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a transition state without any structuresat local energy minima
an intermediate, is formed along the reaction pathway with detectable
structure.
BackgroundTransition-state theory
Large and positive equilibrium constants since ∆G < 0; both reactions are spontaneous.
The reaction canoccur quickly due to the low activation energy,
but it has a small equilibrium constant ∆G
> 0.
Substitution ReactionsInert and Labile Compounds
Substitution ReactionsKinetically Inert/Labile vs Thermodynamically Unstable/Stable Compounds
A labile complex has a very lowactivation energy for ligand substitution.
Compounds that react more slowly are called inert.
Substitution ReactionsKinetically Inert/Labile vs Thermodynamically Unstable/Stable Compounds
KR = 1030; ∆G < 0 ; in fact, the rate of reaction is too slow→ [Co(NH3)6]3+ is thermodynamically unstable, but kinetically inert.
[Ni(CN)4]2- + H2O → no any reactions
→ [Ni(CN)4]2- is thermodynamically stable, but kinetically labile.
Substitution ReactionsDissociation D Association (A)
Interchange (I)
Substitution ReactionsDissociation D
Substitution Reactions
Association (A) 51 5 1 5 2 5
1 55
1 2
5 1 2 52 5 5
1 2
[ ] [ ][ ] [ ] [ ] 0
[ ][ ][ ]
[ ] [ ][ ][ ] [ ][ ]obs
d ML XY k ML X Y k ML XY k ML XYdt
k ML X YML Xk k
d ML Y k k ML X Yr k ML XY k ML X Ydt k k
Substitution ReactionsInterchange (I)
M-Y > M-X: Ia mechanism. M-Y < M-X: Id mechanism
Substitution ReactionsPreassociation Complex
Experimental Evidence in Octahedral SubstitutionDissociation
Experimental Evidence in Octahedral SubstitutionDissociation
The ligand field activation energy (LFAE), defined as the difference between the LFSE of the square-pyramidal transition state and the LFSE of the octahedral reactant
Experimental Evidence in Octahedral SubstitutionDissociation
1. Oxidation state of the central ion: Central atoms with higher oxidation states have slower ligand exchange rates.
2. Ionic radius. Smaller ions have slower exchange rates.
Experimental Evidence in Octahedral SubstitutionLinear Free-Energy Relationships
CFSE (+), the rate(-) Stable bond (+), the rate(-)
Experimental Evidence in Octahedral SubstitutionThe Kinetic Chelate Effect
The ∆H associated withremoval of the first boundatom is larger than forsubsequent reattachment .
Experimental Evidence in Octahedral Substitution
The rate constants do not vary significantly with the substituting anion Y, as would be expected for a D or Id mechanism
Experimental Evidence in Octahedral Substitution
Ia and A reactions are less common with octahedral complexes due to steric hindrance.The rates depend on significantly nature of incoming ligands.
Association
Substitution Reactions of Square-Planar ComplexesSolvent-assisted substitution
rate determining step,Rate constant = kY
rate determining step, Rate constant = kS
A or Ia mechanism
Substitution Reactions of Square-Planar ComplexesSolvent-assisted substitution
Substitution Reactions of Square-Planar ComplexesSolvent-assisted substitution
The rates of reactions depend on nature of incoming L A mechanism
Substitution Reactions of Square-Planar ComplexesSolvent-assisted substitution
- The rate depends on significantly L.
- The rate of incoming L.
- The rate of leaving X.
Substitution Reactions of Square-Planar ComplexesKY path way – A or Ia - geometry preservation
Substitution Reactions of Square-Planar ComplexesKS path way – geometry preservation
Substitution Reactions of Square-Planar ComplexesThe trans effect
The trans effect: A ligand affects an other ligand placed in trans position, which trans-placed ligand is more flexible to leave for substitution reaction.
Substitution Reactions of Square-Planar ComplexesThe trans effect
The trans effect does not always determine product.
Substitution Reactions of Square-Planar ComplexesThe trans effect
Substitution Reactions of Square-Planar ComplexesThe explanation of trans effect - the trans influence
Sigma‐Bonding Effects
Poor trans effect: low ground state,
high transition state
σ Bonding effect: higher ground state
(trans influence)
π Bonding effect: lower transition state,
(trans effect)
Substitution Reactions of Square-Planar ComplexesThe explanation of trans effect - the trans influence
Sigma‐Bonding Effects
Substitution Reactions of Square-Planar ComplexesThe explanation of trans effect - the trans influence
Sigma‐Bonding Effects
The Pt‐T bond is strong, it uses a larger contribution of px,dx2-y2 orbitals of Pt atoms the Pt‐X bond is weaker, its ground state (sigma-bonding orbital) is higher in energy .
The trans influence on the basis of the relative σ donor properties of the ligands:
Substitution Reactions of Square-Planar ComplexesThe explanation of trans effect - the trans influence
Pi‐Bonding Effects A strong π-acceptor Pt-T charge is removed from Ptmetal center more electrophilic and more susceptible to nucleophilic attack. formation of the 5-coordinate intermediate with a relatively strong Pt—Ybond, stabilizing the intermediate by increase in energy due to the M—X bond breaking. The energy of the transition state is lowered, reducing the activation energy.
Oxidation–Reduction ReactionsInner-sphere vs Outer-sphere reaction
Inner-sphere reaction Outer-sphere reaction
Electrons transfer through bridging ligand.
The electron exchange may occurbetween two separate coordination
spheres.
Bridge ligand must have 2 pairs of electrons at least. Ligand is unable to bridge anothers.