Reaction Mechanisms Flashcards
What’s the difference between thermodynamics and kinetics?
Thermodynamics look at how FAVOURABLE forming a product from a reactant is
Kinetics looks at how FAST a reaction occurs
What does labile/inert mean?
Labile = weak, substitution occurs quickly
Inert = resists substitution and so rate of substitution is slow
What makes kinetics slow?
High Ea, therefore complex is kinetically inert and a large CFSE
What makes reaction favourable?
A decrease in ∆G, means reactant is less stable then product
What is ‘dissociative’ analogous to?
SN1 carbon chemistry
What is ‘associative’ analogous to?
There is no analogy in carbon chemistry
What is interchange?
Where a new M-L bond is being formed whilst an old M-L bond is being broken at the same time.
Of the 3 types of substitution of ligands, which has an intermediate that cannot exist independently?
Interchange - undetectable bonds
A/D intermediates both have independent existence
What is ‘interchange’ analogous to?
SN2 carbon chemistry
How many transition states do A/D mechanisms have?
2
How many transition states does interchange mechanism have?
1 - single, undefined intermediate
What is dissociative interchange?
Id is where the M-X bond is more advanced in breaking than the new M-Y bond is in formation
What is associative interchange?
Ia is when the M-Y bond is more advance at transition state than the M-X bond breaking
What mechanisms do most octahedral TM complexes follow?
Ia or Id
How do most organometallic (18e-) complexes react?
Via D mechanisms
How do most square planar complexes react?
Via A mechanisms
Why is TM chemistry hard to study?
Because usually, M-L bonds are fairly weak, and so reactions occur very quickly
What is the RDS in D/Id mechanisms?
M-X bond breaking is key process
So rate depends on identity of X (leaving ligand)
What is the RDS of A/Ia mechanisms?
Bond formation is key process
So rate depends on identity of Y (incoming ligand)
What will increasing steric bulk of other ligands do to rate of mechanism?
Bulkier ligands speed up D/Id reactions - easier for X to leave
Will slow down A/Ia reactions - harder for Y to approach M
What does positive ∆S indicate about the reaction mechanism?
Indicates D/Id mechanism as there is more disorder at the transition state - greater entropy w bonds breaking
What does negative ∆S indicate about the reaction mechanism?
Indicates A/Ia mechanism as there is less disorder at transition state - bonds forming means less entropy
What’re the units of
cm^3 mol-1
What does the molar volume, ∆V, decreasing at the TS mean?
Decreasing ∆V at TS means applying pressure will increase rate
Indicates A/Ia mechanism
What does the molar volume, ∆V, increasing at the TS mean?
Applying pressure will decrease rate
Indicates D/Id mechanism
Why is measuring ∆V and rate difficult?
Hard to get significant variation in k with pressure, very high pressure needed
Following reactions at high pressure is hard to do
What does the Eyring equation give when linearised?
Slope = -∆H/R
Intercept = ln(kB/h) + ∆S/R
How is the Eyring equation linearised? (What’s plotted against what?)
Ln(k/T) vs 1/T is plotted
Should get a straight line
Why is varying Temp, M, L, X or Y too much a bad idea?
Because mechanism may change if the change is too drastic
How should the rate of a reaction be determined?
Establish rate law by varying [LM-X] and [Y] and measuring effect on rate, k.
Vary M, L, X and Y
Measure effect of Temp on rate
- ∆H, ∆S
Measure effect of pressure on rate
- ∆V
Why is Pt(II) good to study?
Very slow reaction - minutes or more at room temp
Means reactions are easier to study - similar to organic reactions
Where do two-term rate laws come from for Pt(II) complexes?
Slow RDS is water subbing in for X
Y subbing in for water is then very fast - as water is a poor L for Pt(II)
How does we know H2O is involved in ligand exchange for Pt(II) complexes?
Study reaction in MeOH instead of water.
MeOH is an even worse L for Pt(II)
- so k1 is smaller in MeOH but k2 remains roughly the same for a given Y
Value of k2 doesn’t depend on solvent (MeOH/H2O). But k1 DOES depend on solvent
What rate mechanism is suggested in substitution of Pt(II) ligands in water/MeOH?
Over 10^4 difference in k2 when Y is varied
Less than 10^1 difference in k2 if X is varied
Suggests A/Ia as bond formation (of solvent) is important in transition state/intermediate.
Bonding ligand is most important
What type of ligands does Pt(II) prefer? Give an example.
Soft ligands, as Pt(II) is also soft
E.g. thiourea
What effect do bulky ligands have on the rate of Pt(II) complexes substitution?
They decrease the rate of substitution
Suggests A/Ia reaction mechanism
What does a reduction in ∆S and ∆V at TS for Pt(II) complexes?
Means there is less disorder and less volume at TS
All values are much larger when compared to other TM reactions
Suggests A/Ia
What is the proposed mechanism for Pt(II) complexes?
Are Pt(II) 5-coordinate complexes common or rare? Compare this to other 4d/5d metals.
Extremely rare
For other 4d/5d square planar ions, Pd(II), Rh(I), Ir(I), are much more common
What is the trans effect?
That some ligands increase the rate of substitution when positioned trans to the leaving ligand
It a KINETIC effect - says nothing about thermodynamics…
What is the order of the trans effect series?
What is trans influence?
Lengthening/weakening of the bond trans to it (M-X) in ground state
Thermodynamic effect
How do ligands like C2H4 and CO exert high trans effect?
They have very low measured trans influence
- there is extra e- density on 5-coordinate TS (more lone pairs)
Pi-acceptor ligands can relieve this, hence stabilising TS and speeding up reaction
Why is solvent exchange used to determine how 3d metal ions undergo substitution?
Because there is no net chemical change (∆G = 0 regardless of M)
Therefore there is no confusion between Thermo/kinetic effects
How does rate relate to bond strength of ligand sub?
Rate is inversely proportional to bond strength
Therefore larger ions with smaller charge exchange fastest
- Cr(III) exchanges slowest
How does CFSE affect rate of ligand sub?
More CFSE means slower rate
What do most inert metal ions have in common?
≥3+ and all have large CFSE
- such as Cr(III) which is d3, or low-spin d5/d6 metals
Why are activation parameters -ve on left of Periodic Table but +ve on right?
Because mechanism changes from Ia to Id from left to right
Why does mechanism of ligand sub change from Ia to Id left to right of PT?
Early TM ion are larger - more space for associative mechanism
- also have at least one empty t2g orbital
This it’s important as H2O needs an empty orbital to donate its lone pair into
How does large CFSE slow down reactions?
Consider Cr(III): -1.2∆oct CFSE
If mechanism was A, intermediate would be 7-coordinate and would lose CFSE
If mechanism is D, intermediate would be 5-coordinate and also lose CFSE
Therefore the rate is slowed regardless
(The larger the CFSE of GS, the larger the loss of CFSE on change of geometry)
What 3d ions have the largest loss of CFSE?
d3 and d6
What 4d/5d ions have the largest loss in CFSE?
d5 and d6 ions for 4d/5d metals - since all are low spin
What 3d ions have the smallest loss of CFSE?
d0, d5 and d10
- as these all have ZERO CFSE
Why are Cr(II) and Cu(II) so labile?
Because of J-T distortion
- 2 waters weakly-bonded, these exchange fast
Give 2 examples of metals ions which are used to study rates of ligand exchange - bar Pt(II)?
Mainly Co(II) - cheap, inert, low-spin d6
Cr(III) - cheap, inert d3
What does +ve ∆V at TS indicate?
+ve ∆V at TS indicates D/Id mechanism
What can happen in a reaction to increase ∆V at TS?
Anion is generated (solvated)
Positive charge on metal increases - solvation increases
These can make ∆V at TS +ve even for reaction that are not D/Id
If a reaction is dissociative, what should it depend on on?
The identity of X, not Y, as bond breaking is RDS - key step
What is the significance of the linear free energy relationship for Co(III) sub?
A bigger ∆G at TS means a smaller ∆G (driving force)
This is how kinetics and thermodynamics relate to each other in the free energy relationship
F- dissociates slowest as both L and M are hard for Co(III)
What is the mechanism for Co(III) ligand sub?
Essentially pure D
- based on the free energy relationship
What is required for acid catalysis of ligand substitution?
That X is the salt of a weak acid, and has an additional lone pair
How does acid catalysis of ligand substitution work?
M-X bond is weakened by protonation of second lone pair of X (salt of a weak acid)
Does acid catalysis work best at higher or lower pH?
Lower
At what pH does base catalysis of ligand substitution work best?
Higher pH
For X = Cl-, kobs is 10^6 times faster at pH 10 than at pH 7.
What role does OH- have in base catalysis?
It removes a proton from an amine group, producing a common intermediate
- despite changing Y, a competing nucleophile, the rate of both reactions increases
What is the name given to base catalysed hydrolysis?
Dcb - dissociative via conjugate base mechanism
What is the mechanism for Dcb?
