In this question, we explore the differences between metal coordination by monodentate and bidentate ligands. Formation constants, K_f, for [Ni(NH₃)₆]²⁺(aq) and [Ni(en)₃]²⁺(aq) are as follows:

The difference in K_f between these complexes indicates a higher thermodynamic stability for the chelated complex, caused by the chelate effect. Recall that K is related to the standard free energy of the reaction by Δ_rG° = −RT ln K and Δ_rG° = Δ_rH° − TΔ_rS°. We know from experiment that Δ_rH° for the NH₃ reaction is −109 kJ/mol-rxn, and Δ_rH° for the ethylenediamine reaction is −117 kJ/mol-rxn. Is the difference in Δ_rH° sufficient to account for the 1010 difference in K_f? Comment on the role of entropy in the second reaction.

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Ni²+ (aq) + 6 NH3(aq) →→ [Ni(NH3)²+ (aq) Ni²+ (aq) + 3 en(aq) →→→ [Ni(en)]²+ (aq) Kf = 108 K= 10¹8