To learn what is HSAB principle and characteristics of Hard-Soft Acids and Bases…Read my previous post on HSAB principle

1.Stability of Complexes:

Consider general lewis acid base reaction:-
Acid base

The complex AB whould be more stable if A and B are either both hard or both soft .
On the other hand,the complex whould be least stable if A is hard and B is Soft or visa versa.

For example:- AgI2 exists as stable compound but AgF2 does not.

Reason:-Ag+ ion is soft acid and it combine with I ion ,a soft base to form a stable complex AgI2 (soft-soft) whereas its combination with F ion ( hard base ),forms an unstable complex AgF2 (soft-hard).

Similarly, we can explain why [CoF6]3- is more stable than [CoI6]3- .

•It may also be noted that combination of hard ligands or soft ligands with metal ions is more preferable than a ombination of mixture of hard and soft ligands.

For example, the complexes [Co(CN)5I]3- and [Co(NH3)5F]2+ are stable where as [Co(CN)5F]3- and [Co(NH3)5I]2+ are unsable .bcoz CN and I are both soft bases while NH3 and F are both hard bases.

2 .Prediction of coordination in complex of ambidentate ligands.

Ambidentate ligand is a mono dentate ligand which can coordinate to metal through more than one coordinating atoms.

Example SCN ,can coordinate through N or S.
It forms stable complexes( with border Line metals like Co,Ni,Zn,Cu) of type [Co(NCS)4]2- by cordinating through N and softer acid cations Rh,Ir,Pd,Pt,Au tends to form complexes of type
[Pd(SCN)4]2- by coordinating through S.

This is explained as Pd2+ is a soft acid and therefore it prefers to coordinate through softer S atom to form Pd-SCN bond ,on the other hand Co2+ is a hard acid so it prefers to coordinate through the harder N atom to form Co-NCS bond.

Note:-there are many examples in which both SCN and NCS bonding to same metal .

3.Prediction of Hardness and softness:

Consider a base B whose hardness or softness is to be predicted .

If the equilibrium :

Acid base equilibrium

Shift towards the right ,then B is a soft base because it shows more affinity for soft acids [CH3Mg]+ (soft -soft interaction).

If equilibrium shifts towards the left ,then it is a hard base as it has more affinity for hard acid H+ (hard -hard interaction).

4.Occurrence of ores and minerals :

can be explained on basis of HSAB

Hard metal ions such as Mg2+, Ca2+,Al3+ occur mostly as their oxides ,carbonates or halides.this is due to preferred hard acid-hard base interactions. These cannot occur as sulphides because S2- is a soft base and would not form stable compounds.

Similarly soft acids occurs as sulphides because of soft-soft interactions.

Borderline acids like Ni2+ ,Fe2+, Co2+ etc can occour both as sulphides as well as oxides or carbonates due to their intermediate charcter.

5.Predicting Feasibility of reaction .

Rxn between LiI and CsF always proceed to give LiF and CsI .it is an intresting example of preferential combination of soft-soft and hard-hard species.

Acid base example

Theoretical basis of hardness and softness-HSAB

1.Electrostatic interactions

According to this theory ,the hard acids and hard bases form purely ionic compounds.
i.e. hard-hard interactions are purely ionic or electrostatic .

For example, hard acids such as Li+ ,Na+ ,K+ and hard bases such as OH, F,O2- are known to form ionic compounds.

The electrostatic energy between a positive ion and a negative ion is inversely perportional to the interatomic distance. Therefore, smaller the ion ,lesser whould be the internuclear distance and greater whould be the electrostatic attraction between the ions.

Consequently ,the resulted complex formed by hard acid and hard base whould be highly stable.

2. Polarizing power and polarizability :

Electrostatic intractions cannot explain the soft-soft interactions and it is assumed that these interactions are covelent in nature.
Most of soft acids are transition metels with 6 or more d-electrons .these d-electrons have more polarizing power and soft bases such as I, S2- are easily polarizable.
Thus bonding between soft acids and soft bases are largely covelent.

3.π-bonding contribution:-

the soft acids are transition metals in low oxidation state containing loosely held outer d-electrons ,these can be easily donated to ligands .
Most soft bases are π-bond acceptors and most soft acids are π-bond donors.polarizability of soft acids and bases further favours π-bonding.

4.Electronegativity & Hardness and Softness

Species with high electronegativity are hard and those with low electronegativity are soft.
This explains why CF3 is considerably hardet than the CH3 group.


Soft ligands have tendency to combine with a metal ion already having soft ligands and hard ligands have tendency to combine with a metal ion already having hard ligands.
This tendency is termed as symbiosis.

Due to symbiosis, the symmetrical substituted compounds are preferred than unsymmetrical substituted compounds. Therefore ,the compounds such as BF3H and BH3F ,having mixed substituents intract spontaneously to yield BH4 and BF4 i.e. compounds with same substituents.


Similarly the fluorinated methane reactsin similar manner:-


Limitations of HSAB principle:

1.This principle is general and does bot provide any quantitative scale of measurement.

2. Hard -soft factor is independent of acidic and basic character of compounds.these factors works independently .however there are many examples with interdependence of two concepts.
In some cases ,hard-hard or soft-soft complexes change to more stable hard-soft system.

stable hard-soft

This rxn occurs (Keq=104) though it violetes the HSAB principle.this is due to the fact that stronger soft base SO32- displaces the weak hard base F from the hard acid H+.