Chem. Soc.   85  (22): 3533–3539.  doi : 10.1021/ja00905a001 4.R . G. Pearson, Chemical Hardness - Applications From Molecules to Solids, Wiley- VCH , Weinheim , 1997, 198 pp