Unit 4 Acid-base and donor-acceptor chemistry Hard and soft acids and bases Miessler/Tarr Ch. 6

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

2. 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

3. 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

4. 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)

5. Frontier orbitals and acid-base reactions Remember the NH3 molecule

6. and BF3

7. adduct base acid Acids and bases (the Lewis concept) A base is an electron-pair donor An acid is an electron-pair acceptor Metal ions as acids; Lewis acid-base adducts involving metal ions: coordination compounds

8. Frontier orbitals and acid-base reactions Remember the NH3 molecule

9. New LUMO (non-bonding) New HOMO (bonding) Frontier orbitals and acid-base reactions The protonation of NH3 (Td) (C3v)

10. 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?

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

12. 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

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

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

15. 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-

16. The MO basis for hydrogen bonding F-H-F- LUMO HOMO We can ignore px and py lone pairs of both F- and HF since there are no matching orbitals on H atom HOMO

17. 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

18. 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

19. 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

20. Hard and soft acids and bases What is hard and what is soft? Hard acids or bases are small and non-polarizable Soft acids and bases are larger and more polarizable

21. 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

22. Class (b) soft metals have d electrons available for p-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 rightof 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

24. 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

25. How is this related to chemical behavior?

26. If these guys are looking for a girlfriend Which one would you say is the most likely candidate?

27. Two groups of friends are going out for a drink on Friday night What is the most likely composition of the two groups?

28. Hard-soft considerations allow us to make reasonable predictions 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 But there is more to it… 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

29. 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

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

32. Energy levels • for halogens • and relations between • , hand HOMO-LUMO energies Ionization E decreases going down in a group