Unit 4Acid-base and donor-acceptor chemistry

Hard and soft acids and basesMiessler/Tarr Ch. 6

Graduate CenterAdvanced Inorganic Chemistry

(Fall 2010)

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 generaland can be interpreted in terms of MO’s

Rememberthat frontier orbitalsdefine the chemistry

of a molecule

-+C O

C OM

C O M

CO is a -donor anda -acceptor

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 ionsare called coordination compounds (or complexes)

acid baseadduct

Frontier orbitals and acid-base reactions

Remember the NH3 molecule

and BF3

Acids and bases (the Lewis concept)

A base is an electron-pair donor An acid is an electron-pair acceptor

acid baseadduct

Metal ions as acids; Lewis acid-base adducts involving metal ions: coordination compounds

Frontier orbitals and acid-base reactions

Remember the NH3 molecule

The protonation of NH3

Frontier orbitals and acid-base reactions

(C3v)(Td)

(non-bonding)

(bonding)New HOMO

New LUMO

In most acid-base reactions HOMO-LUMO combinationslead 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?

When symmetries match several reactions are possible,depending on the relative energies

Frontier orbitals and acid-base reactions

A base has an electron-pairin a HOMO of suitable symmetry

to interact with the LUMO of the acid

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-Dadducts form

The MO basis for hydrogen bonding

F-H-F-

Bonding e

Non-bonding e

MO diagram derived from atomic orbitals(using F…….F group orbitals + H orbital)

[F-H-F]-

But it is also possible from HF + F-

Non-bonding(no E match)

Non-bonding(no symmetry match)

HOMO-LUMO of HF for interaction

The MO basis for hydrogen bonding

F-H-F-

HOMO

LUMOHOMO

We can ignore px and py lonepairs of both F- and HF sincethere are no matching orbitalson H atom

Similarly for any unsymmetrical B-H-A

producing H-bonding

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

the energies of reactants

Poor energy match, little or no H-bonding

e.g. CH4 + H2O

Good energy match,strong H-bonding

e.g. CH3COOH + H2O

Very poor energy matchno adduct formed

H+ transfer reactione.g. HCl + H2O

HYDROGEN BONDING FOR F, O AND N

• When A highly EN: F, O or N

• HOMO A lower energy than 1s H orbital (H more positive charge)

• Hydrogen bonding interaction favored as the overall energy of MO in HA is lowered and the overlap with B orbital is improved

H+...A-BHA

HOMO

B

LUMO

When reactant HA has an structure close to H+….A- hydrogen bonding more likely

Hard and soft acids and bases

Hard acids or bases are small and non-polarizableSoft acids and bases are larger and more polarizable

What is hard and what is soft?

Class (a) (hard) and class (b) (soft) metals according to Chatt

Class (b) or soft always

Borderline cases (depends on oxidation state)Others (blank) are class (a) or hard

Class (b) soft metals have d electrons available for -bonding

High oxidation states of elements to the right of transition metals have more class b (soft) character(Tl(III) > Tl(I) ,two 6s electrons shield the 5d making them less available for π-bonding)

For transition metals: high oxidation states and position to the left of periodic table are related to hardlow oxidation states and position to the right of periodic table are related to 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

The hard-soft distinction is linked to polarizability, the degree to which a moleculeor 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+ o 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 available for π-bonding

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

How is this related to chemical behavior?

If these guys are looking for a girlfriend

Which one would you say is the most likely candidate?

Two groups of friends are going out for a drink on Friday night

What is the most likely composition of the two groups?

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

Hard-soft considerations allow us to make reasonable predictions

But there is more to it…

Tendency to complex with hard metal ions

N >> P > As > SbO >> S > Se > Te

F > Cl > Br > I

Tendency to complex with soft metal ions

N << P > As > SbO << S > Se ~ Te

F < Cl < Br < I

Quantitative measurements

2AI

2

AI

Absolute hardness(Pearson)

Mulliken’s absolute electronegativity(Pearson)

1

Softness

Hard acid or base has a large I-AI E(HOMO) and A E(LUMO)

Energy levelsfor halogensand relations between, and HOMO-LUMO energies

Ionization Edecreases goingdown in a group