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 +4 V +5 Cr +6 Mn +7 Fe Co Ni Cu Zn Cu is the only element which affords CuI compounds without -acceptor ligands

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.

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]

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.

oxalate ion * ethylenediamine *

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.

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 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 b = 8.6 [Ni(H2O)6]+2 + 3 NH2CH2CH2NH2 (en) [Ni(en)3]2+ + 6H2O log b = 18.3 [Ni(NH3)6]2+ + 3 NH2CH2CH2NH2 (en) [Ni(en)3]2+ + 6NH3 log b = 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

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

Cd2+ + 4 MeNH2  [Cd(MeNH2)4]2+ 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 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

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 H = -46 kJ/mol S = -8.4 J/K/mol [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

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- + CN- Very 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) DMPS is a effective chelator with two groups thiols - for mercury, lead, tin, arsenic, silver and cadmium.

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

Important Chelating Ligands EDTA *

EDTA: another view Anticoagulant

Important Chelating Ligands Macrocylic Ligands