1. Inorganic chemistry B.Sc III
Hard acid and soft acid and bases

2. Hard and Soft Acids and Bases
1965- 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 by Pearson to
rationalize a variety of chemical information.
1983 – the qualitative definition of HSAB was
converted to a quantitative one by using the idea of
polarizability.
A less polarizable atom or ion is “hard” and a more
easily polarized atom or ion is “soft”

3. Hard acid: Hard base: High positive charge Small size
Not easily polarizable
Hard base:
Low polarizability
High electronegativity
Not easily oxidized

4. Soft acid: Soft base: Low positive charge Large size; easily oxidized
Highly polarizable
Soft base:
High polarizability
Diffuse donor orbital
Low electronegativity
Easily oxidized

5. Hard and soft acids and bases
Hard acids or bases are small and non-polarizable
Soft acids and bases are larger and more polarizable
Halide ions increase in softness:
fluoride < chloride<bromide<iodide
Hard-hard or soft-soft interactions are stronger (with less soluble salts)
than hard-soft interactions (which tend to be more soluble).

7. Most metals are classified as Hard (Class a) acids or acceptors.
Exceptions shown below: acceptors metals in red box are always soft (Class b).
Other metals are soft in low oxidation states and are indicated by symbol.
Class (b) or soft always
Solubilities: AgF > AgCl > AgBr >AgI
But…… LiBr > LiCl > LiI > LiF

8. since there are electrons outside the d shell.
Chatt’s explanationClass (b) soft metals have d electrons available for p-bonding
Model: Base donates electron density to metal acceptor. Back donation, from acid to base, may occur from the d electrons of the acid metal into vacant orbitals on the base.
Higher oxidation states of elements to the right of transition metals have more class b character
since there are electrons outside the d shell.
Ex. (Tl(III) > Tl(I), has two 6s electrons outside the 5d making them less available for π-bonding)
For transition metals:
high oxidation states and position to the left of periodic table are hard
low oxidation states and position to the right of periodic table are soft
Soft donor molecules or ions that are readily polarizable and have vacant d or π* orbitals
available for π-bonding react best with class (b) soft metals

12. In general, hard-hard combinations are energetically
Hard acids tend to react better with hard bases and soft acids with soft bases, in order to produce hard-hard or soft-soft combinations
In general, hard-hard combinations are energetically
more favorable than soft-soft
An acid or a base may be hard or soft
and at the same time it may be strong or weak
Both characteristics must always be taken into account
e.g. If two bases equally soft compete for the same acid,
the one with greater basicity will be preferred
but if they are not equally soft, the preference may be inverted

14. With lower solubility, color and short interionic distances,
Fajans’ rules
For a given cation, covalent character increases
with increasing anion size.
For a given anion, covalent character increases
with decreasing cation size.
The covalent character increases
with increasing charge on either ion.
Covalent character is greater for cations with non-noble gas
electronic configurations.
A greater covalent character resulting from a soft-soft interaction is related
With lower solubility, color and short interionic distances,
whereas hard-hard interactions result in colorless and highly soluble compounds

15. Application of HSAB principle

17. Mulliken’s absolute electronegativity
Quantitative measurements
Absolute hardness
(Pearson)
Mulliken’s absolute electronegativity
(Pearson)
EHOMO = -I
ELUMO = -A
Softness

19. Electronic Factors
HSAB Concepts
Using HSAB guidelines, reactions between acids and bases can be often be predicted successfully (though not always)
Q: Is OH- or S2- more likely to form an insoluble salt with a +3 transition metal ion?
A: The harder species will bind more strongly. Between OH- or S2-, OH- is the harder species.

21. Electronic Factors
STABILITY OF COMPLEX
Q: Why is AgI(s) very water-insoluble, but LiI very water-soluble?
A: AgI is a soft acid-soft base combination, while LiI is hard-soft. The interaction between Li+ and I- ions is not strong.
AgI(s) + H2O(l)  essentially no reaction
LiI(s) + H2O(l)  Li+(aq) + I-(aq)

22. Qualitative Analysis
In the separation of the group cations carried out this year, HSAB rules were used to separate classes of ions based on different hard and soft interactions
Group II: Hg2+, Cd2+, Cu2+, Sn2+, Sb3+, Bi3+
Group III: Mn2+, Fe2+, Cu2+, Ni2+, Zn2+, Al3+, Cr2+
Group IV: Ca2+, Sr2+, Ba2+, K+, NH4+
soft and
borderline acids
borderline
hard acids

23. Separation of Cations
The soft and borderline cations are separated through reaction with the soft base sulfide, S2-. Group II sulfides are less soluble than group III, so in order to selectively remove group II ions, a low pH is used:
H2S(g) D 2H+(aq) + S2-(aq)
Even at low S2- concentrations, the group II ions precipitate (stronger interactions with the soft base, S2-)
Raising the pH increases the S2- concentration, which allows the precipitation of group III ions
The group IV are then precipitated as hydroxides. These cations are harder and prefer the hard base OH-.

24. Ambidentate Bases
SCN- (thiocyanate) can interact through either its S or N atom with Lewis acids. It can donate an electron pair through more than one atom.
Interaction will be through the S-atom with a soft acid, or through the N-atom when interacting with hard acids.
Cr(III) interacts as Cr-NCS, while Pt(II) does so as Pt-SCN

25. Electronic Factors
Inductive Effects
Electron donating substituents enhance base strength and electron-withdrawing groups enhance electron acceptor (acid) strength
PMe3 stronger base than PH3
gas-phase
base strengths
NMe3 > NHMe2 > NH2Me > NH3
strongest base
weakest base
This plays a role in bond lengths also
Me = methyl; alkyl, aryl groups are electron donating; F, CF3, CN, etc. are e- withdrawing

26. Structural Rearrangement
Structural Factors
Structural Rearrangement
In some cases, a center must adjust its hybridization in order to accommodate the formation of a new bond
Order of Lewis acid strength for BX3 (X = halides) is
BF3 < BCl3 < BBr3
This is due to better p-orbital overlap in BF3 than in BCl3, which is better than BBr3 (B-F bonds are shortest). Thus more energy is needed to change from the sp2-hybridized form of BF3.
opposite order to what is
expected for inductive effect
sp2
sp3

27. Predict which way the following reactions will go.
HI + NaF HF + NaI R
AlI3 + 3NaF AlF3 + 3NaI R
CaS + H2O CaO + H2S R
TiF4 + 2TiI2 TiI4 + 2TiF2 L
CoF2 + HgBr2 CoBr2 + HgF2 L
HgO + H2S HgS + H2O R

29. Predict which way the following reactions will go.
HI + NaF HF + NaI R
AlI3 + 3NaF AlF3 + 3NaI R
CaS + H2O CaO + H2S R
TiF4 + 2TiI2 TiI4 + 2TiF L
CoF2 + HgBr2 CoBr2 + HgF L
HgO + H2S HgS + H2O R