Organometallic Solutions: #12
12.* (1993 F 6) A. Will an inner sphere reaction in which water is the bridging ligand occur more rapidly in strong acid or in strong base? Explain your answer with a mechanism. An inner-sphere process requires a bridging ligand. In this case the bridging ligand would be H2O, which would still have a lone pair of electrons after bonding to a metal. In strong acid, the H2O ligands are protonated, tying up the extra lone pair and eliminating the possibility of bridging. In a strong base, the H2O ligands can be deprotonated to form M–OH species which are great at bridging (OH has two extra pairs of electrons after bonding to a metal). Therefore, the reaction occurs much more quickly in base. B. Other than self exchange rates, what additional factors must be taken into account to predict the approximate rates of inner sphere redox reaction between two different complexes? Factors which are important in predicting rates for inner sphere reactions include: •Because we must substitute a bridging ligand for another ligand, the ligands on the "bridge-acceptor" must be labile (easily substituted.) •Electron transfer through the bridge depends on the conductivity through that ligand. •A strong bridging ligand will form the bridged complex rapidly. •Steric constraints may slow the reaction. (The bridge must be allowed to form.) |