Transition Metal Coordination Compounds
Metal – Ligand Interactions
2+
N
OH2
OH2
H2 O
Fe
NC
OH2
H2O
Ni
CN
C
N
OH2
Octahedral (Oh)
Cl
Square Planar (D4h)
O
C
2-
Co
OC
Cl
2-
C
Cl
Cl
Tetrahedral (Td)
OC
Ru
CO
CO
Square Pyramidal (C4v)
Ligands: Names and Structures
Monodentate
Aqua
Ammine
Carbonyl
Chloro
Bromo
Cyano
Acetato
Bidentate
/
2_
Oxo
Nit ro
Nitrito
Thiocyanato
Isothiocyanato
Hydroxo
Hydrido
Carbonato
OH2
NH3
CO_
Cl _
Br _
CN _
CH3CO2
_O _
NO2 _
_ONO
_
_
NCS _
_ SCN
_
OH_
H 2_
CO3
Multidentate
_
2,2 Bipyridine
(bipy)
N
N
Diethylenetriamine
(dien)
N
N
NH2
NH2
NH
NH2
1,10_ P henanthroline
(phen)
Ethylenediamine
(en)
NH2
Glycinato
(gly)
_
_
O
O
NH2
O
_
O2C
_
O
CO2
N
_
N
_
CO2
O2C
_
Ethylenediaminet et raacetato
(edta)
_
C O
Malonato
(mal)
O
_
C O
O
Acet ylacetonato
(acac)
NH2
O
C C
O
NH
T riethylenet et ramine
(trien)
NH2
O
Oxalato
(ox)
NH
CH3
CH
CH3
N
N
O
O
N
N
O
O
_
O
O
O
T etraazacyclot etradecane
(cyclam)
18-Crown-6
crown ether
Chelates and Macrocycles
O
H2
N
H2N
HO2CCH2
2+ NH2
Co
N
H2
H2N
CH2CO2H
O
HO2CCH2
CH2CO2H
O
O
N
FeEDTA2-
EDTA
NH
Cu2+
O
O
O
O
O
O
NH
O
Cyclen
N
Fe
O
NH2
[Co(en)3]2+
NH
O
O
NCH2CH2N
O
NH
2-
O
[24]crown-8
N
O
O
O
O
O
O
N
cryptand [2,2,2]
Common Geometries of Transition Metal Complexes
4-Coordination
M
M
5-Coordination
M
M
6-Coordination
M
Tetrahedral
Square Planar
Trigonal bipyramidal
Square Pyramidal
Octahedral
Tetragonal
Complexation Equilibria in Water
•
Metallic ions in solution are surrounded by a shell (coordination sphere) of
water molecules, Fe(H2O)63+, Fe(aq)3+
•
Other species present in solution with available lone pairs of electrons
(ligands), that have greater affinity for a metal ion than water, will displace
water ligands from the inner-coordination sphere to form a complex ion or
coordination complex.
•
Such changes are complexation equilibria and an equilibrium formation
constant, Kf (stability constant) describes the ability of the ligand to bind to
the metal in place of water.
Stability Constants
Stepwise (K1, K2, K3…) equilibrium constants, lead to
an overall stability constant (β) for the complex ion.
Factors contributing to metal complex stability
•
•
•
•
•
•
Charge and Size of Metal and Ligand (electrostatic)
Hard-Soft (HSAB) Nature of Metal and Ligand
Chelation
Macrocyclic effects
Electronic Structure of Metal
Solvation Effects
Hard-Soft Acid-Base (HSAB) Concept
•
Hard metals and ligands. Hard cations have high positive charges and are
not easily polarized. e.g. Fe3+. Hard ligands usually have electronegative
non-polarizable donor atoms (O, N ).
The metal-ligand bonding is more ionic
•
Soft metals and ligands. Soft cations (e.g. Hg2+, Cd2+, Cu+) have low
charge densities and are easily polarized. Soft ligands usually have larger,
more polarizable (S, P) donor atoms or are unsaturated molecules or ions.
The metal-ligand bonding is more covalent
•
Borderline metals and ligands lie between hard and soft.
• Hard metals like to bond to hard ligands
• Soft metals like to bond to soft ligands
Hard-Soft Acid-Base Classification of Metals and Ligands
Hard acids
Hard bases
H+, Li+, Na+, K+,
F-, Cl-, H2O, OH-, O2- , NO3-,
Mg2+, Ca2+, Mn2+,
RCO2-, ROH, RO-, phenolate
Al3+, Cr3+, Co3+, Fe3+,
CO3-, SO42-, PO43-, NH3, RNH2
Borderline acids
Borderline bases
Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Sn2+
NO2-, Br-, SO32-, N3-
Pb2+, Ru3+
Pyridine, imidazole,
NH
N
Soft acids
Soft acids
Cu+, Ag+, Au+, Cd2+, Hg2+, Pt2+
I-, H2S, HS-, RSH, RS-, R2S, CN-, CO,
R3P
Stability constant trends for Fe(III) and Hg(II) halides
log K1
X=F
X = Cl
X = Br
Fe3+ + X-
FeX 2+
6.0
1.4
0.5
Hg2+ + X-
HgX +
1.0
6.7
8.9
X=I
12.9
Hard metal formation constants (Kf)
F  Cl  Br  I and O >> S > Se >
Soft metal formation constants (Kf)
F << Cl < Br < I and O << S  Se  Te
HSAB Concept in Geochemistry
•
The common ore of aluminum is alumina, Al2O3 (bauxite) while the most
common ore of calcium is calcium carbonate, CaCO3 (limestone, calcite,
marble). Both are hard acid - hard base combinations. Al3+ and Ca2+ are
hard metals; O2- and CO32- are hard bases.
•
Zinc is found mostly as ZnS (wurtzite) and mercury as HgS (cinnabar). Both
involve soft acid - soft base interactions. Zn2+ and Hg2+ are soft metals; S2is a soft base.
Metal Chelation
[Co(en)3]2+
The Chelate Effect
The replacement of 2 complexed monodentate ligands by one
bidentate ligands is thermodynamically favored since it generates
more particles (increase in disorder) in the solution
The chelate effect is an entropy effect i.e. DS is positive
Thermodynamics of Complexation
Enthalpy (DH) and Entropy (DS) of Complexation.
The Chelate Effect
Chelate Ring Size and Complex Stability
7
ox
log K 1
6
O
O
O
O
ox
5
M
O
mal
O
M
mal
4
O
O
3
O
succ
O
2
M
succ
O
1
O
Mn
Fe
Co
Ni
Cu
Zn
Number of chelate rings and complex stability
20
trien
H2N
NH
NH
NH2
trien
dien
log K 1
15
10
H2N
en
NH NH2
dien
5
H2N
NH2
en
0
Mn
Fe
Co
Ni
Cu
Zn
Solvation Effects
Reaction enthalpy (ΔHReact) and reaction entropy (ΔSReact) for complexation of
M2+ ions by ethylenediamine, glycinate and malonate.
M2+ + Ln- = ML2-n (in kJ/mol. ΔS in J/mol.K.)
O
O
Mn2+
Co2+
Ni2+
Cu2+
Zn2+
NH2
ΔH
-11.7
-28.8
-37.2
-54.3
-28.0
NH2
ΔS
12.5
16.7
23.0
22.6
16.7
NH2
ΔH
-1.3
-11.7
-20.5
-25.9
-13.8
_
O
ΔS
56.4
57.2
49.7
76.9
53.1
_
O
ΔH
15.4
12.1
7.9
11.9
13.1
ΔS
115
113
104
148
117
_
O
O
Solvation Effects
M2+(solv) + L (solv) → ML (solv)
•
Enthalpy changes (ΔHsolv) and entropy changes (ΔSsolv) arising from solvation of the
metal, the ligand and the complex contribute to the overall reaction enthalpy and
entropy of the complexation process.
•
N-donor ligands (ethylenediamine) Complexation is more enthalpy driven than
entropy driven (i.e. large negative ΔH and small positive ΔS).
•
Mixed O- and N-donor ligand (glycinate)
Less negative ΔH, and larger positive ΔS indicates that solvation entropy becomes
more important with O-donors.
•
O-donor ligand (malonate)
The small positive ΔH and large positive ΔS values indicates that the complexation is
entropy driven.
•
O-donor ligands are more strongly solvated by water molecules.
Desolvation of the O-donor ligands, prior to complexation of the metal, reduces the
overall DH for the complexation reaction. i.e. energy is used to remove solvent water
from the O donor atoms before they can bond to the metal. This process also adds to
the reaction entropy, when the water molecules are released to the solvent.
Macrocyclic complexes
Macrocyclic Effect
 Stability constants of macrocyclic ligands are generally higher than those of
their acyclic counterparts.
 Entropy and enthalpy changes provide driving force for the macrocyclic effect
but the balance between the two is complex.
 Metal-ligand bonding is optimized when the size of the macrocyclic cavity and
metal ion radius is closely matched. This promotes a favorable negative DH
for complexation
 For macrocycles, there is minimal reorganization required of the polydentate
ligand structure before coordination to metal. This promotes a more negative
DH for complexation in macrocycles compared to corresponding acyclic open
chain ligands.
 More extensive desolvation of ligand donor atoms may also be involved for
acyclic ligands, which detracts from the overall DH for complexation.