Effect of metal ion on the stability of Complexes

Examination of Equation shows that a large value of K means a high concentration

of MLn+ relative to Mn+ and L; in other words, a large K means a strong preference for

complex formation. The size of K with metal complexes is usually so large that we tend to

report log10 K values. There is also a direct relationship between the stability constant and the free energy of a reaction, expressed in terms of the relationship-

Where R is the gas constant and T the temperature in Kelvin. This means that the higher is K, the more negative is the free energy of the reaction. We feel this usually as a release of heat on complexation, because of the relationship between free energy and reaction enthalpy and  reaction entropy

The overall stability constant (β) represents the stability for a set of sequential complexation steps, rather than for an individual component step. It allows us to represent the stability constant for an overall reaction M+ nL forming MLn, rather than just for a single ligand addition step such as M + L forming ML. As for K, the larger is , the more thermodynamically stable is the assembly.

Factors Influencing the stability of Complexes

 i. The available metal orbitals for bonding

ii. The extent of overlap between the metal orbital and the ligand lone pair orbitals

iii. The strength of the ligand field

iv. The possibility of metal to ligand bonding besides ligand to metal bonding

Ionic Size

      If the metal ion charge remains same, stability of alkali metal complexes increases as the size of the metal ion decreases due to electrostatic field effect. The stability of the complexes is of the order-

                     Li⁺ > Na⁺ > K⁺ > Rb⁺ > Cs⁺     

Electrostatic fields bear some similarity to magnetic  fields. Objects attract if their charges are of opposite polarity (+/-);objects repel if their charges are of the same polarity (+/+ or -/-).

Metal ions with d¹⁰ electrons are exempt from this effect like Zn²⁺   Cu⁺

When the metal ion charge is fixed but the metal ion size is increased, the surface charge

density decreases as the ionic radius increases. This means a less effective attractive force for the ligand applies, which leads to a fall in the size of log K

Ionic Charge

Metal ion that has more than one valency: higher valency ion forms more stable compound

e.g. Co³⁺  > Co²⁺   

Again metal ions having similar size but higher number of oxidation state form stable complexes

As the charge on cations, all of similar size, varies from 1+ to 4+ across the series, the size of log K increases.

As the no of charge increases the electrostatic field around the metal ion also increases which ensure added attraction forces for ligands and make the complex more stable

Metal Class and Ligand Preference

Soft metal prefers hard ligand and vice versa. Electropositive metals (lighter and/or more highly charged ones from the s, d and f block such as Mg2+, Ti4+ and Eu3+, belonging to Class A) tend to prefer lighter p-block donors (such as N, O and F donors).

Less electropositive metals (heavier and/or lower-charged ones such as Ag+ and Pt2+ belonging to Class B) prefer heavier p-block donors from the same families (such as P, S and I donors).

Electronegativity

Though this effect is less significant besides higher electronegative metal atom attracts electrons of donor ligands more extent.

Effect of Ligand on the stability of complexes

Nature of ligand atoms

Size of the ligand plays a role in the electrostatic effect on stability of a complex, particularly for anionic ligands. Since the ligand can be assigned a surface charge density (when anionic), and obviously an anion with a high surface charge density should form stronger complexes from an electrostatic perspective.

For Example, with F⁻ (radius 133 pm) and Cl⁻ (radius 181 pm), Fe³⁺ forms complexes with log K of 6.0 and 1.3 respectively, reflecting the greater charged/surface ratio for the former.

Ligands having more electronegative atoms tend to form strong bond with metal ions.

F⁻  > Cl⁻  > Br⁻  > I⁻ 

Therefore, ligands having higher charge and smaller size form more stable complexes.

Basicity of the  ligand

More basic ligands are more capable of donating electrons. From this perspective, the better

base NH3 should be a better ligand than PH3 or H2O, and F− should be superior to other

 halide ions.

This predicted behaviour holdsvquite well for s-block, lighter (first-row) d-block, and f-block \\

metal ions, which have been defined as ‘hard’ or Class A metal ions.

Unfortunately, when one examines heavier metal ions and those in low oxidation states,

the behaviour is not the same (hence their definition as ‘soft’ or Class B metals). Obviously,

there are electrostatic contributions applying, but other influences are now important. A

classical example is the reaction of the relatively large, low-charged Ag+ ion in formingAgX,

which follows the order (log values of the formation constant in parentheses):

Clearly, this does not follow the trend expected on electrostatic grounds, which should be opposite to that observed. The trend is thought to reflect increased covalent character in the Ag X bond in moving from fluoride to iodide.

Chelation

Ligands those form chelate complex is more stable than complexes of monodentate ligands

e.g.     [Ni(en)₃]²⁺   is more stable than [Ni(NH₃)₆]²⁺

 Size of chelate ring

the size of the chelate ring also influences the size of the stability constant

Stability of complexes increase with the number of chelate rings.

M-EDTA complexes are more stable than M-en complexes

Simply, the size of the stability constant depends on the number of atoms or bonds in the ring. For saturated rings, five-membered rings where ligand donor ‘bite’ and preferred angles within the chelate ring are optimized

are preferred for the lighter metal ions, with smaller or larger rings being of lower stability

The exception to this observed preference for five-membered rings comes when unsaturated conjugated ligands are coordinated, where very stable complexes with six-membered chelate rings can exist with some light metal ions.

When the chelate ring size grows very large, there is no particular stability arising from chelation.

Steric Strain

We can assume that large, bulky groups that interact sterically (that is ‘bump

into’ other ligands) when attempting to occupy coordination sites around a central metal

ion usually leads to lower stability. The strain in such systems is seen in distortions of bond

lengths and angles away from the ideal for the particular stereochemistry applying.

Ammonia- 1.8 x 10-5  kb  Di ethyl – 54 x 10-5   trimethyl amine-  6.7 x 10-5

Thus N(CH3)3, which is a stronger base than NH3, forms weaker complexes due to teric hindrance

Thus coordination of N(CH3))3

will lead to greater steric interaction with other ligands than will −OOC C(CH3)3, as

the ligand bulk is displaced further out in space in the latter case. As a consequence, the

carboxylate is termed a more ‘sterically efficient’ ligand than the amine.

The Macrocycle Effect

the most stable complexes form where the internal diameter of the ring cavity matches the size of the entering cation. The effect can

be significant; for example, the natural antibiotic valinomycin is a macrocycle that binds

potassium ion to form a complex ∼104 times more stable than that formed with the smaller sodium ion, despite the chemical similarity of the cations.

The flexible, long-chain linear molecule must undergo significant translational motion to

‘stitch’ itself onto the metal ion, which is not favoured. However, the cyclic molecule has

the donors pre-organized in more appropriate positions for binding to the metal ion, and

its coordination is thus favoured. This type of enhanced stability for complexation of macrocycles over acyclic analogues is shown by a wide range of

macrocycles of different size, donor type and number, and is well established; a typical

example involving polyamines appears

Entropy Effect

Stability of complexes increase with the number of chelate rings.

M-EDTA complexes are more stable than M-en complexes