chapter 12 coordination chemistry iv
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Chapter 12 Coordination Chemistry IV. Reactions and Mechanisms. Coordination Compound Reactions. Goal is to understand reaction mechanisms - PowerPoint PPT PresentationTRANSCRIPT
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Chapter 12Coordination Chemistry IV
Reactions and Mechanisms
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Coordination Compound Reactions
• Goal is to understand reaction mechanisms
• Primarily substitution reactions, most are rapid
Cu(H2O)62+ + 4 NH3 [Cu(NH3)4(H2O)2]2+ + 4 H2O
but some are slow
[Co(NH3)6]3+ + 6 H3O+ [Co(H2O)6]3+ + 6 NH4+
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Coordination Compound Reactions
• Labile compounds - rapid ligand exchange (reaction half-life of 1 min or less)
• Inert compounds - slower reactions• Labile/inert labels do not imply stability/instability
(inert compounds can be thermodynamically unstable) - these are kinetic effects
• In general:
– Inert: octahedral d3, low spin d4 - d6, strong field d8 square
planar
– Intermediate: weak field d8
– Labile: d1, d2, high spin d4 - d6, d7, d9, d10
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Substitution Mechanisms• Two extremes:
Dissociative (D, low coordination number intermediate)Associative (A, high coordination number intermediate)
• SN1 or SN2 at the extreme limit• Interchange - incoming ligand participates in
the reaction, but no detectable intermediate– Can have associative (Ia) or dissociative (Id)
characteristics
• Reactions typically run under conditions of excess incoming ligand
• We’ll look briefly at rate laws (details in text), consider primarily octahedral complexes
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Substitution Mechanisms
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Substitution MechanismsPictures:
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Substitution Mechanisms
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Determining mechanismsWhat things would you do to determine the mechanism?
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Dissociation (D) Mechanism
• ML5X ML5 + X k1, k-1
ML5 + Y ML5Y k2
• 1st step is ligand dissociation. Steady-state hypothesis
assumes small [ML5], intermediate is consumed as fast
as it is formed
• Rate law suggests intermediate must be observable - no examples known where it can be detected and measured
• Thus, dissociation mechanisms are rare - reactions are more likely to follow an interchange-dissociative mechanism
d[ML5Y]
dt =
k2k1[ML5X][Y]
kĞ1[X] + k2[Y]
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Interchange Mechanism• ML5X + Y ML5X.Y k1, k–1
ML5X.Y ML5Y + X k2 RDS
• 1st reaction is a rapid equilibrium between ligand and complex to form ion pair or loosely bonded complex (not a high coordination number). The second step is slow.
Reactions typically run under conditions where [Y] >> [ML5X]
d[ML5Y]
dt =
k2 K1[M]0[Y]0
1 + K1[Y]0 + (k2 /kĞ1)
k2 K1[M]0[Y]0
1 + K1[Y]0
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Interchange Mechanism• Reactions typically run under conditions where [Y] >>
[ML5X][M]0 = [ML5X] + [ML5X.Y] [Y]0 [Y]
• Both D and I have similar rate laws: • If [Y] is small, both mechanisms are 2nd order
(rate of D is inversely related to [X])
If [Y] is large, both are 1st order in [M]0, 0-order in [Y]
d[ML5Y]
dt =
k2 K1[M]0[Y]0
1 + K1[Y]0 + (k2 /kĞ1)
k2 K1[M]0[Y]0
1 + K1[Y]0
d[ML5Y]
dt =
k2k1[ML5X][Y]
kĞ1[X] + k2[Y]
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Interchange MechanismD and I mechanisms have similar rate laws: Dissociation Interchange
ML5X ML5 + X k1, k-1 ML5X + Y ML5X.Y k1, k–1
ML5 + Y ML5Y k2 ML5X.Y ML5Y + X k2 RDS
• If [Y] is small, both mechanisms are 2nd order (and rate of D mechanism is inversely related to [X])
• If [Y] is large, both are 1st order in [M]0, 0-order in [Y]
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Association (A) Mechanism
ML5X + Y ML5XY k1, k-1
ML5XY ML5Y + X k2
• 1st reaction results in an increased coordination number. 2nd reaction is faster
• Rate law is always 2nd order, regardless of [Y]• Very few examples known with detectable
intermediate
d[ML5Y]
dt =
k1k2[ML5X][Y]
kĞ1 + k2
k[ML5X][Y]
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Factors affecting rate• Most octahedral reactions have dissociative
character, square pyramid intermediate
• Oxidation state of the metal: High oxidation state results in slow ligand exchange[Na(H2O)6]+ > [Mg(H2O)6]2+ > [Al(H2O)6]3+
• Metal Ionic radius: Small ionic radius results in slow ligand exchange (for hard metal ions)[Sr(H2O)6]2+ > [Ca(H2O)6]2+ > [Mg(H2O)6]2+
• For transition metals, Rates decrease down a group Fe2+ > Ru2+ > Os2+ due to stronger M-L bonding
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Dissociation Mechanism
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Evidence: Stabilization Energy and rate of H2O exchange.
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Small incoming ligand effect = D or Id mechanism
Entering Group Effects
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Entering Group Effects
Close = Id mechanismNot close = Ia mechanism
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Activation Parameters
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RuII vs. RuIII substitution
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Conjugate base mechanism: complexes with NH3-like or H2O ligands, lose H+, ligand trans to deprotonated ligand is more likely to be lost.
Conjugate Base Mechanism
[Co(NH3)5X]2+ + OH- ↔ [Co(NH3)4(NH2)X]+ + H2O (equil)
[Co(NH3)4(NH2)X]+ [Co(NH3)4(NH2)]2+ + X- (slow)
[Co(NH3)4(NH2)]2+ + H2O [Co(NH3)5H2O]2+ (fast)
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Conjugate base mechanism: complexes with NR3 or H2O ligands, lose H+, ligand trans to deprotonated ligand is more likely to be lost.
Conjugate Base Mechanism
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Reaction Modeling using Excel Programming
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• Associative or Ia mechanisms, square pyramid intermediate
• Pt2+ is a soft acid. For the substitution reaction
trans-PtL2Cl2 + Y → trans-PtL2ClY + Cl– in CH3OHligand will affect reaction rate:
PR3>CN–>SCN–>I–>Br–>N3–>NO2
–>py>NH3~Cl–>CH3OH
• Leaving group (X) also has effect on rate: hard ligands are lost easily (NO3
–, Cl–) soft ligands with electron density are not (CN–, NO2
–)
Square planar reactions
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Trans effect
• In square planar Pt(II) compounds, ligands trans to Cl are more easily replaced than others such as ammonia
• Cl has a stronger trans effect than ammonia (but Cl– is a more labile ligand than NH3)
• CN– ~ CO > PH3 > NO2– > I– > Br– > Cl– > NH3 > OH–
> H2O
• Pt(NH3)42+ + 2 Cl– PtCl42– + 2 NH3
• Sigma bonding - if Pt-T is strong, Pt-X is weaker (ligands share metal d-orbitals in sigma bonds)
• Pi bonding - strong pi-acceptor ligands weaken P-X bond
• Predictions not exact
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Trans Effect:
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Trans Effect: First steps random loss of py or NH3
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Trans Effect:
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Electron Transfer Reactions
Inner vs. Outer Sphere Electron Transfer
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Outer Sphere Electron Transfer Reactions
Rates Vary Greatly Despite Same Mechanism
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Nature of Outer Sphere Activation Barrier
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Nature of Outer Sphere Activation Barrier
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Inner Sphere Electron Transfer
Co(NH3)5Cl2+ + Cr(H2O)62+ (NH3)5Co-Cl-Cr(H2O)5
4+ + H2O
Co(III) Cr(II) Co(III) Cr(II)
(NH3)5Co-Cl-Cr(H2O)54+ (NH3)5Co-Cl-Cr(H2O)5
4+
Co(III) Cr(II) Co(II) Cr(III)
H2O + (NH3)5Co-Cl-Cr(H2O)54+ (NH3)5Co(H2O)2+ + (Cl)Cr(H2O)5
2+
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Inner Sphere Electron Transfer
Co(NH3)5Cl2+ + Cr(H2O)62+ (NH3)5Co-Cl-Cr(H2O)5
4+ + H2O
Co(III) Cr(II) Co(III) Cr(II)
(NH3)5Co-Cl-Cr(H2O)54+ (NH3)5Co-Cl-Cr(H2O)5
4+
Co(III) Cr(II) Co(II) Cr(III)H2O + (NH3)5Co-Cl-Cr(H2O)5
4+ (NH3)5Co(H2O)2+ + (Cl)Cr(H2O)52+
Nature of Activation Energy:
Key Evidence for Inner Sphere Mechanism:
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Example
[CoII(CN)5]3- + CoIII(NH3)5X2+ Products
Those with bridging ligands give product [Co(CN)5X]2+.