The reaction [Ni(H₂O)₆]²⁺ + 3 en → [Ni(en)₃]²⁺ + 6 H₂O has a much larger formation constant than [Ni(H₂O)₆]²⁺ + 6 NH₃ → [Ni(NH₃)₆]²⁺ + 6 H₂O, despite both involving six N-donor atoms. What is the primary thermodynamic reason?
AThe N-H bonds in ethylenediamine are stronger than those in ammonia
BThe entropy change is more favorable for the chelate reaction because three reactant particles produce seven product particles (1 complex + 6 H₂O), whereas the ammonia reaction involves seven reactant particles producing seven product particles
CEthylenediamine is a better sigma-donor than ammonia due to its carbon backbone
DThe chelate complex has a lower enthalpy due to the strain energy stored in the five-membered rings
The chelate effect is primarily entropic. In the ethylenediamine reaction, 4 particles on the left (1 complex + 3 en) produce 7 particles on the right (1 complex + 6 H₂O) — a net increase of 3 free particles, giving a large positive ΔS. In the ammonia reaction, 7 particles on the left produce 7 on the right — no net change in particle count. Since ΔG = ΔH − TΔS and the enthalpy changes are similar (both involve six Ni-N bonds of comparable strength), the more favorable entropy drives the larger Kf for the chelate complex.
Question 2 True / False
The macrocyclic effect states that cyclic polydentate ligands form even more stable complexes than analogous open-chain chelating ligands with the same donor atoms.
TTrue
FFalse
Answer: True
The macrocyclic effect is an extension of the chelate effect. Cyclic ligands like porphyrins and crown ethers are pre-organized — their donor atoms are already positioned in roughly the geometry needed for coordination, reducing the entropic cost of organizing the ligand around the metal. Additionally, the cyclic structure makes dissociation kinetically more difficult because the ligand cannot peel off one end at a time; it must completely dissociate in one concerted step. Both the thermodynamic and kinetic advantages make macrocyclic complexes extraordinarily stable — hemoglobin's iron-porphyrin complex is a biological example.
Question 3 True / False
A complex with a large formation constant Kf is always kinetically inert — it exchanges its ligands slowly.
TTrue
FFalse
Answer: False
Thermodynamic stability (measured by Kf) and kinetic inertness (measured by the rate of ligand exchange) are independent properties. A complex can be thermodynamically stable (large Kf, products are heavily favored at equilibrium) but kinetically labile (ligands exchange rapidly because the activation barrier is low). For example, [Cu(NH₃)₄]²⁺ has a large Kf but is kinetically labile — its ligands exchange rapidly. Conversely, [Cr(NH₃)₆]³⁺ is both thermodynamically stable and kinetically inert. The distinction depends on whether you are asking 'where does the equilibrium lie?' (Kf) or 'how fast does the system reach equilibrium?' (rate constant).
Question 4 Short Answer
Five-membered chelate rings (formed by ligands like ethylenediamine) are typically more stable than four- or six-membered chelate rings. Explain why, in terms of both strain and entropy.
Think about your answer, then reveal below.
Model answer: Five-membered chelate rings strike an optimal balance between ring strain and conformational flexibility. Four-membered rings have severe angle strain because the bite angle is forced to be very small (~70°), compressing the M-L-L bond angles far from their ideal values. Six-membered rings have minimal strain but are more flexible, adopting multiple conformations — this flexibility means a larger entropic penalty upon ring closure compared to five-membered rings. Five-membered rings, with typical M-N-C-C-N torsion angles, experience modest strain while being rigid enough that the entropic cost of cyclization is low. This explains why ethylenediamine (forming five-membered chelate rings) is one of the most effective chelating agents.
The preferred ring size also depends on the metal ion radius. Larger metals accommodate slightly larger bite angles and may favor six-membered rings. But the five-membered ring preference is a robust general trend across most transition metals.