Download 5.Hard and Soft Acid and Bases

Hard and Soft Acid and Bases
Lewis Acids & Bases
A base is an electron-pair donor, and an acid is an electron-pair
acceptor. The two combine to form an adduct.
A + :B  A-B
In complexes metals are electron acceptors hence they are acids and Ligands
give lone pair of electrons and are bases.
Metals were divided into two categories
Class A/Type I
1.Alkali and alkaline earths
2. TM in higher OS
3. Small size
4. Highly electropositive
Class B/Type II
Transition metals lower OS
Large size
Less electropositive
Hard and Soft Acids
R.G. Pearson (1963) categorized acids and bases as either hard or soft (using Kf
values).
Hard acids
1.
Alkali and alkaline earths
2.
TM in higher OS
3.
Highly electropositive and very few electrons
4.
They are not very polarisable and prefer less polarisable ligands.
5.
Prefer complexes with ligands having donor atoms like N, O, F
6.
high charge density
7.
Their d orbitals are often unavailable to engage in π bonding.
8.
Hard Acids prefer to bond with Hard Bases
Hard and Soft Acids
Soft acids
•Transition metals lower OS
•Large size
•Less electropositive
•Polarisable
•Their d orbitals are available for π bonding.
•often 2nd and 3rd row transition metals with a +1 or +2 charge
• filled or nearly filled d orbitals
•Prefer complexes with ligands having donor atoms like P, S, Cl
•Soft Acids prefer to bond with Soft Bases.
Hard Bases
1. Ligands having highly electronegative donor atoms – F, O, N
2. They prefer complexes with hard acids.
3. Less polarisable
4. Small size
5. Strength of hard bases –
F>Cl> Br >O > S> Se>N> P>
Soft Bases
1. Donor atom is less electronegative – P,As,
2. Large size
3. High polarisability
4. They prefer complexes with soft acids
Acids
Hard Acids
H+, Li+, Na+, K+
Borderline
Soft Acids
BBr3,B(CH3)3
Cu+,Ag+, Au+,
Be2+, Mg2+, Ca2+
Cd2+,Hg22+,
BF3, BCl3, B(OR)3 BH3,Tl+, Tl(CH3)3
Hg2+, Pd2+,Pt2+,
Al3+, AlCl3,AlH3
Pt4+
Cr3+,Mn2+, Fe3+, Co3+
Fe2+,Co2+,Ni2+
Cu2+,Zn2+,Rh3+
Ir3+, Ru3+, Os2+
Bases
Hard Bases
Borderline
Soft Bases
F-, Cl-
Br-
H-, I-
H2O, OH-,O2-
NO2-, N3- , N2
H2S, HS-, S2-
ROH, RO-, R2O, CH3CO2-
SO32-
RSH, RS-, R2S
NO3-, ClO4-
SCN-, CN-, CO
CO32-,SO42-, PO43-
S2O32-
NH3, RNH2
C6H5NH2
R3P, C6H6
HSAB principle
•Hard Acids prefer to bond with Hard Bases
•Soft Acids prefer to bond with Soft Bases.
•HA and HB are not polarisable so the bond is mostly ionic.
•SA and SB is highly polarisable so the bond is covalent.
Applications of Hard/Soft Theory
1. Stability of a complex –
A complex would be more stable if metal and ligand, both are hard or soft.
eg – 1. AgI2- is stable but AgF2- is not.
Ag+ + 2I-  AgI2SA
SB
Stable complex
Ag+ + 2F-  AgF2SA
HB
Unstable
2. [CoF6] 3- is more stable than [CoI6] 3HA-HB
HA-SB
2. Feasibility of a reaction
A reaction is favourable if the product follows HSAB rule, like combinations.
Eg.
LiI
+
CsF

LiF
+ CsI
HA-SB
SA-HB
HA-HB
SA-SB
CaS
+
HA-SB
H 2O
HB

CaO
+
HA-HB
H 2S
SB
3. The Qual Scheme, a series of chemical reactions used to separate
and identify the presence of dozens of metal ions, is based largely on the
hard and soft properties of the metal ions.
The softer metals are precipitated out as chlorides or sulfides,
with the harder ions formed as carbonates.
4.Thiocyanate, an ambidentate ligand:
SCN- vs. NCS- Thiocyanate (SCN-) is a particularly interesting ligand. It is
ambidentate, and can bind to metal ions either through the S or the N.
Obviously, it prefers to bind to soft metal ions through the S, and to hard
metal ions through the N. This can be seen in the structures of [Au(SCN)2]and [Fe(NCS)6]3- in the Figure below:
Thiocyanate
Complexes showing
a) N-bonding in the
[Fe(NCS)6]3complex with the hard
Fe(III) ion, and
b) S-bonding in the
[Au(SCN)2]- complex
(CSD: AREKOX) with
the soft Au(I) ion
5. Evidence in Nature
The chalcophile elements are typically found as sulfides or bonded to
Se2- or Te2-. They include: Cd2+, Pb2+, Sb3+, and Bi3+. These are soft Lewis
acids.
6.
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