Inorganic chemistryB.Sc III

Hard acid and soft acid and bases

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”

• Hard acid:• High positive charge• Small size• Not easily polarizable

• Hard base:• Low polarizability• High electronegativity• Not easily oxidized

• Soft acid: • Low positive charge• Large size; easily oxidized• Highly polarizable

• Soft base: • High polarizability• Diffuse donor orbital• Low electronegativity• Easily oxidized

Hard and soft acids and bases

Hard acids or bases are small and non-polarizableSoft acids and bases are larger and more polarizableHalide 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).

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

Chatt’s explanationClass (b) soft metals have d electrons available for -bonding

Higher oxidation states of elements to the right of transition metals have more class b charactersince 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 hardlow 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* orbitalsavailable for π-bonding react best with class (b) soft metals

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.

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 energeticallymore favorable than soft-soft

An acid or a base may be hard or softand 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

Fajans’ rules

1. For a given cation, covalent character increases with increasing anion size.2. For a given anion, covalent character increases with decreasing cation size.3. The covalent character increases

with increasing charge on either ion.4. Covalent character is greater for cations with non-noble gas electronic configurations.

A greater covalent character resulting from a soft-soft interaction is relatedWith lower solubility, color and short interionic distances,

whereas hard-hard interactions result in colorless and highly soluble compounds

Application of HSAB principle

Quantitative measurements

2

AI

2

AI

Absolute hardness(Pearson)

Mulliken’s absolute electronegativity(Pearson)

1

Softness

EHOMO = -I

ELUMO = -A

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.

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.

Electronic Factors

AgI(s) + H2O(l) essentially no reaction

LiI(s) + H2O(l) Li+(aq) + I-(aq)

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 andborderline acids

borderline

hard acids

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) 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-.

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

Inductive Effects• Electron donating substituents

enhance base strength and electron-withdrawing groups enhance electron acceptor (acid) strength

Electronic Factors

NMe3 > NHMe2 > NH2Me > NH3

strongest base weakest base

Me = methyl; alkyl, aryl groups are electron donating; F, CF3, CN, etc. are e- withdrawing

PMeMe

Me

PHH

H

PMe3 stronger base than PH3

This plays a role in bond lengths also

gas-phasebase strengths

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) isBF3 < 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.

B F

F

FN

H

HH B

F

F

FN

H

H

H+

Structural Factors

sp2 sp3opposite order to what isexpected for inductive effect

• 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

•

Predict which way the following reactions will go.HI + NaF ----------- HF + NaI RAlI3 + 3NaF AlF3 + 3NaI RCaS + H2O CaO + H2S RTiF4 + 2TiI2 TiI4 + 2TiF2 LCoF2 + HgBr2 CoBr2 + HgF2 LHgO + H2S HgS + H2O R