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Element
Fe
Cu
S
Mo
V
reducing
environment
Fe2+ (high)
Cu•sulfides
(low)
HS- (high)
[MoOnS4-n]2MoS2 (low)
V3+,
V4+sulfides
oxidizing
environment
Fe3+ (low)
Cu2+ (moderate)
SO42- (high)
MoO42(moderate)
VO43-(moderate)
Note switch in relative availability of Cu and Fe.
A.-F. Miller, 2008, pg
1
Frausto da Silva & Williams Table 1.6
Inorganic Chemistry Concepts for Bio.
Thermodynamics
Hard-soft acids and bases.
The chelate effect.
Ligand pKa depression.
Redox potential tuning.
Kinetic considerations.
Ligand exchange rates.
Electron transfer.
Electronic and geometric structures.
Reaction of coordinated ligands.
A.-F. Miller, 2008, pg
2
Hard and Soft Metal
Larger, more polarizable metal ions gain extra stabilization
from this capability if the ligands also share this possibility.
Harder smaller metal ions’ electrostatic interactions are
stronger, so bonds with similarly hard ligands are stronger.
A.-F. Miller, 2008, pg
3
Soft and hard metals and ligands
Metal ions
Hard
H+, Na+, K+
Mg2+, Mn2+, Ca2+,
Al3+, Cr3+, Co3+, Fe3+
Fe2+, Co2+, Ni2+,
Intermed. Cu2+, Zn2+
Ligands
PO43-, CO32-, ROPO32-,
OH-, CH3CO2-, Cl-, RO-,
NO3-, H2O, NH3,
NO2-, SO3-, Br-, N3-,
N2,
NH
N
Soft
A.-F. Miller, 2008, pg
Cu+, Au+,
Cd2+, Pb2+, Hg2+
4
Lippard & Berg, Table 2.1
NH2
RS-, CN-, SCN-, H-,
R2S, RSH, R3P, CO, NO
Metallothionein: a soft-ligand protein.
1/3 of amino acids are Cys.
Binds Cd2+, Hg2+, Pb2+, thus protecting the cell from them.
Those metal ions otherwise bind to critical SH groups and
displace other metal ions from soft ligands.
A.-F. Miller, 2008, pg
5
Ag-bound metallothionein. Armitage et al. 1AOO.pdb
Calmodulin, a hard-ligand protein
A.-F. Miller, 2008, pg
Ad Bax 2HF5.pdb
6
The chelate effect
M+L
ML
M + 2L
L L
M
M + L~L
A.-F. Miller, 2008, pg
7
L~L
M
Kb = e-ΔGb/RT, ΔGb = ΔHb -TΔSb
ΔSb is -ve
K’b = e-ΔG’b/RT, ΔG’b =2ΔHb-2TΔSb
K’b = e-ΔG’b/RT, ΔG’b =2ΔHb-TΔSb
ΔG’b is more favourable by TΔSb
ethylenediaminetetraacetic acid = EDTA
O
O
OH
H2C
N
HO
CH2
O
A.-F. Miller, 2008, pg
8
HO
CH2
H2
C
C
H2
OH
N
C
H2
O
www.3dchem.com/molecules.asp?ID=89
EDTA
Based on "YMCA" by The Village People
I
Ligands, there’s no need to feel down,
I said ligands, when you’re floating around,
You don’t have to stay there, free and unbound
–
There’s no need to be uncomplexed!
Ligands, you’ve got electron pairs,
They’re not bonded - and they’re just sitting
there;
A cation - if you’re willing to share –
Could accept your spare electrons!
Chorus I
You’ve got to complex like EDTA,
You’ve got to complex like EDTA;
It’s got everything to be hexadentate!
It’s got six lone pairs to donate!
You’ve got to complex like EDTA,
You’ve got to complex like EDTA!
It’s ethylene-dia-mine-tetra-acet-ate!
It’s a ligand that can chelate!
II
Ligands you might bond to class b
Metals - if you’re polarised easily
(As are sulphur, phosphorus, iodine)
And form more covalent compounds;
Ligands, if you’re hard (like fluorine)
You’re electronegative, so you’ll be
Bound to harder metals like Al (III)
A.-F. Miller, 2008, pg
9
With elec-tro-stat-ic bonding!
Chorus II
You’ve got to complex like EDTA,
You’ve got to complex like EDTA;
It replaces all six H2Os separately,
So the entropy must increase!
You’ve got to complex like EDTA,
You’ve got to complex like EDTA;
It’s the chelate effect! It’s a favoured process!
It’s a positive delta S!
III
Ligands can you act as a pi
Donor? They can even stabilise high
Oxidation states – they’re weak field and high
Spin – the delta value’s smaller;
But if the pi* are empty
They’re acceptors lowering t2g
And increasing the gap in energy:
The ligand field splitting’s larger!
Repeat Choruses I and II until bored
Aimee Hartnell, February 2002
http://www.geocities.com/le_chatelier_uk/song_index.html
Deprotonation of ligands
The metal ion competes with protons, both are cations.
Ligand & rxn.
Metal ion
pKA (25°C, 0.1 M)
H2O +
M2+
M-OH +H+
NH3 + M2+
M-NH2 + H+
none
Ca2+,
Mn2+,
Cu2+,
Zn2+
none
Co2+,
Ni2+,
Cu2+
none
Mg2+,
CH3COOH +
Ca2+,
Ni2+,
M-OOCCH3 +H+
Cu2+
none
HN
2+
Co2+,
NH + M
HN
+
Ni2+,
+
H
N
Cu2+
A.-F. Miller, 2008, pg
10
Lippard & Berg, Table 2.2
M2+
14.0
13.4
11.1
10.7
10.0
35.0
32.9
30.7
32.2
4.7
4.2
4.2
4.0
3.0
7.0
4.6
4.0
3.8
Formation of Fe clusters coupled to
deprotonation of coordinated OHH2O
H2O
H2O
H2O
H2O
H2O
-
OH
Fe3+
OH
HO-
H2O
Fe3+
HO-
H2O
H2O
H2O
pKA ~ 6
A.-F. Miller, 2008, pg
11
-
OH
Fe3+
Fe3+
H2O
H2O
O2-
H2O
H2O
O2-
H2O
H2O
H2O
Fe3+
H2O
H2O
H2O
Fe3+
O2-
H2O
H2O
H2O
H2O
+ H2O
Coordination to a metal ion also
makes ligands more susceptible to
nucleophilic attack.
H2
N
H2
N
OH-
M
O
M
O
OR
H2
N
O
C
H
O
O
A.-F. Miller, 2008, pg
12
O
-
H
His-N
Zn2+
H
N-His
+ H+
O
-
2+
Zn
His-N
C
+
His-N
+ ROH
O
OR
O
His-N
M
OH
N-His
Proximity or template effect.
O
Reduction Midpoint Potentials
Cu2+(O-sal)2en + e- → Cu+(O-sal)2en
Em = -1.21 V
Em = -ΔGreduction/nF , F is Faraday’s constant 96.5 J/V•mol,
n is the number of eso Em corresponds to ΔG/electron transferred.
-0.74 V
Cu2+(iPr-sal)2 + e→ Cu+(iPr-sal)2
Cu+(O-sal)2en + Cu2+(iPr-sal)2
Em
-1.21 V
→ Cu
Cu2+(O-sal)2en + e→ Cu+(O-sal)2en
A.-F. Miller, 2008, pg
13
2+(O-sal)
2en
+ Cu+(iPr-sal)2
Em = -0.74 V + 1.21 V = 0.47 V
Ligands tune the metal’s Em: sterics and
hard-soft effects
Compound
Em vs. NHE*
Cu(O-sal)2en
-1.21 V
Cu(Me-sal)2
-0.90 V
Cu(Et-sal)2
-0.86 V
Cu(S-sal)2en
-0.83 V
Cu(i-Pr-sal)2
-0.74 V
Cu(t-Bu-sal)2
-0.066 V
A.-F. Miller, 2008, pg
14
R
O
N
Cu
N
O
R
Cu(R-sal)2
X
X
Cu
N
N
Cu(X-sal)2en
Lippard and Berg, Table 2.4
*NHE: normal H electrode: 2H+ + 2 e-
H2
Redox tuning tools
Coordination geometry
Ligand natures
Polarization of ligands by H-bonds
Local dielectric (in the event of net charge change).
Proteins can impose a coordination geometry / ligand
ID on a metal ion, paying the energetic cost of
doing so from the energetic stability of the overall
protein structure.
A.-F. Miller, 2008, pg
15