1. Chapter 6 Lecture 2 Hard-Soft Acid-Base Concepts
Hard and Soft Acids and Bases
Factors other than acid/base strength determine acid/base reactivity
Silver Halide solubility
AgX(s) H2O Ag+(aq) X-(aq)
Ksp’s: AgF = 205, AgCl = 1.8 x 10-10, AgBr = 5.2 x 10-13, AgI = 8.3 x 10-17
Explanation of Solubility
Solvation: F- much better solvated (small, high charge)
Degree of Ag—X Interaction must also play a role
HSAB Theory can help explain this data
Hard acids/bases are small and nonpolarizable
Soft acids/bases are large and polarizable
Hard/Hard and Soft/Soft interactions are the most favorable
Polarizable = easily distorted by other charged ions
AgX data and HSAB
Ag+ is large and polarizable = Soft
Softness of Halides: I- > Br- > Cl- > F-
AgI has the strongest interaction, thus the lowest solubility
Softness is also associated with covalent bonds, not ionic bonds

2. 5) Coordination of Thiocyanate (SCN-) to metal ions
SCN- binds to large, polarizable metals, through S: Hg2+----SCN
SCN- binds to smaller, less polarizable metals through N: Zn2+----NCS
Explanation: Hard/Hard and Soft/Soft interactions are favored
Exchange Reactions of [CH3Hg(H2O)]+
a) [CH3Hg(H2O)] HCl CH3HgCl H3O+ K = 1.8 x 1012
[CH3Hg(H2O)] HF CH3HgF H3O K = 4.5 x 10-2

3. 7) LiX solubility: LiBr > LiCl > LiI > LiF
Li+ is a hard ion
LiF would be expected to be very ionic and soluble
Very favorable hard-hard LiF interaction even overcomes solubility
LiBr, LiCl are more soluble because of less favored interactions
LiI is out of order because of poor I- solvation
Pearson’s Hard and Soft Acids and Bases (1963)
Most metal ions are hard acids (class a), some borderline depending on charge
Large polarizable metal ions are soft (class b)
Lewis bases can also be categorized as hard or soft
Reactions favor hardness matches
Hard/hard more energetically favored than soft/soft interactions
Soft and borderline
soft metal ions

4. 6) Polarizability = degree to which an atom’s electron cloud is distorted by interactions with other ions
Hard = small, compact charge, nonpolarizable = M3+, O2-
Soft = large, polarizable = M0, S2-
Comparison is easiest within a column of the periodic table

5. Pearson’s Absolute Hardness = h
Quantitative method to measure hardness and softness, predict matches
Formula uses Ionization energy (I) and Electron Affinity (A)
Related to Mulliken’s definition of Electronegativity
Defines Hardness as a large difference between I and A
I = HOMO energy
A = LUMO energy
Softness = s = 1/h
Halogens as an example
a) Trend in h parallels HOMO
energy (LUMO’s are about the same)
b) F = most electronegative, smallest,
least polarizable = hardest
c) ClBrI h decreases as
HOMO energy increases
Problem: h doesn’t always match
reactivity (hard, but still weak acid)

7. Drago’s Quantitative Approach
-DH = EAEB + CACB
DH = enthalpy of reaction for A + B AB
E = capacity for ionic interactions (calculated from various reactions)
C = capacity for covalent interactions (for various reactions)
I2 used as reference E = 1.00, C = 1.00
Primarily covalent, so most E values > 1, most C values < 1

8. Frontier Orbitals and Acid-Base Chemistry
Example: I C6H I2 • C6H6
-DH = (1.00 x x 0.525) = kcal/mol (weak adduct)
Advantages of Drago’s System
Emphasis on the 2 factors involved in acid-base strength
Electrostatics
Covalency
Pearson’s Hardness only considers covalency
Good predictability if E and C have been tabulated
If no data is available, Pearson’s HSAB method still allows for a rough prediction of the strength of an acid-base reaction
Frontier Orbitals and Acid-Base Chemistry
HOMO-LUMO Combination
Acid-Base Reactions result in new product frontier orbitals
Example: NH H NH4+
NH3 lone pair reside in a1 MO = HOMO (base)
H+ has only an empty 1s AO = LUMO (acid)

9. Combine these two orbitals to form a Bonding/ Antibonding pair
Lone pair is stabilized in the new bonding orbital
NH4+ is more stable than the separate NH3 and H+
Pick orbitals of similar symmetry and energy to combine
If there is no match, there is no acid-base adduct formed

10. 3) Symmetry, Energy, and Occupation of Frontier Orbitals allow us to predict the result of a given reaction
a1 b1 b2

11. 2 H2O Ca Ca H2(g) OH-
Ca HOMO >> H2O LUMO
Not matched well for acid-base adduct to form
Electron transfer (oxidation of Ca by H2O) is the predicted reaction
n H2O Cl [Cl(H2O)n]-
Correct energy match for acid-base adduct to form and be stable
Water is the acid; its LUMO is used along with the base (Cl-) HOMO
6 H2O Mg [Mg(H2O)6]2+
Water is the base; its HOMO is used along with the LUMO of acid Mg2+
2 H2O F F H O2
Water HOMO >> F2 LUMO
Not matched well for an acid-base adduct to form
Electron transfer (reduction of F2 by H2O) is the predicted reaction

12. 4) Restating the Lewis Acid-Base Definition:
a) Base: has e- pair in HOMO of correct energy and symmetry
b) Acid: has LUMO of correct energy and symmetry
HSAB using HOMO-LUMO
a) Hard/Hard = simple electrostatics, little change in HOMO-LUMO energies, which remain far apart
Soft/Soft = HOMO and LUMO close in energy combine to form energetically favorable new MO’s
Hard/Hard is usually more favored than it appears due to +/-charge attraction

13. Carbon Monoxide as a Lewis Base
1) Electronegativity suggests O is the e- pair donor
2) In fact, C is always the donor
a) Formal Charge
b) MO Frontier Orbitals
i. HOMO that is involved in bonding is mostly on C
ii. C-like HOMO donated to the M Lewis acid

14. Hydrogen Bonding
FHF- (symmetric, equivalent bonds to H)
In chapter 5, we described the MO’s as H + F•F group orbitals
Using a HOMO-LUMO Acid-Base description, we can use HF F-
F- HOMO (base) HF LUMO and HOMO (acid) give 3 new MO’s
Filled bonding MO of lowest energy
Filled nonbonding (node through H) orbital
Empty antibonding (nodes between all atoms) orbital of highest energy
3-center 2 electron bond, with ½ bond order per H—F bond

15. Unsymmetric BHA molecules are simplest example of Hydrogen Bonding
Similar MO picture to that of FHF-
Overall lower energy for the 2 e- pairs when bonded than unbonded
3 different possibilities for the energy match
(a) Poor match: H2O CH4 = overall higher energy; no H-bonding
(b) Good match: H2O HOAc = overall lower energy; strong H-bond
(c) Very poor match: H2O HCl = H atom transfer
Why are electronegative atoms good at H-bonding?
Low energy H—A HOMO similar in energy to H+ 1s orbital

16. D. Electronic Spectra of I2 (acid) + Donor (base) adducts

17. I2 gas: violet due to pg. su
I2 gas: violet due to pg*su* transition (blue and red transmitted = purple)
Nondonor hexane solvent causes no change = purple
Donor solvents benzene and and methanol
Donor HOMO interacts with su* LUMO of I2
New HOMO/LUMO energies of adduct shift spectrum
Color (light transmitted) is changed
Red-violet in benzene
Yellow-brown in methanol
KI/H2O solution with I2 forms I2—I- adduct I3-
pg*su* energy increases, LUMO higher in energy toward blue
ss* transition appears = Charge Transfer appears whenever a good donor interacts with I2
Charge transfer is so named because the transition occurs between an orbital mostly composed of one of the partners to an orbital mostly composed of the other partner

18. Charge Transfer in [Fe(H2O)5X]2+ complexes
HOMO of X- increases in energy from F- to I-
F- complex has closest HOMO-LUMO energy match forming a strong complex with stabilized new HOMO = colorless (all UV)
Cl- complex has less interaction, new HOMO not stabilized as much, yellow color
Br- even less interaction, less stabilization, yellow-brown color
I- has no interaction, charge transfer only
2 Fe I Fe I2 (purple color of I2 only)