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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)
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