Unit 4 Acid-base and donor-acceptor chemistry Hard and soft acids and bases Miessler/Tarr Ch. 6 Graduate Center Advanced Inorganic Chemistry (Fall 2010)

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  base 1 + acid 2 (conjugate pairs) H 3 O + + NO 2 -  H 2 O + HNO 2 NH 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 COM COM CO is a  -donor and a  -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 ions are called coordination compounds (or complexes) acid baseadduct

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

and BF 3

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 NH 3 molecule

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

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?

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

A base has an electron-pair in 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-D adducts 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 LUMO HOMO We can ignore p x and p y lone pairs of both F - and HF since there are no matching orbitals on 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. 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

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 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-polarizable Soft 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 hard low 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* orbitals available for π-bonding react best with class (b) soft metals

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+ 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 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

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 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 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 > Sb O >> S > Se > Te F > Cl > Br > I Tendency to complex with soft metal ions N As > Sb O Se ~ Te F < Cl < Br < I

Quantitative measurements Absolute hardness (Pearson) Mulliken’s absolute electronegativity (Pearson) Softness Hard acid or base has a large I-A I  E(HOMO) and A  E(LUMO)

Energy levels for halogens and relations between ,  and HOMO- LUMO energies Ionization E decreases going down in a group