Pearson's classification of Lewis acids and Lewis bases into hard and soft - acids and basesR.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 and bases are called borderline acids or borderline bases. The principal characteristics of hard and soft acids and bases along with their examples are given in Table. 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 hand 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). This principle does not state, however, that hard-soft or soft-hard combinations cannot exist. It only states that if there is a choice, a hard-hard or a soft-soft combination would be preferred to a soft-hard or hard-soft combination. Table : Classification of and borderline acids and bases. Lewis Acids (acceptors) H a r d adds [Ahrland and Chart (1958) bare arbitrarily called bard acids as, class (a) metal ions or metal acceptors] (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). Examples: H+, Li+, Na+, K+, Be2+, Mg2+, Ca2+, Sr2+, Mn2+ , Ag2+, A13+, Sc3+, Ga3+, In3+, La3+ , N3+, Cl3+, Gd3+, Lu3+, Cr3+, Co3+, Fe3+, As3+, CH3Sn3+, Si4+, Ti4+, Zr4+, Th4+, Ln4+, Pu4+, Ce3+, Hf4+, Wo4+, Sn4+, UO22+, MoO3+, VO22+, BeMe2, BF2, B(OR)3, Al(CH3)3, AlCl3, AlH3, RPo2, SO3, Phenol, I7+, I5+, Cl5+, Cr6+, RCO+, Fe6+ , Pt6+, CO2, NC+, HX (hydrogenbonding molecules). Lewis Bases (Donors or ligands) Lewis acids and Lewis bases into hard, soft

Soft acids [These have been called class (b) metal ions or metal acceptors] (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.

Borderline acids

(intermediate)

The characteristics of borderline acids are intermediate between those of hard acids and soft acids.

Examples: Cu+, Ag+, Au+, Tl+, Hg22+, Cs+, Pd2+, Cd2+, Pt2+, Hg2+, CH3Hg+, Co(CN)52-, Pt4+, Te4+, Tl3+, Tl(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+, HO+, RO2, M0(metal atoms), carbenes.

Examples: Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Pb+, Sn2 + , Sb3+, Bi3+, Rh3+, Ir3+, B(CH3)3, SO2, NO+, Ru2+, Os 2+, R3C+, C6H5+, GaH3.

Hard bases (Hard ligands). The donor atom of a hard base: (i) has high electronegativity. (ii) holds its valence electrons strongly and hence cannot be polarised (removed 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-(coordination through N-atom), F-, Cl-.

Soft bases (Soft ligands) ligands) hgandv) soft base: The donor atom of a (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.

Borderline (intermediate) bases These bases have intermediate properties.

Examples: R2S, RSH, RS-, SCN- Examples: (coordination, through S-atom), C5H5N, N3-, S2-, R3P, R3As, I-, CN-, H-, R-, SO32-, N2. S2O32-, (RO)3P, RNC, CO, C2H4, C6H6, CH3-.

C6H5NH2, Br-, NO2-,

It may be pointed out that all the species mentioned in any category are not equally hard or soft. Thus, Cu2+ ion behaves as a harder acid than Mn2+ ion because of the smaller size of the former. Similarly, Li+ ion is a harder acid than Cs+ ion because of the smaller size of the former. Similarly, bases such as C6H5NH2, (C6H5)2NH, N3-, NH3, C6H5N, all containing nitrogen, are not equally hard. For example, NH3 is harder than C6H5NH2 which in turn, is harder than (C6H5)2NH. In fact, (C6H5)2NH is a soft base. Hard acids are Lewis acids which are small in size and whose electron charge clouds are not easily polarizable. These are mostly light metal ions generally associated with high positive oxidation states. Soft acids are Lewis acids which are comparatively larger in size and whose electron charge clouds are easily polarizable. These are mostly heavy metal ions generally associated with low (or even zero) positive oxidation states. Hard bases are Lewis bases which prefer to coordinate with hard acids. These are anions or neutral molecules which are not easily polarizable. Soft bases are Lewis bases which prefer to coordinate with soft acids. These are anions or neutral molecules which are easily polarizable. It is important to note, however, that there is no sharp line of demarcation between soft and hard species and a number of borderline cases also exist. For instance, transition metal ions such as Fe2+, Co2+, Ni2+, Cu2+, Zn2+ behave as borderline acids. Metals and non-metals in lower oxidation states behave as soft acids while the same species in higher oxidation states behave as hard acids. Bonding in Hard-Hard and Soft-Soft Combinations : Since hard acids have generally vacant d orbitals, they can accept electrons from hard bases so that the bonding between hard acids and hard bases is predominantly ionic. Interaction between soft acids and soft bases occurs mostly through pi bonding so that the bonding between soft acids and soft bases is largely covalent. It must be clearly understood that hardness and softness of acids and bases refer only to hardhard and soft-soft interactions. It has no relation with their strengths. For instance, both OH- and Fions are hard bases but OH- ion is several million times more basic than F- ion. Similarly, SO32- ion and Et3P are both soft bases but Et3P is 107 times more basic than SO32-. 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) AgI2- is stable while AgF2- 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 AgI2- is obtained by the combination of soft acid (Ag+) and soft base (I-) and AgF2- results by the interaction of a soft acid (Ag+) and a hard base (F-), AgI2- ion is stable but AgF2- does not exist. Ag+ + 2IAgI2Soft acid Soft base Stable complex

Ag+ + 2FSoft acid Hard base

AgF2Unstable complex

(b) CoF 6 3 - (hard acid + hard base) is more stable than CoI63- (hard acid + soft base). (c) (CH3)2 N PF2 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 N (CH 3)2 is a hard base. It is also known that BH 3 is a soft acid and PF2 is a soft base. On applying the principle that (hard acid + hard base) combinations and (soft acid + &oft 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 molecule 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 to form the complex. SCN- ion is an ambidentate ligand since it can coordinate 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-atorn while in [Pd(SCN)4]2- ion, Pd2+ is coordinated with the ligand through Satom. Thus the complex ions given by Co2+ and Pd2+ ions 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 coordinate with N-atom of the hard ligand, NCS-. On the other, Pd2+ ion is a 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 a 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 with I2. On the other hand RSCN will give more stable complex with 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, if the ligands are of different nature, the complex compound would be unstable. This point maybe illustrated by the following examples : (a) Since in [Co(NH3)5F]2+ (I) both the ligands viz., NH3 molecule and F- ion are hand ligands and in [Co(NH3)5I]2+ (II) NH3 is a hard ligand and I- ion is a soft ligand, (I) is a stable complex ion while (II) is unstable. (b) [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 prefer to get attached with a centre which is already linked with soft ligands. Similarly hard ligands prefer 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 (F3BNH3) adduct and BH4- ion. Hard ligand like NH3 coordinates with B-atom of BF3 molecule to form (F3BNH3) adduct, since F- ions which are already attached with B-atom in BF3 molecule are also hard ligands. Thus: Hard acid

Similarly the formation of BH4- ion by the combination of BH3 (in which H atoms are soft ligands) and H- ions (soft ligands) can also be explained :

The formation of F3BNH3 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 F3BNH3 adduct. F3B (Hard acid) + NH3 (Hard base) F3BNH3 (Stable adduct) - ion is a soft base, their combination gives a stable Similarly since BH3 is a soft acid and H - : BH4 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) is more stable than Hg(OH)2 (soft acid + hard base). More stability of HgS than that of Hg(OH)2 is 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. The 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+, Al3+ ions with hard bases namely CO32- and O2- ions while MgS, CaS and Al2S3 are obtained by the combination of hard acids (Mg2+, Ca2+, A13+ 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+ ions 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 occur in nature both as carbonates and sulphides. 7. Jorginsen has also pointed that hard solvents tend to dissolve hard solutes and vice versa. 8. Predicting Course or feasibility 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 reactions. For example : LiI + CsF LiF + CsI(hard acid + soft base) (soft acid + hard base) (hard acid + hard base) (soft acid + soft base}

HgF2(soft acid + hard ban)

+

BeI2(hard acid + soft base)

BeF2

+

HgI2(soft acid + soft bow)

(hard acid + hard base)

9. Prediction of Hardness and Softness : Consider a base B whose hardness or softness is to be predicted. If the equilibrium : [BH]+ + [CH3Mg]+ [CH3MgB]+ + H+Soft acid Hard acid

shifts towards the right, then B is a soft base because it shows more affinity for the soft acid [CH3Mg] + (soft-soft interaction). If the equilibrium shifts towards the left, then it is a hard base since it exhibits more affinity for the hard acid H+ (hard-hard interaction). Basis for Hard-Hard and Soft-Soft Interactions : Several views have been put forth to explain the basis for hard-hard and soft-soft interactions. However, no single view is completely satisfactory. According to the most accepted view, the cause of hard-hard interaction is an electrostatic interaction.

As already discussed, bonding between hard acids and hard bases is predominantly ionic. The electrostatic energy between a positive and a negative ion pair is inversely proportional to the internuclear distance. Therefore, the smaller the ions, the lesser would be the inter-nuclear distance and the greater would be the electrostatic attraction between the two ions. Consequently, the resulting compound would be highly stable. Electrostatic interaction cannot explain the soft-soft interaction because the sizes of soft species are comparatively very large. Polarisation of the species, however, plays an important role in explaining their interactions. Most of the soft acids have 6 to 10 d electrons in their electronic configurations. These electrons get easily polarised favouring covalent bonding between them and the soft bases which are also easily polarisable (recall covalent bonding between Li+ and I- ions due to polarisation of I- ion). Thus, bonding between soft acids and soft bases is assumed to be largely covalent. Considering hard-hard interactions as ionic and soft-soft interactions as covalent, Misons and coworkers (1967) proposed the following relation which can tell whether a given species is hard or soft : pK = - logK = aX + bY + c where K is the equilibrium constant for the dissociation of the metal-ligand (i.e., acid-base) complex ; X, Y are parameters for the metal ions, i.e., acids ; a, b are parameters for the ligands i.e., bases and c is a constant required to adjust the pK values in such a way that all of them lie on the same scale. The values of parameter Y for some of the cations are given below : Hard acids : Li+ Al3+ Mg2+ Na+ Ca2+ Fe3+ Co2+ Cs+ Co2+ 0.36 0.70 0.87 0.93 1.62 2.37 2.67 2.73 2.96 Soft acids : Sn2+ Tl3+ Cu+ Pb2+ Tl+ Hg2+ Au+ 3.17 3.2 3.45 3.58 3.78 4.25 5.95 If the value of Y is less than 2.8 , the acid is hard and if the value is more than 3.2 , the acid is soft. The value of parameter b for some of the bases are given below : Limitations of HSAB principle Although (hard + hard) and (soft + soft) combination is a useful principle, yet many reactions cannot be explained with the help of this principle. For example, in the reaction : SO32- + HF HSO3+ For SO32- + H+F[H]+[SO3]2+ Fsoft base (hard acid + hard base) (hard acid + soft base)

which proceeds towards right, hard acid (H+) combines with soft or borderline base (SO32- ) to form [H+][SO32-] or HSO3- ion which is a stable ion. (Hard acid + soft base) combination is against the HSAB principle.