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 The d block consists of three horizontal series in periods 4, 5 & 6periods 4, 5 & 6– 10 elements in each series10 elements in each series– Chemistry is “different” from other elementsChemistry is “different” from other elements– Special electronic configurations importantSpecial electronic configurations important

• Differences within a group in the d block are less sharp Differences within a group in the d block are less sharp than in s & p blockthan in s & p block

• Similarities across a period are greaterSimilarities 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 a transition metal is defined as being an element which forms at least as being an element which forms at least one ion with a partially filled sub-shell of d one ion with a partially filled sub-shell of d electrons.electrons.– In period 4 only Ti-Cu are TM’s!– Note that when d block elements form ions the

s electrons are lost first

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 +7

Fe +1 +2 +3 +4 +5 +6

Co +1 +2 +3 +4 +5

Ni +1 +2 +3 +4

Cu +1 +2 +3

Zn   +2

Cu is the only element which affords CuI compounds without -acceptor ligands

Tm complex: Variable valence

ComplexesComplexes: Have Have metalmetal ion (can be zero oxidation state) ion (can be zero oxidation state) bonded to number of bonded to number of ligandsligands..

Complex contains central metal ion bonded to one Complex contains central metal ion bonded to one or more molecules or anionsor more molecules or anions

Lewis acid = metal = center of coordinationLewis acid = metal = center of coordination

Transition metals can act as Lewis acidTransition metals can act as Lewis acid

Lewis base = ligand = molecules/ions covalently Lewis base = ligand = molecules/ions covalently bonded to metal in complexbonded to metal in complex

The term ligand (ligare [Latin], to bind) was first used by The term ligand (ligare [Latin], to bind) was first used by Alfred Stock in 1916 in relation to silicon chemistry. The first use 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 of the term in a British journal was by H. Irving and R.J.P. Williams in in NatureNature, 1948, 162, 746., 1948, 162, 746.

For a fascinating review on 'ligand' in chemistry - Polyhedron, 2, 1983, 1-7.

Coordination compound

Compound that contains 1 or more complexes

Example

[Co(NH3)6]Cl3

[Cu(NH3)4][PtCl4]

[Pt(NH3)2Cl2]

Ligand: Lewis base – contain at least one nonbonding pair of electrons

NiNi2+2+(aq) + 6NH(aq) + 6NH33(aq) (aq) Ni(NH Ni(NH33))662+2+(aq)(aq)

Lewis acidLewis acid Lewis baseLewis base Complex ionComplex ion

• LigandsLigands– 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.

Teeth of a ligand ( teeth dent)

oxalate ion

CC

O

O O

O 2-

* *

ethylenediamine

CH2H2N

CH2

NH2* *

Coordination Equilibria & Chelate effect

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

"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+

[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 & 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 and 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

Coordination Equilibria & Chelate effect

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(NH3)6]2+ + 3 NH2CH2CH2NH2 (en)

[Ni(en)3]2+ + 6NH3

log = 9.7

[Ni(H2O)6]+2 + 3 NH2CH2CH2NH2 (en)

[Ni(en)3]2+ + 6H2O

log = 18.3

NH3 is a stronger (better) ligand than H2O

O NH3 > O H2O

[Ni(NH3)6]2+ is more stable

G = H - TS (H -ve, S 0)

G for the reaction is negative

Complex Formation: Major Factors

[Ni(H2O)6] + 6NH3

[Ni(NH3)6]2+ + 6H2O

Chelate Formation: Major Factors

en and NH3 have similar N-donor environment but en is bidentate and chelating ligand rxn proceeds towards right, G negative G = H - TS (H -ve, S ++ve) rxn proceeds due to entropy gain S ++ve is the major factor behind chelate effect

[Ni(NH3)6]2+ + 3 NH2CH2CH2NH2 (en)

[Ni(en)3]2+ + 6NH3

Cd2+ + 4 NH3 [Cd(NH3)4]2+

Cd2+ + 2 en [Cd(en)2]2+

Chelate Formation: Entropy Gain

Ligands

4 NH3

4 MeNH2

2 en

GkJmol-1

-42.5

-37.2

-60.7

HkJmol-1

- 53.2

-57.3

-56.5

SJK-1mol-1

- 35.5

- 67.3

+13.8

log

7.44

6.52

10.62

Cd2+ + 4 MeNH2 [Cd(MeNH2)4]2+

Reaction of ammonia and en with Cu2+

[Cu(H2O)6]2+ + en [Cu(en)(H2O)4]2+ + 2 H2O

Log K1 = 10.6 H = -54 kJ/mol S = 23 J/K/mol

[Cu(H2O)6]2+ + 2NH3 [Cu(NH3)2(H2O)2]2+ + 2 H2O

Log 2 = 7.7 H = -46 kJ/mol S = -8.4 J/K/mol

Chelate Formation: Entropy Gain

Kinetic stability

Inert and labile complexes

Thermodynamically stable complexes can be labile or inert

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.”

[Hg(CN)4]2- Kf= 1042 thermodynamically stable

[Hg(CN)4]2- + 4 14CN- = [Hg(14CN)4]2- + CN-

Very fast reaction LabileLabile

Chelating agents: Chelating agents:

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

(2) Selective removal of Hg(2) Selective removal of Hg2+2+ and Pb and Pb2+2+ from body when poisoned. from body when poisoned.

(3) Prevent blood clots.(3) Prevent blood clots.

(4) Solubilize iron in plant fertilizer.(4) Solubilize iron in plant fertilizer.

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.

Mn+

Important Chelating Ligands

HOOH

O

OSH

SH

(R,S)-2,3-dimercaptosuccinic acid

As, Cu, Pb, Hg

HS OH

SHM+

S

SOH

M

Dimercaprol

AsHgAuPb

D-Penicillamine

ZnAsHgAuPb

CH2N

CH2

CH2

C

C

CH2 N

CH2

CH2 C

C

O

O

O

O

O O

OO

EDTA

*

* *

*

**

Important Chelating Ligands

AnticoagulantCa2+

EDTA: another view

Macrocylic Ligands

Important Chelating Ligands