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CHELATE AND MACROCYCLIC EFFECT

CHELATE AND MACROCYCLIC EFFECT. 충남대학교 자연과학대학 화학과 201450094 강정란 2014.04.01.Tue. Common ligands. Ligand : any group (ion or molecule) having in its structure an atom with an unshared electron pair may act as a coordinating agent. Common ligands. Chelate ligand.

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CHELATE AND MACROCYCLIC EFFECT

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  1. CHELATE AND MACROCYCLIC EFFECT 충남대학교 자연과학대학 화학과 201450094 강정란 2014.04.01.Tue

  2. Common ligands • Ligand : any group (ion or molecule) having in its structure an atom with an unshared electron pair may act as a coordinating agent.

  3. Common ligands

  4. Chelate ligand • Chelate ligand : when ligand can occupy more than a single coordination position, the ring structure is formed. Such ring-forming groups are referred to as chelating groups. (from the Greek χηλή, chēlē, clab’s claw)

  5. Chelate effect • Chelate effect : observation that chelating ligands form more stable complexes (higher β values) than do an equivalent number of related monodentateligands.

  6. Chelate effect • Why are chelate complexes preferred? [Ni(NH3)6]2+ + 3 en [Cu(en)3]2+ + 6 NH3 ΔG° = – RTlnK = – 67kJ mol-1= ΔH° – TΔS° ΔH° = – 12kJ mol-1 – TΔS° = – 55kJ mol-1

  7. Chelate effect • ΔH is dependent on the nature of the ligand andmetalas well as ligand size but here NH3 and en (H2NCH2CH2NH2) are very similar to one another so ΔH is unlikely to vary much. • Change in entropy and it makes ΔG negative for this reaction. chelate ligands are far less likely to be displaced by water molecules, even in cases where water is a better ligand for the metal than the particular donor in the chelate, because binding 6 waters causes an unfavourable decrease in entropy.

  8. Chelate effect

  9. Chelate effect • the Chelate Effect is ENTROPY driven!!

  10. Chelate effect(1) • The common thermodynamic rationalization for the chelate effect points out the increase in entropy associated with chelate binding as compared to the binding of separate monodentate donors. In turn, this arises because there is an increase in the total number of particles in the former.

  11. Chelate effect(2) • A second explanation of the chelate effect is the increased effective concentration9,12 of the second donor, because its distance from the metal ion is fixed by the link to the bound first donor. This distance is short compared to an unlinked second donor, whose average distance from the metal ion will depend primarily on its concentration.

  12. Chelate effect(3) • A variation of this rationalization of the chelate effect is that the formation of the second M-N bond is abnormally fast, compared to an unlinkedsecond donor. The dissociation of the individual donors, in a flexible chelate, is as fast as that for a corresponding monodentateligand.

  13. Chelate-kinetic effect • Substitution for a chelated ligand is generally a slower reaction than that for a similar monodentate ligand.

  14. Macrocyclic effect • an impressive large additional stabilization occurs from linking the terminal donors into a ring; this phenomenon was termed themacrocyclic effect. The stabilization is much larger than can be explained by the simple addition of one more chelate ring. An accepted explanation for the macrocyclic effect is the difficulty in dissociation of the first donor atom from the metal ion in what is expected to be a stepwise dissociation of the polydentateligand.

  15. Macrocyclic effect • Structures of common crown ethers • ( 12-crown-4, 15-crown-5, 18-crown-6, dibenzo-18-crown-6, and diaza-18-crown-6)

  16. Macrocyclic effect • A polydentatechelate can dissociate from a metal ion through successive SN1 replacement steps, beginning with a terminal donor. The macrocyclicligand can’t dissociate through a similarly simple mechanism because their is no end group.

  17. Macrocyclic effect • Tetraazamacrocyclic complex of Ni(II)

  18. Macrocyclic effect • Template effect : Complexation to a metal ion stabilizes one component of a mixture, shifting the equilibrium in favour of production of a metal complex.

  19. CHELATE AND MACROCYCLIC EFFECT • Stability follows the order: Macrocyclic > Chelate > Monodentate

  20. Reference • Gary L, Miessler, Donald A. Tarr, 2011, Inorganic Chemistry 3thed, Pearson Education, p299-309, pp420-429 • Moeller, Therald, 1952, Inorganic Chemistry : an advanced textbook, Wiley, pp234-242, pp256-257 • Myeonggi Do, 2000, Inorganic chemistry, Scitech, pp209-247, pp291-295

  21. THANK YOU.

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