Labile and inert metal ions - Kinetic effects

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Labile and inert metal ions - Kinetic effects
Water exchange rate constants (s-1) for selected
metal centers
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Approximate half-lives for exchange of water
molecules from the first coordination sphere of
metal ions at 25 oC
Metal ion t1/2 , sec Metal ion t1/2 , sec Metal ion t1/2 , sec
Li 2 x 10-9 V2 9 x 10-3 Sn2 lt 7 x 10-5
Na 1 x 10-9 Cr2 7 x 10-10 Hg2 2 x 10-9
K 7 x 10-10 Mn2 3 x 10-8 Al3 0.7
Mg2 1 x 10-6 Fe2 2 x 10-7 Fe3 4 x 10-3
Ca2 2 x 10-9 Co2 2 x 10-7 Cr3 3 x 105
Ba2 3 x 10-10 Ni2 2 x 10-5 Co3 7 x 105
Cu2 7 x 10-10
Zn2 3 x 10-8
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Relative Stability of 3d Transition Metal
Complexes The Irving-Williams Series. The
stability order of complexes formed by divalent
3d transition metal ions.
Mn2 lt Fe2 lt Co2 lt Ni2 lt Cu2 gt Zn2
M2 L ? ML2 (K1)
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• Mn2 Fe2 Co2 Ni2
  Cu2 Zn 2
• dn d5 d6 d7 d8 d9
  d10
• LFSE (?o) 0 2/5 4/5 6/5 3/5 0

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Ligand field stabilization energy (LFSE)
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M2(g) nH2O M(H2O)62
DHhydration
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Spontaneous loss of degeneracy of eg and t2g
orbitalsfor certain dn configurations
Jahn-Teller Effect
Octahedral
Tetragonal
Some metal ions (e.g. Cu(II), d9 and Cr(II),
high-spin d4) attain enhanced electronic
stability when they adopt a tetragonally
distorted Oh geometry rather than a regular Oh
geometry. They therefore undergo a spontaneous
tetragonal distortion (Jahn-Teller effect). The
net stabilization of the eg electrons for Cu(II),
is shown above.
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Jahn-Teller effect in crystalline CuCl2 lattices
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Electronic spectrum of Ti3 (d1)

• Dynamic Jahn-Teller effect in electronic excited
  state of d1 ion

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Redox Potentials of Metal Complexes

• A redox potential reflects the thermodynamic
  driving force for reduction.
• Ox e? Red Eo (Reduction
  potential)
• Fe3 e? Fe2
• It is related to the free energy change and the
  redox equilibrium constant for
• the reduction process
• ?G ? nDEo F - 2.3 RT logK
• The redox potential of a metal ion couple
  (Mnn/M(n-1)) represents the relative stability
• of the metal when in its oxidized and reduced
  states.
• The redox potential for a metal ion couple will
  be dependent on the nature of
• the ligands coordinated to the metal.
• Comparison of redox potentials for a metal ion in
  different ligand environments provides
• information on factors influencing the stability
  of metal centers.

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The effect of ligand structure on the reduction
potential (Eored) of a metal couple

• Ligands the stabilize the higher oxidized state
  lower Eo (inhibit reduction)
• Ligands that stabilize the lower reduced state
  increase Eo (promote reduction)
• Ligands that destabilize the oxidized state raise
  Eo (promote reduction)
• Ligands that destabilize the reduced form
  decrease Eo (inhibit reduction)
• Hard (electronegative) ligands stabilize the
  higher oxidation state
• Soft ligands stabilize the lower oxidation state
• Negatively charged ligands stabilize the higher
  oxidation state

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Fe(phen)33 e?
Fe(phen)32 Eo 1.14 V Fe(H2O)63 e?
Fe(H2O)62 Eo 0.77
V Fe(CN)63? e?
Fe(CN)64? Eo 0.36 V Heme(Fe3) e?
Heme(Fe2) Eo 0.17
V Fe(III)cyt-c e-
Fe(II)cyt-c Eo 0.126 V


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• Soft 1,10-phenanthroline stabilizes Fe in the
  softer lower Fe(II) state - i.e. it provides
  greater driving force for reduction of Fe(III) to
  Fe(II)
• Hard oxygen in H2O favors the harder Fe(III)
  state. - resulting in a lower driving force for
  reduction of Fe(III) to Fe(II)
• Negatively charged CN- favors the higher Fe(III)
  oxidation state (hard - hard interaction) - i.e.
  it provides a lower driving force for reduction.

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Latimer Diagrams
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• Changes in free energy are additive, but Eo
  values are not.
• If ?Go(3) ?Go(1) ?Go(2),
• since ?Go - nEoF,
• n3 (Eo)3F n1(Eo)1F n2(Eo)2F,
• and hence
• (Eo)3 n1(Eo)1 n2(Eo)2
• n3

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Dependence of Reduction Potential on pH

O2 4 H 4 e- 2 H2O
Eo 1.23 V (1.0 M H)


E 0.82 V (pH 7)
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2 H 2 e- H2 Eo 0.00 V (1.0 M
H)

E -0.413 V (pH 7)