Acid-base and donor-acceptor chemistry
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Acid-base and donor-acceptor chemistry Hard and soft acids and bases. Classical concepts. Arrhenius : acids form hydrogen ions H + (hydronium, oxonium H 3 O + ) in aqueous solution bases form hydroxide ions OH - in aqueous solution acid + base  salt + water

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Acid-base and donor-acceptor chemistry Hard and soft acids and bases

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Acid base and donor acceptor chemistry hard and soft acids and bases

Acid-base and donor-acceptor chemistry

Hard and soft acids and bases


Acid base and donor acceptor chemistry hard and soft acids and bases

Classical concepts

  • Arrhenius:

    • acids form hydrogen ions H+ (hydronium, oxonium H3O+) in aqueous solution

    • bases form hydroxide ions OH- in aqueous solution

    • acid + base  salt + water

    • e.g. HNO3 + KOH  KNO3 + H2O

  • Brønsted-Lowry:

    • acids tend to lose H+

    • bases tend to gain H+

    • acid 1 + base 1  base 1 + acid 2 (conjugate pairs)

    • H3O+ + NO2-  H2O + HNO2

    • NH4+ + NH2-  NH3 + NH3

    • In any solvent, the reaction always favors the formation of the weaker acids or bases

The Lewis concept is more general

and can be interpreted in terms of MO’s


Acid base and donor acceptor chemistry hard and soft acids and bases

d+

d-

C

C

C

O

O

O

M

M

Remember

that frontier orbitals

define the chemistry

of a molecule

CO is a s-donor and

a p-acceptor


Acid base and donor acceptor chemistry hard and soft acids and bases

adduct

base

acid

Acids and bases (the Lewis concept)

A base is an electron-pair donor

An acid is an electron-pair acceptor

Lewis acid-base adducts involving metal ions

are called coordination compounds (or complexes)


Acid base and donor acceptor chemistry hard and soft acids and bases

Frontier orbitals and acid-base reactions

Remember the NH3 molecule


Acid base and donor acceptor chemistry hard and soft acids and bases

New LUMO

(non-bonding)

New HOMO

(bonding)

Frontier orbitals and acid-base reactions

The protonation of NH3

(Td)

(C3v)


Acid base and donor acceptor chemistry hard and soft acids and bases

In most acid-base reactions HOMO-LUMO combinations

lead to new HOMO-LUMO of the product

But remember that there must be useful overlap (same symmetry)

and similar energies to form new bonding and antibonding orbitals

What reactions take place if energies are very different?


Acid base and donor acceptor chemistry hard and soft acids and bases

Frontier orbitals and acid-base reactions

Even when symmetries match several reactions are possible,

depending on the relative energies


Acid base and donor acceptor chemistry hard and soft acids and bases

Frontier orbitals and acid-base reactions

Very different energies like A-B ó A-E

no adducts form

Similar energies like A-C ó A-D

adducts form

A base has an electron-pair

in a HOMO of suitable symmetry

to interact with the LUMO of the acid


Acid base and donor acceptor chemistry hard and soft acids and bases

The MO basis for hydrogen bonding

F-H-F-


Acid base and donor acceptor chemistry hard and soft acids and bases

Bonding e

Non-bonding e

MO diagram derived from atomic orbitals

(using F…….F group orbitals + H orbitals)


Acid base and donor acceptor chemistry hard and soft acids and bases

HOMO-LUMO of HF for s interaction

Non-bonding

(no symmetry match)

Non-bonding

(no E match)

But it is also possible from HF + F-


Acid base and donor acceptor chemistry hard and soft acids and bases

The MO basis for hydrogen bonding

F-H-F-

LUMO

HOMO

HOMO

First take bonding and antibonding combinations.


Acid base and donor acceptor chemistry hard and soft acids and bases

Similarly for unsymmetrical B-H-A

Total energy of B-H-A lower than the sum of the energies of reactants


Acid base and donor acceptor chemistry hard and soft acids and bases

Good energy match,

strong H-bonding

e.g. CH3COOH + H2O

Poor energy match, little or no H-bonding

e.g. CH4 + H2O

Very poor energy match

no adduct formed

H+ transfer reaction

e.g. HCl + H2O


Acid base and donor acceptor chemistry hard and soft acids and bases

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).


Acid base and donor acceptor chemistry hard and soft acids and bases

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.

Solubilities: AgF > AgCl > AgBr >AgI

But…… LiBr > LiCl > LiI > LiF

Class (b) or soft always


Acid base and donor acceptor chemistry hard and soft acids and bases

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 rightof 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


Acid base and donor acceptor chemistry hard and soft acids and bases

Tendency to complex with hard metal ions

N >> P > As > Sb

O >> S > Se > Te

F > Cl > Br > I

Tendency to complex with soft metal ions

N << P > As > Sb

O << S > Se ~ Te

F < Cl < Br < I


Acid base and donor acceptor chemistry hard and soft acids and bases

The hard-soft distinction is linked to polarizability, the degree to which a molecule

or ion may be easily distorted by interaction with other molecules or ions.

Hard acids or bases are small and non-polarizable

Soft acids and bases are larger and more polarizable

Hard acids are cations with high positive charge (3+ or greater),

or cations with d electrons not available for π-bonding

Soft acids are cations with a moderate positive charge (2+ or lower),

Or cations with d electrons readily availbale for π-bonding

The larger and more massive an ion, the softer (large number of internal electrons

Shield the outer ones making the atom or ion more polarizable)

For bases, a large number of electrons or a larger size are related to soft character


Acid base and donor acceptor chemistry hard and soft acids and bases

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


Acid base and donor acceptor chemistry hard and soft acids and bases

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


Acid base and donor acceptor chemistry hard and soft acids and bases

Quantitative measurements

Absolute hardness

(Pearson)

Mulliken’s absolute electronegativity

(Pearson)

EHOMO = -I

ELUMO = -A

Softness


Acid base and donor acceptor chemistry hard and soft acids and bases

  • Energy levels

  • for halogens

  • and relations between

  • , hand HOMO-LUMO energies


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