INORGANIC SUBSTITUTION Flashcards
What is thermodynamics
Questioning whether the reaction will happen at all
What is kinetics
Questioning how fast the reaction will occur
When are forward and backwards rates equal
At equilibrium
Why does the rate of the forward reaction have a negative coefficient
As this will cancel out the depletion of the reactant.
What does rate (forward) equal
-d[A]/dt = kf[A][B]
What does rate (backward) equal
d[A]/dt = kb[C][D]
kf / kb =
K
equilibrium constant
What does inert mean
slow rate of decomposition
What does labile mean
More rapid equilibration
Can a reactant be unstable and inert?
Yes, can be unstable thermodynamically, but kinetically the reaction is so slow that it is considered inert
What are the characteristics of an inert complex
high charge density
small bond length/ high bond strength
high stability (low LFSE, chelate effect, no distortion)
small ion size
not s block (except for Be2+ and Mg2+)
4d and 5d (not 3d)
What is a dissociative mechanism. Is intermediate detectable?
dissociation occurs first
Intermediate detectable
What is an associative mechanism. Is intermediate detectable?
association occurs first
Intermediate detectable
What is an interchange mechanism. Is intermediate detectable?
Dissociation and association occur at the same time
Intermediate not detectable
(Energy profile has association first)
What does associatively activated mean
Rate determining step is association
What does dissociatively activated mean
Rate determining step is dissociation
Why aren’t intermediates detectable for Aa or Dd mechanisms
The first step of each is activated. Therefore the high energy means the intermediate forms and unforms so quickly that it is undetectable
In general
1st order
2nd order
mechanism…
1st order: generally D or Id
2nd order: generally A or Ia
What is the rate of a square planar substitution? What is the equation
Pseudo first order
kobs[metal]
kobs = k1 + k2[ligand]
What is a nucleophilicity parameter
Defined in terms of the reaction rates of a specific platinum complex.
Increase means that nucleophile will increase reaction rate
What are the characteristics of a hard lewis acid
small radii
high positive charge
high energy LUMOs
What are the characteristics of a hard lewis base
small radii
high electronegativity
high energy HOMOs
What are the characteristics of a soft lewis acid
large radii
low positive charge
low energy LUMOs
What are the characteristics of a soft lewis base
large radii
moderate electronegativity
low energy HOMOs
What are the 3 factors on a square planar reaction
nucleophilicity of entering group
trans effect
steric effects
Explain the trans effect
Trans influence: trans ligands that are good sigma donors cause weakening of other trans bond encouraging dissociation
Transition state effect: entering ligands with goof pi acceptance can accomodate electron density and therefore better at association
Explain the steric effects on a square planar reaction
steric crowding blocks nucleophiles
therefore, inhibits associative mechanism and favours dissociative mechanism.
Sterics on trans to substitution site is better than cis
Explain the stereochemistry of a square planar substitution
Passes through a trigonal bipyramidal transition state
Preserves original geometry
two cis ligands are out of trigonal plane and therefore don’t affect in reaction
What sign of S and V show associative mechanism
negative
What are the most common mechanism of octahedral substitution and why
Mostly interchange, then dissociative
7 coordinate of associative is unlikely
What is an encounter pair
Reactants that have come into contact but have not yet successfully reacted
What is the rate equation of octahedral substitution
kobs[M]tot[ligand]
kobs = kKE
Only for when KE[ligand] «_space;1
What are the 2 main effects on octahedral reactions
Spectator ligands and steric effects
Explain effect of spectator ligands on octahedral reactions
sigma donors increase e density forcing weak ligand to dissociate
sigma donation stabilises the reduced coordination transition state
Explain the steric effects on octahedral reactions
Big ligands either chiral or achiral, whatever one more sterically hindering will have a faster rate due to favouring dissociation to reduced hinderance
increased cone angle = increased rate
If 2 ligands are sterically the same but the rate is different…
likely due to electronic properties
What is ligand field activation energy (LFAE)
LFAE =LFSE(ts) - LFSE
Comes from change in geometry from octahedral to square pyramidal or trigonal bipyramidal
Discuss radius and electron density on octahedral activation
decreased radius and increased electron density make it hard for nucleophilic attack.
Therefore harder for nucleophilic attack going from 3d3 to 3d8
Discuss the stereochemistry of octahedral substitution
cis reactant goes through square pyramidal and produces cis
trans goes through trigonal bipyramidal and produces cis or trans
stronger pi donor equatorial and trans favours isomerisation
Discuss the D-CB mechanism
Base hydrolysis, replacement of ligand by OH-, overall a proton transfer
Deprotonation decreases charge which increases lability
OH takes proton from attached ligand. Forms H2O
Weak ligand dissociates
H2O replaces ligand and H is given back to initial ligand
Outline an inner sphere mechanism
electron and ligand transfer by forming bridge
one reactant is labile (e donor)
other reactant has bridging ligand
reactants are not too bulky (therefore not biological reactions)
Outline an outer sphere mechanism
e transfer by forming encounter pair
increased charge = small bond length = stretches
decreased charge = long bond length =compresses
Lengths distort to halfway between bond lengths
What does a high and low Ea mean with respect to an outer sphere mechanism
High Ea = more different bond lengths, more distortion needs to occur
Low Ea = similar bond lengths or already distorted (e.g. Jahn Teller)
How can bond length be determined by eg orbitals
eg orbitals are anti bonding, therefore more electrons means longer bond length
t2g orbitals are non bonding therefore don’t really affect bond length
