HARD AND SOFT ACIDS AND BASES
R.G. Pearson (1963) has classified the Lewis acids and Lewis bases as hard and soft acids and bases.
A third category whose characteristics are intermediate between those of hard and soft acids/bases are called borderline acids or borderline bases.
Pearson’s hard and soft acids and bases principle (HSAB Principle)
On the basis of experimental data of various complexes obtained by the combination of Lewis acids and Lewis bases, Pearson (1963) discovered a principle known as Hard and Soft Acids and Bases Principle (HSAB Principle) (some chemists prefer the abbreviation SHAB instead of HSAB used by Pearson).
This principle states that a hard Lewis acid prefers to combine with a hard Lewis base and similarly a soft Lewis acid prefers to combine with a soft Lewis base, since this type of combination gives a more stable product.
Thus we can say that (hard acid + hard base) and (soft acid + soft base) combinations give more stable products than the (hard acid + soft base) and (soft acid + hard base) combinations.
The combination of hard acid and hard base occurs mainly through ionic bonding as in Mg(OH)2 (Mg2+ = hard acid, OH− = hard base) and that of soft acid and soft base takes place mainly by covalent bonding as in HgI2 (Hg2+ = soft acid, I− = soft base.)
Table: Classification of Lewis acids and Lewis bases into hard, soft and borderline acids and bases
Lewis Acids (acceptors)
|Hard acids [Ahrland and Chatt (1958) have arbitrarily called hard acids as class (a) metal ions or metal acceptors]||Soft acids [These have been called class (b) metal ions or metal acceptors]||Borderline (intermediate) acids|
|(i) They have acceptor metal atom of small size.
(ii) they have acceptor with high positive charge (oxidation state)
(iii) The valence-electrons of the acceptor atom of these acids cannot be polarised (or distorted or removed) easily (i.e., they have low polarisability), since they are held strongly and it is for this reason that these Lewis acids have been called hard acids (or hard metal ions) by Pearson (1963).
|(i) They have acceptor metal atom of large size.
(ii) They have acceptor atom with low or zero positive charge
(iii) The valence-electrons of the acceptor atom of these acids can be polarised easily (i.e., they have high polarisability), since they are held weakly and for this reason these Lewis acids have been called soft acids (or soft metal ions) by Pearson.
|The characteristics of borderline acids are intermediate between those of hard acids and soft acids.|
|Examples: H+, Li+, Na+, K+, Be2+, Mg2+, Ca2+, Sr2+, Mn2+, Ag2+, Al3+, Sc3+, Ga3+, In3+, La3+, N3+, Cl3+, Gd3+, Lu3+, Cr3+, Co3+, Fe3+, As3+, CH3Sn3+, Si4+, Ti4+, Zr4+, Th4+, Li4+, Pu4+, Ce3+, Hf4+, Wo4+, Sn4+, UO22+, MoO3+, BeMe2, BF2, B(OR)3, Al(CH3)3, AlCl3, AlH3, RPo2, SO3. Phenol, I7+, I5+, Cl7+, Cr6+, RCO+, Fe6+, Pt6+, CO2, NC+, HX (hydrogen-bonding molecules)||Examples: Cu+, Ag+, Au+, Ti+, Hg+, Cs+, Pd2+, Cd2+, Pt2+, Hg2+,CH3Hg+, Co(CN)52−, Pt4+, Te4+, Ti3+, TI(CH3)3, BH3, Ga(CH3)3, GaCl3, GaI3, InCl3, RS+, RSe+, RTe+, I+, Br+, I2, Br2, ICN, trinitrobenzene, chloranil, quinones, tetracyanoethylene O, Cl, Br, I, N, RO, RO2 M (metal atoms), CH2 (carbene)||Examples: Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Pb2+, Sn2+, Sb3+, Bi3+, Rh3+, Ir3+, B(CH3)3, SO2, NO+, Ru2+, Os2+, R3C+, C6H5+, GaH3.|
Lewis Bases (Donors or ligands)
|Hard bases (Hard ligands)||Soft bases (Soft ligands)||Borderline (intermediate bases|
|The donor atom of a hard base:||The donor atom of soft base:||These bases have intermediate properties.|
|(i) has high electronegativity.
(ii) holds its valence-electrons strongly and hence cannot be polarised (re-moved or deformed) easily, i.e., the donor atom of hard base has low polarisability.
(iii) has filled orbitals
Examples: H2O, OH−, ROH, R2O, RO−, CH3COO−, PO43−, SO42− RCO2−, CO32−, ClO4−, NO3−, O2−, C2O42−, (co-ordination through O-atom), NH3, NR3, NHR2,. NH2R, N2H4, NCS− (co-ordination through N-atom) F−, Cl−.
|(i) has low electronegativity.
(ii) holds its valence-electrons weakly and hence can be polarised easily, i.e., the donor atom of a soft base has high polarisability.
(iii) has partially filled orbitals
Examples: R2S, RSH, RS−, SCN− (co-ordination through S-atom). S2−, R3P, R3As, I−, CN−, H−, R−, S2O32−, (RO)3P, RNC, CO, C2H4, C6H6, CH3−.
|These base have intermediate properties.
Examples: C6H5NH2, C5H5N, N3−, Br−, NO2−, SO32−, N2.
Applications of HSAB principle
HSAB principle is extremely useful in explaining the following:
1. Stability of complex compounds, having the same ligands.
This application can be understood by considering the following examples :
(a) is stable while does not exist. We know that Ag+ is a soft acid, F− ion is a hard base and I− ion is a soft base. Thus, since is obtained by the combination of a soft acid (Ag+) and soft base (I−) and results by the interaction of a soft acid (Ag+) and a hard base (F−), ion is stable but does not exist.
(b) CoF63− (hard acid + hard base) is more stable than CoI63− (hard acid + soft base).
(c) (CH3)2 molecule acts as a bidentate ligand, since it has two lone pairs of electrons one of which is on N-atom and the other is on P-atom. Both BH3 and BF3 molecules combine with this ligand and forms an adduct. With the help of HSAB principle we can predict the structure of this adduct. We know that BF3 is a hard acid and (CH3)2 is a hard base. It also known that BH3 is a soft acid and − is a soft base. On applying the principle that (hard acid + hard base) combinations and (soft acid + soft base) are preferred, the structure of the adduct should be that in which N-atom donates its lone pairs of electrons to B-atom of BF3 molecules and P-atom donates its lone pair of electrons to B-atom of BH3 molecule. Thus the structure of the adduct is:
2. To predict the nature of bonding in complex ions given by ambidentate ligands
(a) With the help of HSAB principle we can predict which atom of an ambidentate ligand will combine with metal ion too form the complex, SCN− ion is an ambidentate ligand since it can co-ordinate to the metal ion either through its S-atom or through N-atom. It has been found that Co2+ and Pd2+ both combine with four SCN− ligands to form the complex ion, [M(SCN)4]2− (M = Co2+, Pd2+). With the help of HSAB principle it can be shown that in [Co(SCN)4]2− ion, Co2+ is linked with the ligand through N-atom while in [Pd(SCN)4]2− ion, Pd2+ is co-ordinated with the ligand through S-atom. Thus the complex ions given by Co2+ and Pd2+ ion should be represented as [Co(NCS)4]2− and [Pd(SCN)4]2− respectively. The reason for this is that since Co2+ ion is a hard acid, it prefers to co-ordinate with N-atom of the hard ligand, NCS−. On the other, Pd2+ ion is soft acid and hence combines with the S-atom of the soft ligand, SCN−.
(b) We know that phenol is a hard acid and I2 is soft acid. It is also known that alkyl thiocyanate, RSCN (S-atom acting as a donor) is a soft ligand and alkyl iso-thiocyanate, RNCS (N-atom acting as a donor) is a hard ligand. Thus, if RSCN and RNCS are complexed with phenol and I2, RNCS will form more stable complex with phenol due to hard acid (phenol) – hard ligand (RNCS) combination than that with I2. On the other hand RSCN will give more stable complex I2 due to soft acid (I2) – soft ligand (RSCN) combination.
3. Stability of complex compounds having different ligands. Jorgensen has pointed out that in a complex compound having different ligands, if all the ligands are of the same nature, i.e., if all the ligands are soft ligands or hard ligands, the complex compound will be stable. On the other hand, of the ligands are of different nature, the complex compound would be unstable. This point may be illustrated by the following examples :
- in [Co(NH3)5F]2+ both the ligands , NH3 molecule and F− ion are hard ligands
- in [Co(NH3)5I]2+ (II) NH3 is a hard ligand and I− ion is a soft ligand, therefore (I) is a stable complex ion while (II) is unstable.
- [Co(CN)5I]3− (I) is more stable than [Co(CN)5F]3− (II) because
- in (I) both the ligands are soft ligands
- while in (II) CN− ions are soft ligands and F− ion is a hard ligand.
4. Symbiosis. Soft ligands prefers to get attached with a centre which is already linked with soft ligands. Similarly hard ligands prefers to get attached with a centre which is already linked with hard ligands. This tendency of ligands is called symbiosis and can be explained by considering the formation of (F3B ← NH3) adduct and BH4− ion. Hard ligand like NH3 co-ordinates with B-atom of BF3 molecule to form (F3B ← NH3) adduct, since F− ions which are already attached with B-atom in BF3 molecule are also hard ligands. Thus :
Similarly the formation of BH4− ion by the combination of BH3 (in which H atoms are soft ligands) and H− ion (soft ligands) can also be explained
The formation of F3B ← NH3 adduct can also be explained on the basis of the fact that since BF3 and NH3 are hard acid and hard base respectively, they combine together to form a stable F3B ← NH3 adduct.
F3B (hard acid) + NH3 (Hard base) → F3B ← NH3 (Stable adduct)
Similarly since BH3 is soft acid and H− ion is a soft base, their combination gives a stable BH4− ion.
BH3 (Soft acid) + H− (Soft base) → BH4− (Stable ion)
5. Solubility of compounds:
This point would be more clear when we compare the relative stability of HgS and Hg(OH)2 in acidic aqueous solution. HgS (soft acid + soft base) in more stable than Hg(OH)2 (soft acid + hard base).
More stability of HgS than that of Hg(OH)2 explains why Hg(OH)2 dissolves readily in acidic aqueous solution but HgS does not.
6. Occurrence of metals in nature.
The occurrence of some metals in nature as their ores can be explained with the help of HSAB principle.
This following examples illustrate this point :
(a) We know that since MgCO3, CaCO3 and Al2O3 are obtained by the combination of hard acids viz., Mg2+, Ca2+ and Al3+ ion with hard bases namely CO32− and O2− ions while MgS, CaS and Al2S3 are obtained by the combination of hard acids (Mg2+, Ca2+, Al3+ ions) and soft base viz., S2− ion, Mg, Ca and Al occur in nature as MgCO3, CaCO3 and Al2O3 respectively and not as their sulphides (MgS, CaS and Al2S3).
(b) Since Cu2S, Ag2S and HgS are obtained by the combination of soft acids namely Cu+, Ag+ and Hg2+ ion and soft base viz., S2− ion while Cu2CO3, Ag2CO3 and HgCO3 result by the interaction of soft acids (Cu+, Ag+, Hg2+) and hard base viz., (CO32−, Cu, Ag and Hg occur in nature as their sulphides (Cu2S, Ag2S and HgS) and not as their carbonates.
(c) Ni2+, Cu2+ and Pb2+ ions which are borderline (intermediate) acids occurs in nature both as carbonates and sulphides.
7. Jorginsen has also pointed that hard solvents tend to dissolve hard solutes and vice versa.
8. Course of reaction. The principle of (hard acid + hard base) and (soft acid + soft base) combination has also been used to predict the course of many reaction. For example :
Limitations of HSAB principle
Although (hard + hard) and (soft + soft) combination is a useful principle, yet many reaction cannot be explained with the help of this principle. For example in the reaction:
Which proceeds towards right, hard acid (H+) combines with soft or borderline base to form [H+] or ion which is a stable ion. (Hard acid + soft base) combination is against the HSAB principle.