Download Acids and bases

Graduate Center
Advanced Inorganic Chemistry
(Fall 2010)
Unit 4
Acid-base and donor-acceptor chemistry
Hard and soft acids and bases
Miessler/Tarr Ch. 6
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
Remember
that frontier orbitals
define the chemistry
of a molecule
d+
C
dO
M
C
O
C
O
M
CO is a s-donor and
a p-acceptor
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 NH3 molecule
and BF3
Acids and bases (the Lewis concept)
A base is an electron-pair donor
An acid is an electron-pair acceptor
acid
adduct
base
Metal ions as acids; Lewis acid-base adducts involving metal ions: coordination compounds
Frontier orbitals and acid-base reactions
Remember the NH3 molecule
Frontier orbitals and acid-base reactions
The protonation of NH3
New LUMO
(non-bonding)
New HOMO
(bonding)
(Td)
(C3v)
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
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-
[F-H-F]MO diagram derived from atomic orbitals
(using F…….F group orbitals + H orbital)
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
We can ignore px and py lone
pairs of both F- and HF since
there are no matching orbitals
on H atom
LUMO
HOMO
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. CH4 + H2O
Good energy match,
strong H-bonding
e.g. CH3COOH + H2O
Very poor energy match
no adduct formed
H+ transfer reaction
e.g. HCl + H2O
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
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
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 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 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-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
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
Quantitative measurements
IA

2
Absolute hardness
(Pearson)
s
1

Softness
IA

2
Mulliken’s absolute electronegativity
(Pearson)
Hard acid or base has a large I-A
I  E(HOMO) and A  E(LUMO)
Energy levels
for halogens
and relations between
,  and HOMOLUMO energies
Ionization E
decreases going
down in a group