1. Substitution reactions at octahedral complexes:
the search for mechanism

2. Begin by determining whether the intimate mechanism is a or d.
Table 1
Aquation refers to the reaction
[Co(NH3)5X]n+ + H2O  [Co(NH3)5(H2O)]3+ + X
Rate constants vary by 6 orders of manitude
 Strongly dependent on the nature of the leaving group
Anation refers to the reaction
[Co(NH3)5(H2O)]3+ + Y  [Co(NH3)5(H2O)]n+ + H2O
Rate constants vary by a factor of 10
 Weakly dependent on the nature of the entering group
 d activation

3.  probably under a activation
Table Data for Ru3+
Rate more sensitive to the nature of the entering group than the leaving group
anation reactions vary by 3 orders of magnitude
aquation reactions vary by at most 2 orders of magnitude
 probably under a activation

4.  d activation Steric effects If one crowds the metal ion:
speed up reactions under d activation
retard reactions under a activation
Table Data for Co3+ complexes of the type
As bulk of the equatorial ligand increases, so does the rate of the reaction
 d activation

5.  d activation Electronic effects
If the inert ligands stabilise a 5 coordinate intermediate, and the reaction proceeds faster, then we conclude the reaction is under d activation
Table 4
The saturated complex (cyclam) reacts slowly
bis(dmg) complex reacts faster
-unsaturated, with electron-withdrawing substituents
trans[14]diene reacts fastest
- unsaturated; electron donating group (CH2) on N
So, increasing the donation of electron density to the metal ion stabilises the loss of the chloride axial ligand
 d activation

6. Table 5
The reactivity of cis versus trans complexes
displacement of Cl- by H2O
H2O
NH3
 donors only
Cl-
OH-
low down in the spectrochemical series   donors
cis complexes where these are present are quite reactive

7. This accords with a mechanism under d activation
Cl- departing
p orbital of a  donor like Cl- of OH- in the cis position
donates electron density into emerging vacant metal orbital

8. orthogonal orbitals (no net overlap) Cl- departing
 donor in the trans position
orthogonal orbitals
(no net overlap)

9. rearrange (slow)
 donation

10. Consider an aqua complex.
We saw in Chapter 3 that...
D saturating rate constant = k1
I = k
A = only saturates at the
diffusion limit
rate of dissociation of departing X
interchange rate constant of X and Y

11. Hence, for a D mechanism, ksat = k1 and the limit is set by the rate of water exchange
For an Id mechanism, ksat = k, the rate constant for the exchange of departing H2O and entering Y
But [H2O] = 55 M in aqueous solution
since [H2O]outer sphere >> [Y]outer sphere, the rate is also limited by the rate of water exchange
For an Ia mechanism, ksat = k, the rate constant for the exchange of departing H2O and entering Y. But this is dominated by bond forming between entering Y and the metal
rate could be greater than the rate of H2O exchange

12. Hence:
for d actication, rate cannot be > rate of H2O exchange
for a activation, the rate may be greater than the rate of H2O exchange
Table 6
Rh3+ and Ir3+ complexes under associative activation

13. [Cr(H2O)5X]n+  [Cr(H2O)5]m+ + X (X = H2O, OH-)
Effects of charge
See Table 7
For d activation:
[Cr(H2O)5X]n+  [Cr(H2O)5]m+ + X (X = H2O, OH-)
(This is a D process; Id would have Y involved as X departs.)
As the charge on the metal complex increases, the stronger the MX bond
 rate decreases
Rate is faster when X = H2O (n+ = 3+) than when X = OH- (n+ = 2+)
Cr3+ data is in line with a d intimate mechanism

14. Electrostriction
Ordering or disordering of solvent molecules around the metal centre during a chemical reaction
Effect is predominantly seen in values of S‡
Charge density has been increased in the transition state
S‡ < 0, as the solvent becomes more ordered around the system

15. The ordering of the solvent is largely unaffected and the contribution to S‡ will be close to zero.
There is charge neutralisation in the transition state; the solvent will be less ordered and the electrostriction contribution to S‡ > 0

16. Corrections for electrostriction effects should be made before any definitive statements concerning mechanism based on values of S‡.
After correction for electrostriction effects:
S‡ > 0  d
S‡ < 0  a
S‡  0  no conclusions can be reached