6.14: The Chelate Effect

The term ligand come from the latin word ligare, which meaning to bind) was first used by Alfred Stock in 1916 in relation to silicon chemistry. The first use of the term in a British journal was by H. Irving and R.J.P. Williams in Nature, 1948, 162, 746 in their paper describing what is now called the Irving-Williams series. Ligands can be characterised as monodentate, bidentate, tridentate etc. where the concept of teeth (dent) is introduced. The term chelate was first applied in 1920 by Sir Gilbert T. Morgan and H.D.K. Drew [J. Chem. Soc., 1920, 117, 1456], who stated: "The adjective chelate, derived from the great claw or chela (chely- Greek) of the lobster or other crustaceans, is suggested for the caliperlike groups which function as two associating units and fasten to the central atom so as to produce heterocyclic rings." Metal complexation is of widespread interest. It is studied not only by inorganic chemists, but by physical and organic chemists, biochemists, pharmacologists, molecular biologists and environmentalists.

Ligands like chloride, water, and ammonia are said to be monodentate (one-toothed, from the Greek mono, meaning “one,” and the Latin dent-, meaning “tooth”): they are attached to the metal via only a single atom. Ligands can, however, be bidentate (two-toothed, from the Greek di, meaning “two”), tridentate (three-toothed, from the Greek tri, meaning “three”), or, in general, polydentate (many-toothed, from the Greek poly, meaning “many”), indicating that they are attached to the metal at two, three, or several sites, respectively. Ethylenediamine (H₂NCH₂CH₂NH₂, often abbreviated as en) and diethylenetriamine (H₂NCH₂CH₂NHCH₂CH₂NH₂, often abbreviated as dien) are examples of a bidentate and a tridentate ligand, respectively, because each nitrogen atom has a lone pair that can be shared with a metal ion. When a bidentate ligand such as ethylenediamine binds to a metal such as Ni²⁺, a five-membered ring is formed. A metal-containing ring like that shown is called a chelate ring . Correspondingly, a polydentate ligand is a chelating agent, and complexes that contain polydentate ligands are called chelate complexes.

The Chelate Effect

Experimentally, it is observed that metal complexes of polydentate ligands are significantly more thermodynamically stable than the corresponding complexes of chemically similar monodentate ligands; this increase in stability is called the chelate effect. For example, the complex of Ni²⁺ with three ethylenediamine ligands, [Ni(en)₃]²⁺, should be chemically similar to the Ni²⁺ complex with six ammonia ligands, [Ni(NH₃)₆]²⁺. In fact, the equilibrium constant for the formation of [Ni(en)₃]²⁺ is almost 10 orders of magnitude larger than the equilibrium constant for the formation of [Ni(NH₃)₆]²⁺ (Table E4):

\(\begin{align} & [\mathrm{Ni(H_2O)_6]^{2+}+6NH_3} & \rightleftharpoons \mathrm{[Ni(NH_3)_6]^{2+}+6H_2O(l)} \hspace{3mm} & K_\mathrm f=\mathrm{4\times10^8} \\
& \mathrm{[Ni(H_2O)_6]^{2+}+3en} & \rightleftharpoons \mathrm{[Ni(en)_3]^{2+}+6H_2O(l)} \hspace{3mm} & K_\mathrm f=\mathrm{2\times10^{18}}\end{align} \label{23.10}\) The formation constants are formulated as ligand exchange reactions with aqua ligands being displaced by new ligands (\(NH_3\) or \(en\)) in the examples above.

Chelate complexes are more thermodynamically stable than the analogous complexes with monodentate ligands.

The stability of a chelate complex depends on the size of the chelate rings. For ligands with a flexible organic backbone like ethylenediamine, complexes that contain five-membered chelate rings, which have almost no strain, are significantly more stable than complexes with six-membered chelate rings, which are in turn much more stable than complexes with four- or seven-membered rings. For example, the complex of nickel (II) with three ethylenediamine ligands is about 363,000 times more stable than the corresponding nickel (II) complex with trimethylenediamine (H₂NCH₂CH₂CH₂NH₂, abbreviated as tn):

\(\begin{align} & \mathrm{[Ni(H_2O)_6]^{2+}+3en} & & \rightleftharpoons \mathrm{[Ni(en)_3]^{2+}+6H_2O(l)} & K_\mathrm f=6.76 \times 10^{17} \\ & \mathrm{[Ni(H_2O)_6]^{2+}+3tn} & & \rightleftharpoons \mathrm{[Ni(tn)_3]^{2+}+6H_2O(l)} & K_\mathrm f=1.86 \times 10^{12}\end{align} \label{23.11}\)

*The above measurements were done in a solution of ionic strength 0.15 at 25º C.

Example \(\PageIndex{1}\)

Arrange [Cr(en)₃]³⁺, [CrCl₆]³⁻, [CrF₆]³⁻, and [Cr(NH₃)₆]³⁺ in order of increasing stability.

Given: four Cr(III) complexes

Asked for: relative stabilities

Strategy:

A Determine the relative basicity of the ligands to identify the most stable complexes.

B Decide whether any complexes are further stabilized by a chelate effect and arrange the complexes in order of increasing stability.

Solution

A The metal ion is the same in each case: Cr³⁺. Consequently, we must focus on the properties of the ligands to determine the stabilities of the complexes. Because the stability of a metal complex increases as the basicity of the ligands increases, we need to determine the relative basicity of the four ligands. Our earlier discussion of acid–base properties suggests that ammonia and ethylenediamine, with nitrogen donor atoms, are the most basic ligands. The fluoride ion is a stronger base (it has a higher charge-to-radius ratio) than chloride, so the order of stability expected due to ligand basicity is

[CrCl₆]³⁻ < [CrF₆]³⁻ < [Cr(NH₃)₆]³⁺ ≈ [Cr(en)₃]³⁺.

B Because of the chelate effect, we expect ethylenediamine to form a stronger complex with Cr³⁺ than ammonia. Consequently, the likely order of increasing stability is

[CrCl₆]³⁻ < [CrF₆]³⁻ < [Cr(NH₃)₆]³⁺ < [Cr(en)₃]³⁺.

Entropy

Consider the formation of a complex of Ni²⁺ with ammonia or 1,2-diaminoethane.

In reaction 3 above, three bidentate ligands replace the six monodentate ligands. On the left-hand-side there are 4 species, whereas on the right-hand-side there are 7 species, that is a net gain of 3 species occurs as the reaction proceeds. This represents an increase in entropy since it the disorder of the system increases by the production of additional molecules.

An alternative view comes from trying to understand how the reactions might proceed. To form a complex with 6 monodentates requires 6 separate favorable collisions between the metal ion and the ligand molecules. To form the tris-bidentate metal complex requires an initial collision for the first ligand to attach by one arm but remember that the other arm is always going to be nearby and only requires a rotation of the other end to enable the ligand to form the chelate ring.

If you consider dissociation steps, then when a monodentate group is displaced, it is lost into the bulk of the solution. On the other hand, if one end of a bidentate group is displaced the other arm is still attached and it is only a matter of the arm rotating around and it can be reattached again. Both conditions favor the formation of the complex with bidentate groups rather than monodentate groups.

The Macrocyclic Effect

The macrocyclic effect follows the same principle as the chelate effect, but the effect is further enhanced by the cyclic conformation of the ligand. Macrocyclic ligands are not only multi-dentate, but because they are covalently constrained to their cyclic form, they allow less conformational freedom. The ligand is said to be "pre-organized" for binding, and there is little entropy penalty for wrapping it around the metal ion. For example heme b is a tetradentate cyclic ligand which is strongly complexes transition metal ions, including (in biological systems) Fe⁺².

Heme b

Some other common chelating and cyclic ligands are shown below:

Acetylacetonate (acac^-, right) is an anionic bidentate ligand that coordinates metal ions through two oxygen atoms. Acac^- is a hard base so it prefers hard acid cations. With divalent metal ions, acac^- forms neutral, volatile complexes such as Cu(acac)₂ and Mo(acac)₂ that are useful for chemical vapor deposition (CVD) of metal thin films.

2,2'-Bipyridine and related bidentate ligands such as 1,10-phenanthroline (below, center left) form propeller-shaped complexes with metals such as Ru²⁺. The [Ru(bpy)₃]²⁺ complex (below left) is photoluminescent and can also undergo photoredox reactions, making it an interesting compound for both photocatalysis and artificial photosynthesis. The chiral propellor shapes of metal polypyridyl complexes such as [Ru(bpy)₃]²⁺ coincidentally match the size and helicity of the major groove of DNA. This has led to a number of interesting studies of electron transfer reactions along the DNA backbone, initiated by photoexcitation of the metal complex.

Prof. Jacqueline Barton (Caltech) has used metal polypyridyl complexes to study electron transfer reactions that are implicated in the biological sensing and repair of damage in DNA molecules.

Crown ethers such as 18-crown-6 (below, center right) are cyclic hard bases that can complex alkali metal cations. Crowns can selectively bind Li⁺, Na⁺, or K⁺ depending on the number of ethylene oxide units in the ring.

The chelating properties of crown ethers are mimetic of the natural antibiotic valinomycin (below right), which selectively transports K⁺ ions across bacterial cell membranes, killing the bacterium by dissipating its membrane potential. Like crown ethers, valinomycin is a cyclic hard base.

Problems

Exercise \(\PageIndex{1}\)

Arrange [Co(NH₃)₆]³⁺, [CoF₆]³⁻, and [Co(en)₃]³⁺ in order of decreasing stability.

Answer

[Co(en)₃]³⁺ > [Co(NH₃)₆]³⁺ > [CoF₆]³⁻

Exercise \(\PageIndex{1}\)

Of \(\ce{[Ni(en)3]^{2+}}\), \(\ce{[Ni(EDTA)]^{2-}}\), and \(\ce{[Ni(NH3)6]^{2+}}\), which would you expect to have the largest \(K_f\) value? The smallest? Explain.

Answer

This is a question involving the effects of chelation. When the complex ion with EDTA is formed, the EDTA ligand displaces six aqua ligands at once. Ethylenediamine ligands displace two at a time and ammine ligands displace one at a time. Based on this we would expect \(\ce{[Ni(EDTA)]^{2-}}\) to be the most stable and \(\ce{[Ni(NH3)6]^{2+}}\) to be the least stable.