Hard Soft Acid Base Theory
H Acids: small, highly (+) ions
low polarizability, fewer e–
e.g. Fe3+ Co3+ Cr3+
H Bases: small, high χ donor atoms
low polarizability
e.g. F–, N- and O-donors
S Acids: large, less (+) ions
S Bases: large, lower χ donor atoms
high polarizability, more e–
high polarizability
e.g. Pt2+ Ag+ Au+
e.g. I–, C- P- and S- donors
Metal ions: softness ↑ as M→row and M↓column, softness ↑ as n+ ↓
Ligands: hardness ↑ as L→row and ↑column
Hard Acids
H+ Gr1 M+ Gr 2 M2+ Gr 13 M3+ Ti4+
V5+ VO2+ Cr3+ Cr2+
Mn2+ Fe3+ Co3+
Hard Bases
F– Cl– HO– RO– RCO2– CO3– NO3–
SO42– ox2– H2O ROH NH3 RNH2
Intermediate Acids
Fe2+ Co2+ Ni2+ Cu2+ Zn2+
Ru3+ Ir3+ Rh3+
Intermediate Bases
py N3– NO2– SO32– Br– NCS–
Soft Acids
Pd2+ Pt2+ Cu+ Ag+ Au+
Cd2+ Hg+ Hg2+ Tl+ Pb2+
Soft Bases
I– H– R– CN– CNR CO
SCN– RS– RSH R2S R3P alkene arene
harder
A qualitative observation:
“Hard likes hard, soft likes soft”
e.g. AHBS + ASBH ⎯→ AHBH + ASBS will have K > 1
e.g. ML bond strengths: F– > Cl– > Br– > I– for Zn2+ F– < Cl– < Br– < I– for Hg2+
softer
ionic radii, Oh M2+
100
95
rion (pm)
90
85
HS
80
75
70
LS
65
60
M2+ Ca Sc Ti V Cr
dn 0 1 2 3 4
CFSE HS (Dq) 0 –4 –8 –12 –6
CFSE LS (Dq)
–16
Mn
5
0
–20
Fe
6
–4
–24
Co Ni Cu Zn
7 8 9 10
–8 –12 –6 0
–18
Hydration Enthalpies, M2+
Irving Williams Series
[M(H2O)6]2+ + L → [M(L)(H2O)n]2+ + n H2O
M2+(g) + 6 H2O → [M(H2O)6]2+(aq)
3000
20.0
18.0
2950
–ΔHhydr (kJ/mol)
16.0
log K1
14.0
12.0
[EDTA]4–
en
NH3
10.0
8.0
6.0
4.0
2900
2850
2800
2750
2.0
2700
0.0
Mn
Fe
Co
M2+
dn
CFSE (Dq)
rion
stability
Ni
Cu
Mn
5
0
Mn
Zn
Fe
6
–4
>
<
Co
7
–8
>
<
Ni
8
–12
>
<
<
≤
Fe
Co
Cu
9
–6
Zn
10
0
<
>
Ni
Cu
Zn