= Solution
A <dissociative substitution> begins with slow CO loss,
$$
\mathrm{M(CO)_5L}\xrightarrow{k_d}\mathrm{M(CO)_4L}+\mathrm{CO},
$$
followed by capture of the incoming <ligand in coordination chemistry>. A more electron-releasing $L$ increases metal-to-CO <pi backbonding> in the six-coordinate reactant, stabilizing it and strengthening its M–CO attachments. Removing CO sacrifices a useful pi acceptor from that electron-rich center. Thus \b[the activation free energy for CO dissociation often rises and the substitution slows]. Within an otherwise matched series, $k_d\propto\exp[-\Delta G^\ddagger/(RT)]$ captures this sensitivity. The rate depends on the relative energies of reactant and dissociation <transition state>, so the trend is conditional: large changes in <steric hindrance>, geometry or mechanism can override the electronic effect.
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