Hard-Soft Acid-Base (HSAB) Theory Classifying Lewis acids and bases into: (i)) Ha (i Hard rd acid (ii) (ii) Soft Soft acid acid Ralph G. Pearson
(iii (i ii)) Ha Hard rd ba base se (iv) (iv) Soft Soft base base
Hard acids (hard metal ions) form more stable complexes with hard bases (hard ligands), while soft acids (soft metal ions) shows a preference for soft bases (soft ligands)
e.g. AgF(s) + H2O
→
Ag+(aq) + F-(aq) K sp = 205
AgCl(s) + H2O
→
Ag+(aq) + Cl-(aq) K sp = 1.8x10-10
AgBr(s) + H2O
→
Ag+(aq) + Br -(aq) K sp = 5.2x10-13
AgI(s) + H2O
→
Ag+(aq) + I-(aq) K sp = 8.3x10-17
K sp = solubility product Reasons:
different so solvation energy of th the anions Interaction with Ag+: I > Br > Cl > F
Lithium Lithium halide halides s have solubi solubiliti lities es roughly roughly in the re reverse ord orde er: LiB LiBrr > Li LiCl > Li LiI > Li LiF LiI is out out of of order order,, proba probably bly bec becaus ause e of the poo poorr soubl soublity ity of the the large iodide ion but it is still 100 times more soluble than LiF
Strong interactions in Li-F overcome the solv solvat ation ion of wate water r
•These interactions interactions between between cations cations and anions can be expressed in term of hard and soft acids and bases (HSAB) Electr tron ons s in in pol polar ariz izab able le mole molecu cule les/ s/io ions ns ca can n be be •Elec attracted or repelled by charges on other molecules
•Hard acids and bases are small and nonpolarizable •Soft acids and bases are larger and more polarizable •Interactions btw 2 hard or 2 soft species are stronger than those btw 1 hard and 1 soft species
Characteristic Properties of Hard and Soft Acids and Bases
Properties
Hard acids
Electronegativity
0.7 -1.6 1.9-2.5
2.1 – 3.0 3.4 – 4.0
Ionic radius (pm)
<90
>170
Ionic Charges
≥
+3
Soft acids
>90 ≤
+2
Soft bases
Hard bases
~120
Explanation for HSAB Principle •Large difference in electronegativity electronegativity btw hard acids acids and and hard bases bases – strong strong ionic ionic bonds bonds •Soft acids acids and bases bases have similar similar electron electrongeat geativity ivity – favors covalent covalent bonds (the large size and and low charge of the atoms, although not favorable for covalent bonding, are even less favorable for ionic bonding) •Soft acid-hard base or hard acid-soft base combinat combinations ions – the relative relative electron electronegat egativity ivity and ionic sizes are not optimal for either ionic or covalent bonding. The bonding is polar covalent. •Polar covalent compounds tend to be reactive if they can react to give an ionic product and a covalent product.
e.g.
R
Polar covalent
ionic
O
covalent
+ H3C
O- MgBr
R2 C
R
Mg
Polar covalent
CH3 Br
Hard acids
Borderline Acids
Soft acids
H+, Li+, Na+, K+, Be2+, Mg2+, Ca2+, Sr2+, BF3, B(OR)3, Al3+, Al(CH3)3, AlCl3, AlH3, Cr3+, Mn2+, Fe3+, Co3+ Ga3+, Sc3+ , In3+, Sc3+,
B(CH3)3, Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Rh3+, Ir3+, Ru3+, Os2+,
BH3, Tl+, Tl(CH3)3, Cu+, Ag+, Au+, Cd2+, Hg22+, Hg2+, CH3Hg+, Pd2+, Pt2+, Br2, I2
Ions with oxidation state of 4 or higher i.e. Ti4+, Zr4+,
Metals with zero oxidation state
Hard bases
Borderline bases
F-, H2O, OH-, O2-, Cl-, Br-, NO2-, N32-, ROH, RO-, R2O, SO32-, C6H5NH2, CH3COO-, NO3-, C5H5N, N2 ClO4-, CO32-, SO42-, PO43-, NH3, RNH2, N2H4
Soft bases H-, I-, H2S, HS-, S2-, RSH, RS-, R2S, CN-, RCN, CO, S2O32-, PR3, P(OR)3, AsR3
Predict whether reactants or products are favored in the following equlibirum
Nb2S5 + 5 HgO
Nb2O2 + 5 HgS
Consider a reaction in which both Lewis acids are soft and both Lewis bases are soft
CdSe + HgS
CdS + HgSe
HSAB principle can be used to predict the position of the equilibrium because it is possible to assign relative softness to different soft acids and bases
Less soft acids tend to combine combine with less soft bases; softer acids prefer softer bases
