Transition Metal Coordination CompoundsMetal – Ligand Interactions

Tetrahedral (Td) Square Pyramidal (C4v)

FeOH2

OH2

OH2

OH2

H2O

H2O

Ni

C

C

CN

N

N

CN

2+ 2-

Octahedral (Oh) Square Planar (D4h)

Ru

C CCC

CO

O

OO

OCo

ClCl

Cl

Cl 2-

CH3 CH CH3

O O

Aqua OH2Ammine NH3

Chloro Cl_

Cyano CN

Carbonyl CO

Acetato

Bromo Br __

_

Glycinato

Oxo O2

Nitro NO2Nitrito ONOThiocyanato NCSIsothiocyanato SCN

_ _

_

_

_

_

_

__

HOH

_HydroxoHydrido

_

_

N N

2,2_Bipyridine

Bidentate

N N

1,10_Phenanthroline

NH2 NH NH2Diethylenetriamine

NH2 NH2Ethylenediamine

Carbonato CO32 _

Oxalato C CO O

O O

__

NH2 O

O

Monodentate

/

Malonato C

C

O

O

O

O

_

_

Multidentate

Acetylacetonato_

(bipy)

(phen)

(en)

(ox)

(gly)

(mal)

(acac)

(dien)

NH2 NH NH NH2

T riethylenetet ramine(trien)

N NCO2

CO2

O2C

O2C

_

_ _

N N

N N

Ethylenediaminetet raacetato(edta)

T etraazacyclotetradecane

(cyclam)

Ligands: Names and Structures

CH3CO2

_

O

O

O

O

O

O

18-Crown-6crown ether

Fe NO

O

N

O

O

O

O

O

O

2-

FeEDTA2-

NCH2CH2N

HO2CCH2

HO2CCH2

CH2CO2H

CH2CO2HCo

NH2

NH2

NH2

H2N

H2N

H2N

2+

NO O

NO OO O

cryptand [2,2,2]

NH NH

NH NH

Cu2+

Cyclen [24]crown-8

O

O O

O

O

OO

O

Chelates and Macrocycles

EDTA[Co(en)3]2+

M

M

M

M

M

4-Coordination

5-Coordination

6-Coordination

Tetrahedral

Square Planar

Trigonal bipyramidal

Square Pyramidal

Octahedral

Tetragonal

Common Geometries of Transition Metal Complexes

Complexation Equilibria in Water

• Metallic ions in solution are surrounded by a shell (coordination sphere) of water molecules, Fe(H2O)6

3+, Fe(aq)3+

• Other species present in solution with available lone pairs of electrons (ligands), that have greater affinity for a metal ion than water, will displace water ligands from the inner-coordination sphere to form a complex ion or coordination complex.

• Such changes are complexation equilibria and an equilibrium formation constant, Kf (stability constant) describes the ability of the ligand to bind to the metal in place of water.

Stability Constants

M + L ML K1 = [ML]/[M][L]

ML + L ML2 K2 = [ML2]/[ML][L]

ML2 + L ML3 K3 = [ML3]/[ML2][L]

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

M + 2L ML2 2 = [ML2]/[M][L]2

M + 3L ML3 3 = [ML3]/[M][L]3

n = K1K2K3...Kn

Stepwise (K1, K2, K3…) equilibrium constants, lead to an overall stability constant (β) for the complex ion.

Factors contributing to metal complex stability

• Charge and Size of Metal and Ligand (electrostatic)• Hard-Soft (HSAB) Nature of Metal and Ligand• Chelation• Macrocyclic effects• Electronic Structure of Metal • Solvation Effects

Hard-Soft Acid-Base (HSAB) Concept • Hard metals and ligands. Hard cations have high positive charges and are

not easily polarized. e.g. Fe3+. Hard ligands usually have electronegative non-polarizable donor atoms (O, N ). The metal-ligand bonding is more ionic

• Soft metals and ligands. Soft cations (e.g. Hg2+, Cd2+, Cu+) have low charge densities and are easily polarized. Soft ligands usually have larger, more polarizable (S, P) donor atoms or are unsaturated molecules or ions.The metal-ligand bonding is more covalent

• Borderline metals and ligands lie between hard and soft.

• Hard metals like to bond to hard ligands• Soft metals like to bond to soft ligands

Hard-Soft Acid-Base Classification of Metals and Ligands

Hard acids Hard bases

H+, Li+, Na+, K+, F-, Cl-, H2O, OH-, O2- , NO3-,

Mg2+, Ca2+, Mn2+, RCO2-, ROH, RO-, phenolate

Al3+, Cr3+, Co3+, Fe3+, CO3-, SO4

2-, PO43-, NH3, RNH2

Borderline acids Borderline bases

Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Sn2+ NO2-, Br-, SO3

2-, N3-

Pb2+, Ru3+ Pyridine, imidazole,

Soft acids Soft acids

Cu+, Ag+, Au+, Cd2+, Hg2+, Pt2+ I-, H2S, HS-, RSH, RS-, R2S, CN-, CO, R3P

N

NH

Stability constant trends for Fe(III) and Hg(II) halides

Fe3+ + X- FeX 2+

Hg2+ + X- HgX +

log K1

X = F X = Cl X = Br X = I

6.0 1.4 0.5

1.0 6.7 8.9 12.9

Hard metal formation constants (Kf)

F Cl Br I and O >> S > Se >

Soft metal formation constants (Kf)

F << Cl < Br < I and O << S Se Te

HSAB Concept in Geochemistry

• The common ore of aluminum is alumina, Al2O3 (bauxite) while the most common ore of calcium is calcium carbonate, CaCO3 (limestone, calcite, marble). Both are hard acid - hard base combinations. Al3+ and Ca2+ are hard metals; O2- and CO3

2- are hard bases.

• Zinc is found mostly as ZnS (wurtzite) and mercury as HgS (cinnabar). Both involve soft acid - soft base interactions. Zn2+ and Hg2+ are soft metals; S2- is a soft base.

Metal Chelation

[Co(en)3]2+

Fe NO

O

N

O

O

O

O

O

O

NCH2CH2N

HO2CCH2

HO2CCH2

CH2CO2H

CH2CO2H

+ Fe2+

2-

FeEDTA2-

Co2+(aq) + 3 NH2Co

NH2

NH2

NH2

H2N

H2N

H2N

2+

NH2

The Chelate Effect

Ni(H2O)62+ + en Ni(H2O)4(en)2+ + 2 H2O log K f = 7.5

Ni(H2O)62+ + 2 NH3 Ni(H2O)4(NH3)2

2+ + 2 H2O log K f = 5.0

en = H2N NH2

The replacement of 2 complexed monodentate ligands by one bidentate ligands is thermodynamically favored since it generates more particles (increase in disorder) in the solution

The chelate effect is an entropy effect i.e. S is positive

G = -2.3RT logK f = H - TS

logK f = -H + S 2.3RT 2.3R

logKf

1/T

slope = -H/2.3R

intercept = S/2.3R

H ref lects strength of metal-ligand interactionS ref lects change in "disorder"accompanying complexation

K f Temp

Thermodynamics of Complexation Enthalpy (H) and Entropy (S) of Complexation.

The Chelate Effect

NiNH3

H3N NH3

H3N

NH3

2+NH3

+ 3 H2N NiNH2

NH2

NH2

H2N

H2N

H2N

NH2 + 6 NH3

2+

Ni(NH3)62+ + 3 en Ni(en)3

2+ + 6 NH3 log K f = 9.7

Go = -55.4 kJ Ho = -29.2 kJ So = +88 J/mol.K

TSo = -26.2 kJ/mol

Cd(H2O)42+ + 4 CH3NH2

logGo = -37.2 kJ/mol,

Ho = -57.3 kJ/mol,

So = -67.3 J./mol. K So = 20.1 kJ/mol

Cd(H2O)42+ + 2 en [Cd(en)2]2++ 4 H2O

logGo = -60.7 kJ/mol,

Ho = -56.5 kJ/mol,

So = +14.1 J./mol. K So = -4.2 kJ/mol

[Cd(NH2CH3)4]2++ 4 H2O

chelation

chelation

Go = -23.5 kJ/mol,

Ho = +0.80 kJ/mol,

So = 81.4 J./mol. K So = -24.3 kJ/mol

+ 2 en [Cd(en)2]2+ + 4 CH3NH2

log

[Cd(NH2CH3)4]2+

chelation

entropy driven reaction

1

2

3

4

5

6

7

Mn Fe Co Ni Cu Zn

log

K1

malO

O

OO

M

oxO

O

O

O

M

Msucc

OO

OO

ox

mal

succ

Chelate Ring Size and Complex Stability M2+(aq) + L ML K1

Number of chelate rings and complex stability

0

5

10

15

20

Mn Fe Co Ni Cu Zn

log

K1

H2N NH2en

NHH2N NH2

dien

NHH2N NH NH2

trien

trien

dien

en

M2+(aq) + L ML K1

MNH

NH2

H2N

2+H2O

H2O

H2O

MNH

NH

H2N

H2N

2+H2O

H2O

MNH2

H2N

2+H2O

H2O

H2OOH2

Mn2+ Co2+ Ni2+ Cu2+ Zn2+

ΔH -11.7 -28.8 -37.2 -54.3 -28.0

ΔS 12.5 16.7 23.0 22.6 16.7

ΔH -1.3 -11.7 -20.5 -25.9 -13.8

ΔS 56.4 57.2 49.7 76.9 53.1

ΔH 15.4 12.1 7.9 11.9 13.1

ΔS 115 113 104 148 117

NH2

NH2

NH2

OO_

OO

OO

_

_

Reaction enthalpy (ΔHReact) and reaction entropy (ΔSReact) for complexation of M2+ ions by ethylenediamine, glycinate and malonate.

M2+ + Ln- = ML2-n (in kJ/mol. ΔS in J/mol.K.)

Solvation Effects

Solvation Effects M2+(solv) + L (solv) → ML (solv)

• Enthalpy changes (ΔHsolv) and entropy changes (ΔSsolv) arising from solvation of the metal, the ligand and the complex contribute to the overall reaction enthalpy and entropy of the complexation process.

• N-donor ligands (ethylenediamine) Complexation is more enthalpy driven than entropy driven (i.e. large negative ΔH and small positive ΔS).

• Mixed O- and N-donor ligand (glycinate) Less negative ΔH, and larger positive ΔS indicates that solvation entropy becomes more important with O-donors.

• O-donor ligand (malonate)The small positive ΔH and large positive ΔS values indicates that the complexation is entropy driven.

• O-donor ligands are more strongly solvated by water molecules. Desolvation of the O-donor ligands, prior to complexation of the metal, reduces the overall H for the complexation reaction. i.e. energy is used to remove solvent water from the O donor atoms before they can bond to the metal. This process also adds to the reaction entropy, when the water molecules are released to the solvent.

Macrocyclic complexes

NH HN

NH2

Cu

2,3,2-tet Cu2+ + 2,3,2-tet log K f = 23.9

NH2

NH HN

NH HN

Cuteta

Cu2+ + teta log K f = 28

acyclic

macrocyclic

NH HN

NH2 NH2

NH HN

NH HN

Macrocyclic Effect

Stability constants of macrocyclic ligands are generally higher than those of their acyclic counterparts.

Entropy and enthalpy changes provide driving force for the macrocyclic effect but the balance between the two is complex.

Metal-ligand bonding is optimized when the size of the macrocyclic cavity and metal ion radius is closely matched. This promotes a favorable negative H for complexation

For macrocycles, there is minimal reorganization required of the polydentate

ligand structure before coordination to metal. This promotes a more negative H for complexation in macrocyclescompared to corresponding acyclic open chain ligands.

More extensive desolvation of ligand donor atoms may also be involved for

acyclic ligands, which detracts from the overall H for complexation.

HN HN

NH2 NH2

Ni2+

HN HN

NH HN

Ni2+

Ligand logK Ho -TS kJ/mol kJ/mol

2,3,2-tet 15.3 -71 -17

cyclam 22.2 -130 -2.5

enthalpy difference opposed by entropy difference

HN NH

NH2 NH2

Cu2+

HN NH

NH NH

Cu2+

Ligand logK Ho -TS kJ/mol kJ/mol

2,2,2-tet 20.1 -91 -24

cyclen 24.8 -77 -64

entopy difference opposed by ethalpy difference