Hard soft acid-base theory

Inorganic Chemistry

1. Yashwantrao Chavan College of Science, Karad Department of Chemistry Dr. S. H. Burungale Hard-Soft Acid-Base Theory

2. Hard-Soft Acid-Base Theory 1.1 Classification of acids and bases as hard and soft. 1.2 Theoretical bases of hardness and softness 1.3 Pearson’s HSAB concept. 1.4 Acid – Base strength and hardness and softness. 1.5 Application and limitations of HSAB principle.

3. Limitations of Lewis Acids and Bases failed to provide a definite and uniform scale to measure the relative strength of acid and base In 1958 this was done by Ahrland , Chatt and Davies by classifying the acids or bases into general categories .Class – “a” and Class- “ b” According to them the two categories of metal ions (Lewis acids)are as follows –

4. Physical meanings behind HSAB 1  ionization energy & electronegativity 2 polarizability 3  HOMO-LUMO gap: large for hard species, small for soft species 4  soft-soft interactions imply covalency ; and hard-hard interactions are for ionic bonding

5. I) Class (a) – The metal ions which prefer to from stable complexes with the ligands having donor atom of the first members of Gr.15 th (N),16 th (O), and 17 th (F) in the periodic table . . Examples are – Alkali metals , Alkaline earth metals and the first row transition metals in high oxidation state (e.g.fe3+ ,Co3+.Etc. belong to class (a) acids ) .

6. ii)Class (b )- The other metal ions which prefer to from their most stable complexes with the ligands having donor atom of the lower members of Gr.15 th ( P,As,Sb ),16 th ( S,Se,Te ), and 17 th ( Cl,Br,I ) in the periodic table . . Lighter transition elements in low oxidation state and heavier transition elements ,say Cu+ , Ag+,Hg+,Pt2+,Pd2+ etc. act as class (b) acids.

7. Ahrland , Chatt and Davies Ahrland ,  Chatt , and Davies  explained the class a and class Class A Borderline Class B H+, Li+ ,Na+, K+ Fe2+ Co2+ Ni2+ Cu+,Ag+,Au+,TI+ Be2+,Mg2 +, Ca2 + Cu2+ ,Zn2+,Pb2+ Hg+,Pd2+,Cd2+, Cr2+,Cr3+,Al3+ SO2,BBr3 Pt2+,Hg2+,BH3 , Br2 SO3,BF3,BCl3 Br+, SO3,BF3,BCl3 HX(H-bonding) M0 (metal atoms) and bulk metals HX(H-bonding)

8. Class A Acceptor atoms are marked by: Class B Acceptor atoms are marked by: 1) Small size 1) Large size 2) High positive oxidation state. 2) Zero or low positive oxidation state 3) Absence of any outer electrons Which are easily excited to higher States. 3) Presence of several excitable valence shell electrons. 4) Absence of d-electrons. 4) With nearly full d-electrons. 5) Usually light metal ions. 5) Mostly heavy metal ions. 6) Know as Lewis acids which Are not easily polarizable Prefer to coordinate with hard 6 6) Known as Lewis acids and are easily polarizable. Prefer to coordinate with soft bases.

9. Hard Borderline Soft F-,OH-,H2O,NH3 NO2,SO3,Br- H-,R-,CN-,CO,I-, CO32-,NO3-,O2-, N3-,N2,C6H5N, SCN-,R3P,C6H6, SO42-, PO43-CIO4-,( Cl -) SCN- R2S. Lewis Base

10. Hard Soft 1) High electronegativity . 1) Low electronegativity 2) Low polarisability . 2) High Polarisability. 3) Presence of filled orbits ; empty orbitals may exist at high energy level. 3) Partially filled orbitals, empty orbitals are low- lying. 4) These are anions or neutral molecule known as Lewis bases or ligands, prefer to co-ordinate with hard acids. 4) these are anions or neutral molecules called similarly as Lewis bases or ligands, prefer to bind with soft acids.

11. PEARSON’S 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.

12. The class –‘a’ metals which are small and less polarizable, prefer to combine with non- metals or ligands which are also small and not very polarizable , pearson called such metals as Hard Acids and the corresponding ligands as hard Bases. the class ‘ b’metals having large size ,more or easily polarisable , prefers to combine with non-metals or ligands having similar properties Pearson called such metals as soft acids and the ligands as soft base The attempt of classification of acidsand bases as hard and soft by Pearson is known as Hard and soft Acids and Bases .(HSAB) or pearson’s concept

13. Hard metals which are small and less polarizable, prefer to combine with non- metals or ligands which are also small and not very polarizable , pearson called such metals as Hard Acids and the corresponding ligands as Hard B ases. Soft ’metals having large size ,more or easily polarisable , prefers to combine with non-metals or ligands having similar properties Pearson called such metals as soft acids and the ligands as soft base The attempt of classification of acidsand bases as hard and soft by Pearson is known as Hard and soft Acids and Bases .(HSAB) or pearson’s concept

14. Theorotical basis of hardness and softness Several theories have been given to explain the stability of complexes Formed by hard-hard and soft-soft interactions. Some important theories are : (a) Ionic and covalent bond theory (b) - Bonding theory ( c) Pitzer’s theory

15. Ionic bond is formed by the interaction of hard acids and hard bases (a) Ionic and covalent bond theory Covalent bond is formed by the interaction of soft acids and soft bases. Electrostatic force of attraction Internulcear distance is smaller Highly stable soft acids and soft bases have larger size. polarization effects stronger covalent bond high polarization

16. (b) - Π Bonding theory : Mulliken (1955) and chatt (1956) soft –soft interaction on the basis of π –bonding Soft acids have low oxidation state and have a large number of d- electrons. strong tendency to form π –bonds The polarization of soft acids and soft base also favour π -bonding.

17. (c) Pitzer’s theory : London dispersion energies stabilize a bond between two large polarizable atom These energies increase with an increase in the size and polarizability . soft-soft interactions are more stable as compared to soft-hard interaction.

18. Hard and Soft Acids and Bases The polarizability of an acid or base plays a role in its reactivity. Hard acids and bases are small, compact, and non- polarizable . Soft acids and bases are larger, with a more diffuse distribution of electrons.

19. Hard and Soft Acids and Bases In addition to their intrinsic strength , Hard acids react preferentially with hard bases, and soft acids react preferentially with soft bases.

20. Aqueous Solubility Silver Halides Compound solubility product AgF 205 AgCl 1.8 x 10 -10 AgBr 5.2 x 10 -13 AgI 8.3 x 10 -17 AgX (s) + H 2 O(l) ↔ Ag + ( aq ) + X - ( aq )

21. Solubility of Lithium Halides LiBr > LiCl > LiI > LiF LiF should have a higher ∆ solv than the other salts, yet it is the least soluble in water. This is due to the strong hard acid (Li + )/hard base (F - ) interaction.

22. Thiocyanate Bonding SCN - displays linkage isomerism as the ligand coordinates to metals via the sulfur or the nitrogen. Mercury (II) ion bonds to the sulfur (a soft-soft interaction) whereas zinc ion bonds to the nitrogen atom.

23. K for ligand exchange reactions Compare: [ MeHg (H 2 O)] + + HCl MeHgCl + H 3 O + K= 1.8 x 10 12 [ MeHg (H 2 O)] + + HF MeHgF + H 3 O + K= 4.5 x 10 -2

24. Hard and Soft Acids & Bases There have been many attempts to categorize various metal ions and anions to predict reactivity, solubility, etc. R.G. Pearson (1963) categorized acids and bases as either hard or soft (using K f values). Hard acids bond in the order: F - >Cl - >Br - >I - Soft acids bond in the order: I - >Br - >Cl - > F -

25. Hard and Soft Acids & Bases Hard acids or bases are compact, with the electrons held fairly tightly by the nucleus. They are not very polarizable. F - is a hard base, and metal ions such as Li + , a hard acid.

26. Hard and Soft Acids & Bases Large, highly polarizable ions are categorized as “soft.” Iodide is a soft base, and transition metals with low charge density, such as Ag + , are considered to be soft acids.

27. Hard and Soft Acids & Bases Hard acids tend to bind to hard bases. Soft acids tend to bind to soft bases.

28. Predict the solubility (high or low) of silver fluoride, silver iodide, lithium fluoride and lithium iodide using the hard-soft acid/base approach. Identify each Lewis acid and Lewis base, and categorize each as hard or soft.

29. Charge Density – Hard Acids Hard acids typically have a high charge density. They are often metal ions with a (higher) positive charge and small ionic size. Their d orbitals are often unavailable to engage in π bonding.

30. Charge Density – Soft Acids Soft acids typically have lower charge Their d orbitals are available for π bonding. Soft acids are often 2 nd and 3 rd row transition metals with a +1 or +2 charge, and filled or nearly filled d orbitals .

31. Acids Hard Acids Borderline Soft Acids H + , Li + , Na + , K + Be 2+ , Mg 2+ , Ca 2+ BF 3 , BCl 3 , B(OR) 3 BBr 3 ,B(CH 3 ) 3 BH 3 ,Tl + , Tl (CH 3 ) 3 Al 3+ ,Al(CH 3 ) 3 ,AlCl 3 ,AlH 3 Cr 3+ ,Mn 2+ , Fe 3+ , Co 3+ Fe 2+ ,Co 2+ ,Ni 2+ Cu + ,Ag + , Au + , Cu 2+ ,Zn 2+ ,Rh 3+ Cd 2+ ,Hg 2 2+ , Ir 3+ , Ru 3+ , Os 2+ Hg 2+ , Pd 2+ ,Pt 2+ , SO 3 SO 2 Pt 4+

32. Acids – Effect of Oxidation state Hard Acids Borderline Soft Acids H + , Li + , Na + , K + Be 2+ , Mg 2+ , Ca 2+ BF 3 , BCl 3 , B(OR) 3 BBr 3 ,B(CH 3 ) 3 BH 3 ,Tl + , Tl (CH 3 ) 3 Al 3+ ,Al(CH 3 ) 3 ,AlCl 3 ,AlH 3 Cr 3+ ,Mn 2+ , Fe 3+ , Co 3+ Fe 2+ ,Co 2+ ,Ni 2+ Cu + ,Ag + , Au + , Cu 2+ ,Zn 2+ ,Rh 3+ Cd 2+ ,Hg 2 2+ , Ir 3+ , Ru 3+ , Os 2+ Hg 2+ , Pd 2+ ,Pt 2+ , SO 3 SO 2 Pt 4+

33. Bases Hard Bases Borderline Soft Bases F - , Cl - Br - H - , I - H 2 O, OH - ,O 2- H 2 S, HS - , S 2- ROH, RO - , R 2 O, CH 3 CO 2 - RSH, RS - , R 2 S NO 3 - , ClO 4 - N O 2 - , N 3 - , N 2 S CN - , C N - ,RNC, CO CO 3 2- ,SO 4 2- , PO 4 3- SO 3 2- S 2 O 3 2- NH 3 , RNH 2 C 6 H 5 NH 2 , pyr R 3 P, C 6 H 6

34. Bases – effect of Oxidation State Hard Bases Borderline Soft Bases F - , Cl - Br - H - , I - H 2 O, OH - ,O 2- H 2 S, HS - , S 2- ROH, RO - , R 2 O, CH 3 CO 2 - RSH, RS - , R 2 S NO 3 - , ClO 4 - N O 2 - , N 3 - , N 2 S CN - , C N - ,RNC, CO CO 3 2- ,SO 4 2- , PO 4 3- SO 3 2- S 2 O 3 2- NH 3 , RNH 2 C 6 H 5 NH 2 , pyr R 3 P, C 6 H 6

35. Effect of Linkage Site SC N - vs. NC S - The nitrogen tends to coordinate with harder acids such as Si, whereas the sulfur tends to coordinate with softer acids such as Pt 2+ .

36. Effect of Oxidation Number Cu 2+ /Cu + on acid hardness SO 3 /SO 2 on acid hardness NO 3 - /NO 2 - on base hardness SO 4 2- /SO 3 2- on base hardness

37. Acid or Base Strength It is important to realize that hard/soft considerations have nothing to do with acid or base strength. An acid or a base may be hard or soft and also be either weak or strong. In a competition reaction between two bases for the same acid, you must consider both the relative strength of the bases, and the hard/soft nature of each base and the acid.

38. Acid or Base Strength Consider the reaction between ZnO and LiC 4 H 9 . ZnO + 2 LiC 4 H 9 ↔ Zn(C 4 H 9 ) 2 + Li 2 O Zinc ion is a strong Lewis acid, and oxide ion is a strong Lewis base.

39. Acid or Base Strength Consider the reaction between ZnO and LiC 4 H 9 . ZnO + 2 LiC 4 H 9 ↔ Zn(C 4 H 9 ) 2 + Li 2 O Zinc ion is a strong Lewis acid, and oxide ion is a strong Lewis base. However, the reaction proceeds to the right (K>1), because hard/soft considerations override acid-base strength considerations. soft -hard hard -soft soft -soft hard -hard

40. The Nature of the Adduct Hard acid/hard base adducts tend to have more ionic character in their bonding. These are generally more favored energetically. Soft acid/soft base adducts are more covalent in nature.

41. As the adduct forms, there is usually a change in geometry around the Lewis acid site. BX 3 + N(CH 3 ) 3  X 3 B-NMe 3 The stability of the adduct is: BBr 3 > BCl 3 > BF 3 This order seems opposite of what would be expected based on halogen size or electronegativity .

42. Other Considerations empty 2p orbital filled orbitals The reactivity pattern suggests some degree of π bonding in BF 3 .

43. Other Considerations Steric factors can play a role. An example is the unfavorable reaction between :N(C 6 H 5 ) 3 and BCl 3 . The large phenyl groups interact with the chlorine atoms on boron to destabilize the product.

44. Applications of Hard/Soft Theory The Qual Scheme , a series of chemical reactions used to separate and identify the presence of dozens of metal ions, is based largely on the hard and soft properties of the metal ions. The softer metals are precipitated out as chlorides or sulfides, with the harder ions formed as carbonates.

45. Evidence in Nature In geochemistry, the elements in the earth’s crust are classified as lithophiles or chalcophiles. The lithophile elements are typically found as silicates (bonded via the O atom): Li + , Mg 2+ , Ti 3+ , Al 3+ and Cr 2+,3+ . These are hard Lewis acids.

46. Evidence in Nature The chalcophile elements are typically found as sulfides or bonded to Se 2- or Te 2- . They include: Cd 2+ , Pb 2+ , Sb 3+ , and Bi 3+ . These are soft Lewis acids. Zinc ion, which is borderline, is typically found as a sulfide.