Acidbase and donoracceptor chemistry Hard and soft acids

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 e. g. HNO 3 + KOH KNO 3 + H 2 O Brønsted-Lowry: • acids tend to lose H+ • bases tend to gain H+ • acid 1 + base 1 + acid 2 (conjugate pairs) H 3 O+ + NO 2 - H 2 O + HNO 2 NH 4+ + NH 2 - NH 3 + NH 3 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

Remember that frontier orbitals define the chemistry of a molecule CO is a s-donor and a p-acceptor d+ C d. O M C O M

Acids and bases (the Lewis concept) A base is an electron-pair donor An acid is an electron-pair acceptor acid adduct base Lewis acid-base adducts involving metal ions are called coordination compounds (or complexes)

Frontier orbitals and acid-base reactions Remember the NH 3 molecule

Frontier orbitals and acid-base reactions The protonation of NH 3 New LUMO (non-bonding) New HOMO (bonding) (Td) (C 3 v)

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?

Frontier orbitals and acid-base reactions Even when symmetries match several reactions are possible, depending on the relative energies

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

The MO basis for hydrogen bonding F-H-F-

MO diagram derived from atomic orbitals (using F……. F group orbitals + H orbitals) Bonding e Non-bonding e

But it is also possible from HF + F- HOMO-LUMO of HF for s interaction Non-bonding (no symmetry match) Non-bonding (no E match)

The MO basis for hydrogen bonding F-H-F- HOMO LUMO HOMO First take bonding and antibonding combinations.

Similarly for unsymmetrical B-H-A 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. CH 4 + H 2 O Good energy match, strong H-bonding e. g. CH 3 COOH + H 2 O Very poor energy match no adduct formed H+ transfer reaction e. g. HCl + H 2 O

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).

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: Ag. F > Ag. Cl > Ag. Br >Ag. I But…… Li. Br > Li. Cl > Li. I > Li. F

Chatt’s explanation. Class (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 6 s electrons outside the 5 d 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

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

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

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

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 related With lower solubility, color and short interionic distances, whereas hard-hard interactions result in colorless and highly soluble compounds

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

Energy levels for halogens and relations between c, h and HOMO-LUMO energies