Transition Metal Coordination Compounds Metal

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Transition Metal Coordination CompoundsMetal
Ligand Interactions
Octahedral (Oh) Square Planar (D4h)

• Tetrahedral (Td) Square Pyramidal (C4v)

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Chelates and Macrocycles
Co(en)32
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Complexation Equilibria in Water

• Metallic ions in solution are surrounded by a
  shell (coordination sphere) of water molecules,
  Fe(H2O)63, 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.

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

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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

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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-, SO42-, PO43-, NH3, RNH2

Borderline acids Borderline bases
Fe2, Co2, Ni2, Cu2, Zn2, Sn2 NO2-, Br-, SO32-, N3-
Pb2, Ru3 Pyridine, imidazole,

Soft acids Soft acids
Cu, Ag, Au, Cd2, Hg2, Pt2 I-, H2S, HS-, RSH, RS-, R2S, CN-, CO, R3P
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Stability constant trends for Fe(III) and Hg(II)
halides
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• Hard metal formation constants (Kf)
• F ?? Cl ? Br ? I and O gtgt S gt Se gt
• Soft metal formation constants (Kf)
• F ltlt Cl lt Br lt I and O ltlt S ? Se ? Te

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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 CO32- 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.

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Metal Chelation

• Co(en)32

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The Chelate Effect
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. DS
is positive
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Thermodynamics of Complexation Enthalpy (DH) and
Entropy (DS) of Complexation.
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The Chelate Effect
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Chelate Ring Size and Complex Stability
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Number of chelate rings and complex stability
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Reaction enthalpy (?HReact) and reaction entropy
(?SReact) for complexation of M2 ions by
ethylenediamine, glycinate and malonate.
M2 Ln- ML2-n (in kJ/mol. ?S in
J/mol.K.)
Solvation Effects

Mn2 Co2 Ni2 Cu2 Zn2
?H -11.7 -28.8 -37.2 -54.3 -28.0
?S 12.5 16.7 23.0 22.6 16.7
?H -1.3 -11.7 -20.5 -25.9 -13.8
?S 56.4 57.2 49.7 76.9 53.1
?H 15.4 12.1 7.9 11.9 13.1
?S 115 113 104 148 117
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Solvation Effects

• M2(solv) L (solv) ? ML
  (solv)
• Enthalpy changes (?Hsolv) and entropy changes
  (?Ssolv) 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 ?H and small positive ?S).
• Mixed O- and N-donor ligand (glycinate)
• Less negative ?H, and larger positive ?S
  indicates that solvation entropy becomes more
  important with O-donors.
• O-donor ligand (malonate)
• The small positive ?H and large positive ?S
  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.

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Macrocyclic complexes
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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 DH for complexation
• For macrocycles, there is minimal reorganization
  required of the polydentate ligand structure
  before coordination to metal. This promotes a
  more negative DH for complexation in macrocycles
  compared 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 DH for complexation.

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