Transcript Document

Coordination Chemistry
Transition elements: partly filed d or f shells
Which elements should be considered as transition elements?
Why do we consider the 1st row separately from others?
The d block:
• The d block consists of three horizontal series in
periods 4, 5 & 6
– 10 elements in each series
– Chemistry is “different” from other elements
– Special electronic configurations important
• Differences within a group in the d block are less sharp
than in s & p block
• Similarities across a period are greater
What is a transition metal?
• Transition metals [TM’s] have characteristic
properties
– e.g. coloured compounds, variable oxidation states
• These are due to presence of an inner incomplete d
sub-shell
• Electrons from both inner d sub-shell and outer s
sub-shell can be involved in compound formation
What is a transition metal?
• Not all d block elements have incomplete d
sub-shells
– e.g. Zn has e.c. of [Ar]3d104s2, the Zn2+ ion
([Ar] 3d10) is not a typical TM ion
– Similarly Sc forms Sc3+ which has the stable e.c
of Ar. Sc3+ has no 3d electrons
What is a transition metal?
• For this reason, a transition metal is defined
as being an element which forms at least
one ion with a partially filled sub-shell of d
electrons.
– In period 4 only Ti-Cu are TM’s!
– Note that when d block elements form ions the
s electrons are lost first
Tm complex: Variable valence
Sc
+3
Ti
+1
+2
+3
+4
V
+1
+2
+3
+4
+5
Cr
+1
+2
+3
+4
+5
+6
Mn +1
+2
+3
+4
+5
+6
Fe
+1
+2
+3
+4
+5
+6
Co
+1
+2
+3
+4
+5
Ni
+1
+2
+3
+4
Cu
+1
+2
+3
Zn
+2
+7
Cu is the only element
which affords CuI
compounds without acceptor ligands
Complexes: Have metal ion (can be zero oxidation state)
bonded to number of ligands.
Complex contains central metal ion bonded to one
or more molecules or anions
Lewis acid = metal = center of coordination
Transition metals can act as Lewis acid
Lewis base = ligand = molecules/ions covalently
bonded to metal in complex
The term ligand (ligare [Latin], to bind) was first used by
Alfred Stock in 1916 in relation to silicon chemistry. The first use
of the term in a British journal was by H. Irving and R.J.P. Williams
in Nature, 1948, 162, 746.
For a fascinating review on 'ligand' in chemistry - Polyhedron, 2, 1983, 1-7.
Ligand: Lewis base – contain at least one nonbonding pair of
electrons
Ni2+(aq) + 6NH3(aq)  Ni(NH3)62+(aq)
Lewis acid
Lewis base
Complex ion
Coordination compound
Compound that contains 1 or more complexes
Example
[Co(NH3)6]Cl3
[Cu(NH3)4][PtCl4]
[Pt(NH3)2Cl2]
Teeth of a ligand ( teeth  dent)
• Ligands
– classified according to the number of donor
atoms
– Examples
chelating agents
• monodentate = 1
•
•
•
•
bidentate = 2
tetradentate = 4
hexadentate = 6
polydentate = 2 or more donor atoms
monodentate, bidentate, tridentate etc. where the concept of teeth (dent)
is introduced, hence the idea of bite angle etc.
oxalate ion
O
O
C
C
2-
ethylenediamine
CH2 CH2
H2N
O
O
*
*
*
NH2
*
Coordination Equilibria & Chelate effect
"The adjective chelate, derived from the great claw or
chela (chely - Greek) of the lobster, is suggested for the
groups which function as two units and fasten to the
central atom so as to produce heterocyclic rings."
J. Chem. Soc., 1920, 117, 1456
Ni2+
The chelate effect or chelation is one of the most important
ligand effects in transition metal coordination chemistry.
Coordination Equilibria & Chelate effect
[Fe(H2O)6]3+ + NCS-  [Fe(H2O)5(NCS)]2+ + H2O
Kf = [Fe(H2O)5(NCS)]2+/ [Fe(H2O)6]3+[NCS-]
Equilibrium constant Kf  formation constant
M + L  ML K1 = [ML]/[M][L]
ML + L  ML2 K2 = [ML2]/[ML][L]
ML2 + L  ML3 K3 = [ML3]/[ML2][L]
MLn-1 + L  MLn Kn = [MLn]/[MLn-1][L]
Coordination Equilibria and Chelate effect
• K1, K2….  Stepwise formation constant.
• To calculate concentration of the final
product, use overall formation constant n:
• n = [MLn]/[M][L]n
• = K1 x K2 x K3 x …. x Kn
Coordination Equilibria & Chelate effect
Example: [Cd(NH3)4]2+
Cd2+ + NH3  [CdNH3]2+
K1 = 102.65
[CdNH3]2+ + NH3  [Cd(NH3)2]2+
K2 = 102.10
[Cd(NH3)2]2++ NH3  [Cd(NH3)3]2+
K3 = 101.44
[Cd(NH3)3]2++ NH3  [Cd(NH3)4]2+
K4 = 100.93
Overall: Cd2+ + 4 NH3  [Cd(NH3)4]2+
β4 = K1 x K2 x K3 x K4 = 10(2.65 + 2.10 + 1.44 + 0.93) = 107.12
What are the implications of the following results?
NiCl2 + 6H2O  [Ni(H2O)6]+2
[Ni(H2O)6]+2 + 6NH3  [Ni(NH3)6]2+ + 6H2O
log  = 8.6
[Ni(H2O)6]+2 + 3 NH2CH2CH2NH2 (en)
log  = 18.3
[Ni(en)3]2+ + 6H2O
[Ni(NH3)6]2+ + 3 NH2CH2CH2NH2 (en)
[Ni(en)3]2+ + 6NH3
log  = 9.7
Complex Formation: Major Factors
[Ni(H2O)6] + 6NH3
[Ni(NH3)6]2+ + 6H2O
 NH3 is a stronger (better) ligand than H2O
 O NH3 > O H2O
 [Ni(NH3)6]2+ is more stable
 G = H - TS
(H -ve, S 0)
 G for the reaction is negative
Chelate Formation: Major Factors
[Ni(NH3)6]2+ + 3 NH2CH2CH2NH2 (en)
[Ni(en)3]2+ + 6NH3
 en and NH3 have similar N-donor environment
 but en is bidentate and chelating ligand
 rxn proceeds towards right, G negative
 G = H - TS
(H -ve, S ++ve)
 rxn proceeds due to entropy gain
 S ++ve is the major factor behind chelate effect
Chelate Formation: Entropy Gain
Cd2+ + 4 NH3  [Cd(NH3)4]2+
Cd2+ + 4 MeNH2  [Cd(MeNH2)4]2+
Cd2+ + 2 en  [Cd(en)2]2+
Ligands
log 
G
kJmol-1
H
kJmol-1
S
JK-1mol-1
4 NH3
7.44
-42.5
- 53.2
- 35.5
4 MeNH2
6.52
-37.2
-57.3
- 67.3
2 en
10.62
-60.7
-56.5
+13.8
Chelate Formation: Entropy Gain
Reaction of ammonia and en with Cu2+
[Cu(H2O)6]2+ + 2NH3  [Cu(NH3)2(H2O)2]2+ + 2 H2O
Log 2 = 7.7
[Cu(H2O)6]2+ + en
Log K1 = 10.6
H = -46 kJ/mol

S = -8.4 J/K/mol
[Cu(en)(H2O)4]2+ + 2 H2O
H = -54 kJ/mol
S = 23 J/K/mol
Kinetic stability
Inert and labile complexes
The term inert and labile are relative
“A good rule of thumb is that those complexes that react
completely within 1 min at 25o should be considered labile and
those that take longer should be considered inert.”
Thermodynamically stable complexes can be labile or inert
[Hg(CN)4]2-
Kf= 1042
thermodynamically stable
[Hg(CN)4]2- + 4 14CN- = [Hg(14CN)4]2- + CNVery fast reaction
Labile
Chelating agents:
(1) Used to remove unwanted metal ions in water.
(2) Selective removal of Hg2+ and Pb2+ from body when poisoned.
(3) Prevent blood clots.
(4) Solubilize iron in plant fertilizer.
Important Chelating Ligands
2,3-dimercapto-1-propanesulfonic
acid sodium (DMPS)
Mn+
DMPS is a effective chelator with two groups thiols - for
mercury, lead, tin, arsenic, silver and cadmium.
SH
O
HO
OH
O
SH
(R,S)-2,3-dimercaptosuccinic acid
D-Penicillamine
As, Cu, Pb, Hg
SH
S
M+
M
HS
OH
Dimercaprol
Zn
As
Hg
Au
Pb
OH
S
As
Hg
Au
Pb
Important Chelating Ligands
EDTA
O
*O
C
*O
C
CH2
*
N
O
CH2
*
CH2 CH2 N
O
CH2 C
O*
CH2 C
O*
O
EDTA: another view
Ca
2+
Anticoagulant
Important Chelating Ligands
Macrocylic Ligands