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Unit 4Acid-base and donor-acceptor chemistry
Hard and soft acids and basesMiessler/Tarr Ch. 6
Graduate CenterAdvanced Inorganic Chemistry
(Fall 2010)
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 generaland can be interpreted in terms of MO’s
Rememberthat frontier orbitalsdefine the chemistry
of a molecule
-+C O
C OM
C O M
CO is a -donor anda -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 ionsare called coordination compounds (or complexes)
acid baseadduct
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
The protonation of NH3
Frontier orbitals and acid-base reactions
(C3v)(Td)
(non-bonding)
(bonding)
New HOMO
New LUMO
In most acid-base reactions HOMO-LUMO combinationslead 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-pairin 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-Dadducts form
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
LUMOHOMO
We can ignore px and py lonepairs of both F- and HF sincethere are no matching orbitalson 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. CH4 + H2O
Good energy match,strong H-bonding
e.g. CH3COOH + H2O
Very poor energy matchno adduct formed
H+ transfer reactione.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
H+...A-BHA
HOMO
B
LUMO
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-polarizableSoft 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 hardlow 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* orbitalsavailable for π-bonding react best with class (b) soft metals
The hard-soft distinction is linked to polarizability, the degree to which a moleculeor 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 electronsshield 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
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 energeticallymore favorable than soft-soft
An acid or a base may be hard or softand 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 > SbO >> S > Se > Te
F > Cl > Br > I
Tendency to complex with soft metal ions
N << P > As > SbO << S > Se ~ Te
F < Cl < Br < I
Quantitative measurements
2
AI
2
AI
Absolute hardness(Pearson)
Mulliken’s absolute electronegativity(Pearson)
1
Softness
Hard acid or base has a large I-AI E(HOMO) and A E(LUMO)
Energy levelsfor halogensand relations between, and HOMO-LUMO energies
Ionization Edecreases goingdown in a group