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Hard and soft acids and bases
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
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)
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
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: AgF > AgCl > AgBr >AgI
But……
LiBr > LiCl > LiI > LiF
Chatt’s explanationClass (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 6s electrons outside the 5d 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
IA

2
Absolute hardness
(Pearson)

1

Softness
IA

2
Mulliken’s absolute electronegativity
(Pearson)
EHOMO = -I
ELUMO = -A
Pearson’s Hard and Soft Acids and Bases
Most of the hard-soft distinction depends on polarizability.
Hard acids and bases are small, compact, and nonpolarizable.
Most metal ions are hard acids (class a ions, Fig. 6-11).
Hardness of a metal ion generally increases with its charge.
Soft acids are generally large and polarizable (class b ions).
Glead to softer behavior.
Soft acids can often be characterized as havenerally, more
electrons and larger sizes ing d electrons/orbitals for p bnding.
Tl(I) versus Tl(III) – Tl(III) has more class be character even though the
charge on the metal is greater. Why?
Pearson’s Principle:
Hard Lewis acids prefer to bind to hard Lewis bases; soft
Lewis acids prefer to bind to soft Lewis bases
Class (a)– hard acids
Class (b)– soft acids
Hard and Soft Acids and Bases (HSAB)
Let A be a Lewis acid, and B a base
Measure log K for the reaction
A + B  AB
If for B = halide, the order of
log K is
I– < Br– < Cl– < F–
then A is called a hard acid
If for B = halide, the order of
log K is
I– > Br– > Cl– > F–
then A is called a soft acid
Hard metal ions form their most stable complexes with
Hard Bases
Hard Bases: contain the smaller electronegative atoms,
especially O, N, F and Cl.
The bonding between a Hard Lewis Acid and a
Hard Lewis Base is predominantly ionic
Soft metal ions form their most stable complexes with
Soft Bases
Soft Bases: contain the larger, more polarisable and less
electronegative atoms, especially S, Se, P, C and As.
The bonding between a Soft Lewis Acid and a
Soft Lewis Base is predominantly covalent
Quantitative measure
Absolute hardness  = (I –A)/2
Mulliken’s definition of electronegativity
 = (I + A)/2
Softness  = 1/
Reactions favor hardness matches
Hard=small, compact charge, and nonpolarizable.
M+3, O-2
Soft=large, polarizable.
M0, S2-
Acid and Base Strength
Binary hydrogen compounds
The acidity increases across a period (the
electronegativity also increases).
HF>H2O>NH3
Explanation: The negative charge of the
conjugate base is spread out over more lone
pairs (electrons) as the base gets larger. The
larger the number of lone pairs, the lower the
attraction for protons.
Acid-Base Strength, Inductive
Effects
• Electronegativity effects can can change the acidity and
basicity.
– PF3 is a weaker base than PH3 due to the electrons being
drawn toward the electronegative fluorines.
NMe3>NHMe2>NH2Me>NH3 (base strength)
Alkyl groups tend to increase the electron density on the
center atom.
Electron-contributing and electron-withdrawing capabilities
of the ligands need to be considered.
Acid-Base Strength, Oxyacids
•
HClO4>HClO3>HClO2>HClO
– pKa  9-7n
• n = number of nonhydrogenated oxygen atoms.
Explanation: Nonhydrogenated oxygens draw electron density from the
central atom. The central atom, in turn, draws electron density away from
he hydrogenated oxygen. The net result is a weaker O-H bnd.
Other explanation: The charge of the conjugate base is spread over all the
nonhydrogenated oxygens. Stability of the conjugate base increases with
the number of these oxygens producing a weaker base.
Acidity of Cations in Aqueous
Solutions
•
•
•
In general, stronger acids form from metal ions with larger charges and smaller radii.
– [Fe(H2O)6]3+ + H2O  [Fe(H2O)5OH]2+ + H3O+
Solubility of the metal hydroxide is also a measure of cation acidity. The stronger the
cation acid, the less soluble the hydroxide.
– 3+ metal ions form hydroxides that precipitate.
• Fe+3(aq) + 3H2O(l)  Fe(OH)3(s) + 3H+(aq)
– 2+ d-block ions (and Mg+2) from hydroxide precipitates in neutral or slightly acidic
solutions
– Alkali (e.g. Na+) and alkaline earth metals produce no pH changes (spectators).
The free metal cation is no longer detectable for highly-charged species.
– UO2+ and CrO42-
Steric Effects
• Repulsions by bulky groups make reactions less favorable
for adduct formation.
• Different types of strain (Brown):
– F (front) – bulky groups interfere directly with the
approach.
– B (back) – bulky groups interfere with each other when
they bend away from the other molecule forming the
adduct.
– I (internal) – electronic differences within similar
molecules.
Examples
Solvation
• In aqueous solution the basicities have the order
– Me2NH>MeNH2>Me3N>NH3 Why?
– The reduced solvation of their protonated cations.
• Solvation energies; RNH3+>R2NH2+>R3NH+ for the
reaction RnH4-nN+(g) + H2O  RnH4-nN+(aq). Solvation
is dependent upon the number of hydrogen atoms
available to hydrogen bond.
– Competition between induction and solvation
produce the observed order for the amines.
Nonaqueous Solvents
• Any acid stronger than H3O+ reacts completely
with H2O to form H3O+. The is called the
leveling effect; acids or bases are brought down
to the limiting conjugate acid or base of the
solvent.
– HCl, HBr, HClO4, and HNO3 all have the same
strength in water.
• Likewise, the strongest base in water is OH-.
– Water can differentiate weak acids and bases.
Nonaqueous Solvents
• Acetic acid can differentiate the strengths of many
strong acids.
– H2SO4 + HOAc  H2OAc+ + HSO4– In acetic acids, HClO4>HCl>H2SO4>HNO3
• Inert solvents allow a broad range of acids and
bases to be differentiated according to strength.
The do not form acid or base species readily.
– Figure 6-17.
Hard and Soft Acids and Bases.