Question

Give two reasons why the Co3+ substitution mechanism is dissociative/ID instead of associative/IA

Answer 1

Co(III), d6 low spin, octahedral, t2g6 configuration, maximum CFSE,

Leaving group effects are expected to be large in Id mechanisms since ∆G‡ is largely determined by breakage of the MX bond. So vary the leaving group X.

there is a relationship between the logarithms of rate constant and equilibrium constant. log k = log K + c Since logarithms of rate constants and equilibrium constants are proportional to free energy changes, an alternative expression of this equation is ∆G‡ = a ∆Go + b This is known as a Linear Free Energy Relationship.

The observation that the reaction shows a LFER has some implications for the reaction mechanism.

• Changing the Co-X function has the same effect on the rate (k) as it has on the equilibrium constant (K).

• Changing the Co-X function has the same effect on activation free energy as on the overall free energy.

• Since the overall reaction will involve solvation of X-, this implies solvation of X- in the activated complex.

• This implies a Dissociative (Id) mechanism.