Substitution reactions at octahedral complexes:the search for mechanism

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

Table 2 - 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

Steric effects

If one crowds the metal ion:

• speed up reactions under d activation• retard reactions under a activation

Table 3 - Data for Co3+ complexes of the type

Co

N

N N

N

Cl

Cl

R R

As bulk of the equatorial ligand increases, so does the rate of the reaction

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

d activation

•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

Table 5

The reactivity of cis versus trans complexes

CoX

Cl N

N

N

N

CoN

N N

N

Cl

X

displacement of Cl- by H2O

H2ONH3

donors only

Cl-

OH-low down in the spectrochemical series donors

cis complexes where these are present are quite reactive

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

Cl- departing

orthogonal orbitals(no net overlap)

donor in the trans position

rearrange (slow)

donation

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

rate of dissociation of departing X

interchange rate constant of X and Y

interchange rate constant of X and Y

Consider an aqua complex.

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

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

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 MX 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

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 stateS‡ < 0, as the solvent becomes more ordered around the system

[ [=o o o o

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

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 dS‡ < 0 aS‡ 0 no conclusions can be reached