Aqueous Equilibria and Light Absorption in Metal Complexes

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Aqueous Equilibria and Light Absorption in Metal
Complexes

• With your Chemistry 102 Host
• Dr. Mike Daniel

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Aqueous Equilibria

• Addition of ligand displaces H2O as ligand
• Cu (H2O)42 NH3 Cu NH3 (H2O)32 H2O K1
• Cu NH3 (H2O)32 NH3 Cu(NH3)2(H2O)22
  H2O K2

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Aqueous Equilibria

• Addition of ligand displaces H2O as ligand
• Cu (NH3)2 (H2O)22 NH3 Cu(NH3)3H2O2
  H2O K3
• Cu (NH3)3 H2O2 NH3 Cu(NH3)42 H2O K4
• Net
• Cu (H2O)42 4 NH3 Cu (NH3)42 4H2O
• K1 K2 K3 K4 Ktotal
  Kform.

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Formation (Stability) Constant

• If Ligand gtgt Metal rxn pushed to right
  (courtesy of Le Chatelier) and we can use Kform
• Cu2 4 NH3 Cu (NH3)42
• Kf 1.1 X 1013
• Kf Cu (NH3)42 / ( Cu2 NH34 )

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Chelate Effect

• Chelates are more stable due to multiple bonds to
  metal gt Kf bigger
• Cu2 4 NH3 ? Cu (NH3)42
• Kf 1.1 X 1013
• Cu2 2 en ? Cu (en)22
• Kf 3.5 X 1019
• Which reaction has more negative delta G

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Chelate Effect

• Cu2 4 NH3 Cu (NH3)42
• Kf 1.1 X 1013
• Cu2 2 en Cu (en)22
• Kf 3.5 X 1019
• Which reaction better at reducing Cu2

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Remember Thermodynamics

• Cu2 4 NH3 Cu (NH3)42
• Cu2 2 en Cu (en)22
• Which reaction has larger delta S?
• Which reaction has larger delta H?

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Which is better at reducing Cd2

• Cd (NH3)42 Kf 1.0 X 107
• Cd (CN)-2 Kf 1.3 X 1017

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d Orbitals
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Relative Energy of Orbitals in Atom
E 0 (e- removed from atom)
3p
3s
2p
Increasing E
2s
1s
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Fe2 e- Configuration

• Fe Ar 4s2 3d6
• Fe2 Ar 4s0 3d6 Fe2 e- Configuration
• Hunds Rule e- in orbitals of E are
  distributed so spin is maximized (Bus Seat Rule)

3d6
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Crystal Field Theory (Octahedral Complex)

• CFT 1st used to explain properties of transition
  metals in crystals
• Theory accounts for colors and magnetic
  properties
• Approximates Ligands as negative point charge and
  metal-ligand bonding as ionic

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Crystal Field Theory (Octahedral Complex)

• Ligands increase energy of d orbitals (an others)
  due to ligand Metal e- repulsion
• Two d orbitals oriented along coordinate axis
  increase in energy more than others.

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d Orbitals
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d Orbital Splitting
F-
F-
Orbitals oriented along coordinate axis increase
in energy more than others
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d Orbital Splitting
D
Crystal Field Splitting
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Crystal Field Splitting

• Size of Splitting Depends on
• Ligand
• Metal Oxidation
• Group Position

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Crystal Field Splitting

• Ligand
• Increasing Strength of Interaction (Increasing
  Field Strength of Ligand) gtGreater D between d
  orbitals
• Spectrochemical Series (in part)
• CN- gt NH3 gt H2O gt F- gt I-

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Ligand Effects on Crystal Field Splitting
Fe (CN)6-4
Fe (H2O)62
Large D Splitting Low Spin
Small D Splitting High Spin Follows Hunds Rule
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Spin

• Diamagnetic A substance with no unpaired e-
  spin, repelled by magnetic field
• Paramagnetic A substance with unpaired e-,
  attracted by magnetic field
• Greater Splitting, more paired e-s, less
  paramagnetic
• 64Gd3 Has 7 unpaired e-, used in Magnetic
  Resonance Imaging (MRI) to contrast tissue

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Crystal Field Splitting

• Size of Splitting Depends on
• Ligand
• Metal Oxidation
• Group Position

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Crystal Field Splitting

• Metal Oxidation
• Greater Oxidation , (e.g. Fe3 vs Fe2)
  attracts Ligand in closer and results in greater
  Ligand-Metal interaction and greater D

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Crystal Field Splitting

• Size of Splitting Depends on
• Ligand
• Metal Oxidation
• Group Position

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Crystal Field Splitting

• Group Position
• Further down group, greater D
• D orbitals more diffuse, interact more with ligand

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Light Absorption

• If E separating d orbitals in visible light
  range,
• and d orbitals are not full
• e- can absorb visible light
• Color perceived is complement of remaining light
• (ROYGBIVlight absorbedcolor perceived)

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Light Absorption
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Light Absorption
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Light Absorption
Color preceived is on Opposite side from Color
absorbed. Cu (H2O)42 absorbs Orange,
appears blue
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Light Absorption Cu2 Complexes

• Cu (CN)6-4 absorbes UV, appears colorless
• Cu (en)22 absorbes red-violet, appears green
• Cu (H2O)42 absorbes orange, appears blue

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Chlorophyll
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Chlorophyll Absorption Spectra
Sun Radiation _at_ Earths Surface
Chlorophyll a b
Absorption
900
400
Wavelength / nm