1. 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?

2. 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

3. 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

4. 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

5. 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

6. Tm complex: Variable valenceSc +3 Ti +1 +2 +4 V +5 Cr +6 Mn +7 Fe Co Ni Cu Zn
Cu is the only element which affords CuI compounds without -acceptor ligands

7. Transition metals can act as Lewis acid
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.

8. Coordination compound
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]

9. Teeth of a ligand ( teeth  dent)
Ligands
classified according to the number of donor atoms
Examples
monodentate = 1
bidentate = 2
tetradentate = 4
hexadentate = 6
polydentate = 2 or more donor atoms
chelating agents
monodentate, bidentate, tridentate etc. where the concept of teeth (dent)
is introduced, hence the idea of bite angle etc.

11. oxalate ion *
ethylenediamine *

12. 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
The chelate effect or chelation is one of the most important ligand effects in transition metal coordination chemistry.

13. 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]

14. 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

15. Coordination Equilibria & Chelate effect
Example: [Cd(NH3)4]2+ Cd2+ + NH3  [CdNH3] K1 = [CdNH3]2+ + NH3  [Cd(NH3)2]2+ K2 = [Cd(NH3)2]2++ NH3  [Cd(NH3)3]2+ K3 = [Cd(NH3)3]2++ NH3  [Cd(NH3)4]2+ K4 = Overall: Cd NH3  [Cd(NH3)4]2+
β4 = K1 x K2 x K3 x K4 = 10( ) =
Coordination Equilibria & Chelate effect

16. What are the implications of the following results?
NiCl2 + 6H2O  [Ni(H2O)6]+2
[Ni(H2O)6] NH3  [Ni(NH3)6]2+ + 6H2O
log b = 8.6
[Ni(H2O)6] NH2CH2CH2NH2 (en)
[Ni(en)3]2+ + 6H2O
log b = 18.3
[Ni(NH3)6] NH2CH2CH2NH2 (en)
[Ni(en)3]2+ + 6NH3
log b = 9.7

17. 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

18. [Ni(NH3)6]2+ + 3 NH2CH2CH2NH2 (en)
Chelate Formation: Major Factors
[Ni(NH3)6] 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

19. Cd2+ + 4 MeNH2  [Cd(MeNH2)4]2+
Chelate Formation: Entropy Gain
Cd NH3  [Cd(NH3)4]2+
Cd MeNH2  [Cd(MeNH2)4]2+
Cd en  [Cd(en)2]2+
Ligands
4 NH3
4 MeNH2
2 en
log b
7.44
6.52
10.62 DG
kJmol-1
-42.5
-37.2
-60.7 DH
kJmol-1
- 53.2
-57.3
-56.5 DS
JK-1mol-1
- 35.5
- 67.3
+13.8

21. Chelate Formation: Entropy Gain
Reaction of ammonia and en with Cu2+
[Cu(H2O)6]2+ + 2NH3  [Cu(NH3)2(H2O)2] H2O
Log 2 = 7.7 H = -46 kJ/mol S = -8.4 J/K/mol
[Cu(H2O)6]2+ + en  [Cu(en)(H2O)4] H2O
Log K1 = H = -54 kJ/mol S = 23 J/K/mol

22. 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] CN- = [Hg(14CN)4]2- + CN-
Very fast reaction Labile

23. 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.

24. Important Chelating Ligands
2,3-dimercapto-1-propanesulfonic acid sodium (DMPS)
DMPS is a effective chelator with two groups thiols - for mercury, lead, tin, arsenic, silver and cadmium.

25. Zn As Hg D-Penicillamine Au Pb M+ As Hg Au Pb Dimercaprol
(R,S)-2,3-dimercaptosuccinic acid
D-Penicillamine
As, Cu, Pb, Hg M+ As Hg Au Pb
Dimercaprol

26. Important Chelating Ligands
EDTA *

27. EDTA: another view
Anticoagulant

28. Important Chelating Ligands
Macrocylic Ligands