Why are polydentate ligands more stable

Kinetic stability is related to the reactivity of the metal complexes in solution and deals with the rate of the reaction, its activation energy, etc. Kinetic stability is also related to how fast a compound reacts rather than how stable it is.

It aids in determining the rate at which the reaction occurs to establish the equilibrium [ 7 ]. The term kinetic stability of complexes is classified into labile and inert by Taube on the basis of rate of the reactions.

When the rate of substitution of ligands is high, the complex is said to be labile. On the other hand, in inert complexes the rate of ligand exchange is very slow, and the ligands are very exchanged with difficulty.

HCl was added to the aqueous solution. However, only one NH 3 ligand was found to be substituted by Cl ligand, when the aqueous solution of the complex was heated with 6M hydrochloric acid. For metal complexes, the stability and reactivity are described in thermodynamic and kinetic terms, respectively. In particular, the terms stable and unstable are related to thermodynamic aspects, whereas labile and inert terms are related to kinetic aspects.

Kinetic stability refers to reactivity or the ability of the metal complex to undergo ligand substitution reactions. Complexes which undergo extremely rapid ligand substitution reaction are referred to as labile complexes, and complexes that undergo extremely slow ligand substitution reaction are referred to as inert complexes. Sometimes the thermodynamic and kinetic stabilities of complexes are parallel to one another, but often they do not. However, no ligand substitution reaction is found when the complex is kept in acidic solution for several days; hence the complex is kinetically inert.

From the above two examples, it can be interpreted that the stability of a complex mainly depends upon the conditions, and it is always recommended to specify the conditions such as pH, temperature, etc. In brief, it is not necessary for a stable complex to be inert and an unstable complex to be labile. All the complexes are thermodynamically stable, but kinetically they behave in a different manner.

The rate of exchange can be measured when carbonlabelled cyanide ions are reacted with metal complexes in solution. There are several factors that can affect the stability of the metal complexes [ 2 , 5 , 8 , 9 ], which include: Nature of the central metal ion.

In metal cations, higher oxidation state forms more stable complex than lower oxidation states with ligands such as NH 3 , H 2 O, etc. The stability of metal complex increases with decrease in size of the metal cations. The order of size of dipositive ions is. Basic character of ligands: The greater is the basic character of ligand, the more easily it can donate its lone pair of electrons to the central metal ion and hence greater is the complex stability.

The nature of metal-ligand bond also affects the stability of metal complexes. The higher the covalent character, the greater will be the complex stability. For example, the stabilities of silver complexes have different halide ligands which are in the following order:.

The chelate effect is that the complexes resulting from coordination of metal ions with the chelating ligand are thermodynamically much more stable than the complexes with non-chelating ligands [ 10 , 11 ]. Chelating ligands are molecules which can bind to single metal ion through several bonds and are also called as multidentate ligands. Simple and common examples include ethylenediamine and oxalate.

Non-chelating ligands are ligands that bond to just one site, such as chloride, cyanide, and water. During the comparison study, the number of coordination should be maintained equal in both the cases, for example, the value obtained while adding a bidentate ligand is compared with the value obtained for two monodentate ligands. Such comparison studies revealed that the metal complex formed from chelating ligands are thermodynamically more stable than the complex formed from monodentate ligand.

A macrocyclic ligand is a cyclic molecule that contains nine or more atoms in the cyclic structure and has three or more potential donor atoms which can coordinate to the metal ion.

It has been observed that the stability of metal complexes in the presence of macrocyclic ligand of appropriate size is higher than the stability of complexes coordinated to open-ended multidentate chelating ligands. Some notable examples of macrocyclic ligands include cyclic crown polyether, heme, etc. Resonance increases the stability of the complexes.

For example, acetylacetonate anion ligand shows resonance, and as a result it forms stable complexes upon reacting with metal ion Figure 1. Resonance structure of acetonylacetonate ligand.

Acetonylacetonate-metal complex. Generating a little bit more during the reaction is going to make no effective difference to the total concentration of the water in terms of moles of water per dm 3. The concentration of the water is approximately constant. The equilibrium constant is defined so that you avoid having an extra unnecessary constant in the expression. With that out of the way, let's go back to where we were - but introduce a value for K 1 :. The value of the equilibrium constant is fairly large, suggesting that there is a strong tendency to form the ion containing an ammonia molecule.

Each of the other equilibria above also has its own stability constant, K 2 , K 3 and K 4. For example, K 2 is given by:. You could keep plugging away at this and come up with the following table of stability constants:. You will often find these values quoted as log K 1 or whatever. All this does is tidy the numbers up so that you can see the patterns more easily. The ions keep on getting more stable as you replace up to 4 water molecules, but notice that the equilibrium constants are gradually getting less big as you replace more and more waters.

This is common with individual stability constants. You can see that overall this is a very large equilibrium constant, implying a high tendency for the ammonias to replace the waters. The "log" value is This overall value is found by multiplying together all the individual values of K 1 , K 2 and so on. To find out why that works, you will need a big bit of paper and some patience!

Write down expressions for all the individual values the first two are done for you above , and then multiply those expressions together. You will find that all the terms for the intermediate ions cancel out to leave you with the expression for the overall stability constant.

Whether you are looking at the replacement of individual water molecules or an overall reaction producing the final complex ion, a stability constant is simply the equilibrium constant for the reaction you are looking at.

The larger the value of the stability constant, the further the reaction lies to the right. That implies that complex ions with large stability constants are more stable than ones with smaller ones. 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.

B Decide whether any complexes are further stabilized by a chelate effect and arrange the complexes in order of increasing stability. 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. Consequently, the likely order of increasing stability is. Sign up to join this community. The best answers are voted up and rise to the top. Stack Overflow for Teams — Collaborate and share knowledge with a private group.

Create a free Team What is Teams? Learn more. Asked 5 years, 2 months ago. Active 5 months ago. Viewed times. Improve this question. Therefore bidentate ligands generally have an advantage over monodentate.

Also, welcome to Chem.