Coordination Chemistry

Transition elements: partly filed d or f shells Which elements should be considered as transition elements?

Why do we consider the 1 st 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]3d 10 4s 2 , the Zn 2+ ([Ar] 3d 10 ) is not a typical TM ion ion – Similarly Sc forms Sc 3+ of Ar. Sc 3+ which has the stable e.c 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 V Cr +1 +1 Mn +1 Fe +1 +2 +2 +2 +2 +2 +3 +3 +3 +3 +3 +4 +4 +4 +4 +4 +5 +5 +5 +5 +6 +6 +6 +7 Co +1 +2 +3 +4 +5 Ni +1 +2 +3 +4 Cu +1 +2 +3 Zn +2 Cu is the only element which affords Cu I 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 Ni 2+ (aq) + 6NH 3 (aq)



Ni(NH 3 ) 6 2+ (aq) Lewis acid Lewis base Complex ion



Coordination compound

 Compound that contains 1 or more complexes  Example  [Co(NH 3 ) 6 ]Cl 3  [Cu(NH 3 ) 4 ][PtCl 4 ]  [Pt(NH 3 ) 2 Cl 2 ]

Teeth of a ligand ( teeth



dent)

•

Ligands

–

classified according to the number of donor atoms

–

Examples

• • •

monodentate = 1 bidentate = 2 tetradentate = 4

chelating agents

•

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.

O

oxalate ion

O C C O * O * 2-

ethylenediamine

H 2 N * CH 2 CH 2 NH 2 *

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 Ni 2+ The chelate effect or chelation is one of the most important ligand effects in transition metal coordination chemistry.

Coordination Equilibria & Chelate effect

[Fe(H 2 O) 6 ] 3+ + NCS -



[Fe(H 2 O) 5 ( NCS )] 2+ + H 2 O K f = [Fe(H 2 O) 5 ( NCS )] 2+ / [Fe(H 2 O) 6 ] 3+ [NCS ] Equilibrium constant K f



formation constant M + L



ML K 1 = [ML]/[M][L] ML + L



ML 2 ML 2 + L



ML 3 K 2 = [ML 2 ]/[ML][L] K 3 = [ML 3 ]/[ML 2 ][L] ML n-1 + L



ML n K n = [ML n ]/[ML n-1 ][L]

Coordination Equilibria and Chelate effect

• •

K

1

, K

2

….



Stepwise formation constant.

To calculate product, use concentration of the

overall formation constant

final



n

:

• •



n

= [ML

n

]/[M][L]

n

= K

1

x K

2

x K

3

x …. x K

n

Coordination Equilibria & Chelate effect

Example: [Cd(NH 3 ) 4 ] 2+ Cd 2+ + NH 3  [CdNH 3 ] 2+ [CdNH 3 ] 2+ + NH 3  [Cd(NH 3 ) 2 ] 2+ [Cd(NH 3 ) 2 ] 2+ + NH 3  [Cd(NH 3 ) 3 ] 2+ K K 2 K 1 3 = 10 = 10 2.65

= 10 2.10

1.44

[Cd(NH 3 ) 3 ] 2+ + NH 3  [Cd(NH 3 ) 4 ] 2+ K 4 = 10 Overall: Cd 2+ + 4 NH 3  [Cd(NH 3 ) 4 ] 2+ 0.93

β 4 =

K 1 x K 2 x K 3 x K 4 = 10 (2.65 + 2.10 + 1.44 + 0.93) = 10 7.12

What are the implications of the following results?

NiCl 2 + 6H 2 O



[Ni(H 2 O) 6 ] +2 [Ni(H 2 O) 6 ] +2 + 6NH 3



[Ni(NH 3 ) 6 ] 2+ + 6H 2 O log



= 8.6

[Ni(H 2 O) 6 ] +2 + 3 NH 2 CH 2 CH 2 NH 2 (en) log



= 18.3

[Ni(en) 3 ] 2+ + 6H 2 O [Ni(NH 3 ) 6 ] 2+ + 3 NH 2 CH 2 CH 2 NH 2 (en) [Ni(en) 3 ] 2+ + 6NH 3 log



= 9.7

Complex Formation: Major Factors [Ni(H 2 O) 6 ] + 6NH 3



[Ni(NH 3 ) 6 ] 2+ + 6H 2 O

 

NH 3



O is a NH 3 stronger (better) ligand >



O H 2 O than H

 

[Ni(NH 3 ) 6 ] 2+ is more stable



G =



H - T



S (



H -ve,



S



0) 2 O

 

G for the reaction is negative

Chelate Formation: Major Factors [Ni(NH 3 ) 6 ] 2+ + 3 NH 2 CH 2 CH 2 NH 2 (en) [Ni(en) 3 ] 2+ + 6NH 3



en and NH 3

    

have similar N-donor environment but en is bidentate and chelating ligand rxn proceeds towards right,



G =



H - T



S



G negative (



H -ve,



S ++ve) rxn proceeds due to entropy gain



S ++ve is the major factor behind chelate effect

Chelate Formation: Entropy Gain Cd 2+ + 4 NH 3



[Cd(NH 3 ) 4 ] 2+ Ligands Cd 2+ + 4 MeNH 2



[Cd(MeNH 2 ) 4 ] 2+ Cd 2+ + 2 en



[Cd(en) 2 ] 2+ log

 

G kJmol -1



H kJmol -1 JK



-1 S mol -1 4 NH 3 4 MeNH 2 7.44

6.52

-42.5

-37.2

- 53.2

-57.3

- 35.5

- 67.3

2 en 10.62

-60.7

-56.5

+13.8

Chelate Formation: Entropy Gain [Cu(H 2 Reaction of ammonia and en with Cu 2+ O) 6 ] 2+ + 2NH 3



[Cu(NH 3 ) 2 (H 2 O) 2 ] 2+ + 2 H 2 O Log



2 = 7.7



H = -46 kJ/mol



S = -8.4 J/K/mol [Cu(H 2 O) 6 ] 2+ + en Log K 1 = 10.6



[Cu( en )(H 2 O) 4 ] 2+ + 2 H 2 O



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 25 o 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 K f = 10 42 thermodynamically stable [Hg(CN) 4 ] 2 + 4 14 CN = [Hg( 14 CN) 4 ] 2 + CN Very fast reaction Labile

Chelating agents: (1) Used to remove unwanted metal ions in water.

(2) Selective removal of Hg 2+ and Pb 2+ 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) M n+ 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 As, Cu, Pb, Hg SH M + HS OH Dimercaprol M S S D-Penicillamine Zn As Hg Au Pb OH As Hg Au Pb

* O * O O

Important Chelating Ligands EDTA

O C CH 2 CH 2 C CH 2 CH 2 C CH 2 CH 2 C O * O * O O

EDTA: another view Anticoagulant Ca 2+

Important Chelating Ligands Macrocylic Ligands