Factors Affecting the Stability of Complexes

Complex stability of coordination chemistry includes metal-ligand interactions that make up the compounds. With some further scanning of various sources, it is easy to say that the term 'complex' in regard to this connection and the respective field is so widely used everywhere. Complexes exist in nature and are synthetic compounds.

This Story also Contains

1. Complex Stability
2. Nature of the Central Metal Atom:
3. Relevance and Applications
4. Some Solved Examples
5. Summary

Complex Stability

Complex stability is the extent to which the structure of the Coordination Compound resists the effects of various factors such as temperature, pressure, and the presence of other ligands. One can separate stability into thermodynamics, which deals with the equilibrium constant of formation of the complexes, and kinetic, referring to the rate of substitution reaction. Those are all of the factors that have an impact on the compounds' stability: the nature of the central metal atom, the characteristics of the ligands, and the overall geometry of the complex.
It can also be put forward that high oxidation state transition metals generally form more stable complexes because of their increased positive charge and hence increased attraction to negatively charged ligands. Moreover, size and charge on the ligand become an important factor, the smaller and more highly charged, then the stronger the bond that can be formed with a metal ion to result in increased stability. Having a good grasp of the dynamics of these is quite useful in understanding the behavior of complexes in different chemical contexts.
Factors that Play a Critical Role on the Stability of Metal Complexes: Broadly, these factors can be divided into two main aspects: nature and characteristic of the ligand.

• The stability of complex compounds increases as the oxidation state of the central metal atom increases.
• The stability of complex compounds increases as the charge on the central metal atom increases.
• The stability of complex compounds increases as the charge density on the central metal atom increases.
• The stability of complex compounds increases as the electronegativity of the central metal atom increases.
• The stability of complex compounds increases as the strength of the ligand increases.
• Chelating ligands which form 5-6 membered ring complex compounds are far more stable than any other complex compound.

Nature of the Central Metal Atom:

Oxidation State:

Generally, the higher the charge on the metal ion, the greater the stability due to an increase in the strength of electrostatic interactions with ligands. For example, Fe³⁺ forms more stable complexes than Fe²⁺.

Size:

Smaller metal ions generally form more stable complexes. With a decrease in the ionic size, the metal ion can have a better grip over the ligands and hence increase in stability.

Character of the Ligands:

Basicity:

The stronger the ligand can easily donate electron pairs more are the stability of the complex that is formed. A case example water is a weaker ligand than ammonia.

Chelation:

The use of multidentate ligands, which can coordinate with the metal at several sites, enhances the stability by a great deal. Chelation is better exhibited in the five-membered rings where there is less strain and hence they are more stable.

Steric Effects:

Large ligands can hinder the incoming of other ligands to the metal. The geometry or the arrangement of atoms around the metal center also impacts the complex's stability.

Relevance and Applications

The factors in control of the stability of complexes are very broad phenomena that have implications in many ways, not only in academic research but in practical applications also. Thus, in biochemistry, the stability of metal ion complexes is an important parameter for explaining enzyme function and the varied ways by which metal ions are transported throughout the biological system. For example, the extent to which oxygen binds to hemoglobin depends on the stability of its iron-containing heme complex and gets modulated by the surrounding ligands.
The importance of metal complex stability in industrial chemistry is in the context of catalysis. In a chemical reaction where catalysts are put at work for increased efficiency in bringing about a chemical change, the requirement is that the metal-ligand complexes be stable. Information on how to improve the stability of a complex would thus apply during the optimization of newly designed catalysts.
Such knowledge also applies to environmental chemistry, mainly in metal ion remediation. For example, chelating agents may have a potential application in agriculture: Those compounds could help fishes to make the increased availability of vital nutrients and, at the same time, reduce toxic metal uptake. Similarly, gas storage and separation in metal-organic frameworks are controlled by the stability of metal-ligand interactions.

Recommended topic video on (Factors Affecting the Stability of Complexes)

Some Solved Examples

Example 1

The metal d-orbitals that are directly facing the ligands in $K_3\left[\mathrm{Co}(\mathrm{CN})_6\right]$ are:

1)$d_{x z}, d_{y z}$ and $d_{z^2}$

2)$d_{x y}$, and $d_{x^2-y^2}$

3) (correct)$d_{x^2-y^2}$ and $d_{z^2}$

4)$d_{x y}, d_{x z}$ and $d_{y z}$

Solution

In $K_3\left[\mathrm{Co}(\mathrm{CN})_6\right]$,

$\mathrm{Co}^{3+}$ has a configuration of $[A r] 3 d^6$.

The complex has octahedral geometry and hybridization

Six co-ordinated complexes have octahedral geometry and the $e_g$ orbitals have a greater energy as they are directly facing the approach of the ligands.

Thus, the $d_{x^2-y^2}$ and $d_{z^2}$ are directly facing the ligands.

Hence, the answer is the option (3).

Example 2

The complex ion that will lose its crystal field stabilization energy upon oxidation of its metal to +3 state is:

1)$\left[\mathrm{Co}(\text { phen })_3\right]^{2+}$

2)$\left[Z n(\text { phen })_3\right]^{2+}$

3) (correct)$\left[\mathrm{Fe}(\text { phen })_3\right]^{2+}$

4)$\left[N i(\text { phen })_3\right]^{2+}$

Solution

As we know, Phenanthrene is a bidentate chelating ligand and hence, it is a strong field ligand capable of causing pairing.

(i) $\left.\mathrm{Co}(\text { phen })_3\right]^{2+} \longrightarrow\left[\mathrm{Co}(\text { phen })_3\right]^{3+}$

$\mathrm{Co}^{2+}=3 d^7 \quad \mathrm{Co}^{3+}=3 d^6$

$C F S E=-1.8 \triangle_0 \quad C F S E=-2.4 \triangle_0$

In this case, the CFSE value increases.

(ii) $\left[\mathrm{Zn}(\text { phen })_3\right]^{2+} \rightarrow \quad\left[\mathrm{Zn}(\text { phen })_3\right]^{2+}$

$Z n^{2+}=3 d^{10} \quad Z n^{3+}=3 d^9$

$C F S E=0 \quad C F S E=-0.6 \triangle_0$

$\therefore C F S E$ value increases.

(iii) $\left[\mathrm{Fe}(\text { phen })_3\right]^{2+} \rightarrow \quad\left[\mathrm{Fe}(\text { phen })_3\right]^{3+}$

$F e^{2+}=3 d^6 \quad F e^{3+}=3 d^5$

$C F S E=-2.4 \triangle_0 \quad C F S E=-2.0 \triangle_0$

CFSE value decreases

(iv) $\left[\mathrm{Ni}(\text { phen })_3\right]^{2+} \rightarrow \quad\left[\mathrm{Ni}(\text { phen })_3\right]^{3+}$

$N i^{2+}=3 d^8 \quad N i^{3+}=3 d^7$

$C F S E=-1.2 \triangle_0 \quad C F S E=-1.8 \triangle_0$

CFSE value increase.

Therefore, option (3) is correct.

Example 3

The magnitude of crystal field stabilization energy in an octahedral field depends on:

(a) the nature of the ligand

(b) the charge on the metal ion

(c) whether the element is in the first, second, or third row of transition elements.

1)only (a) and (b) are correct

2) (correct)(a),(b) and (c) are correct

3)only (b) and (c) are correct

4)only (c) is correct

Solution

Factors affecting the CFT energy in the octahedral field depend on the nature of the ligand, the charge on the metal ion, and whether the element is in the first, second, or third row of transition elements.

Hence, the answer is the option (2).

Example 4

The magnitude of $\Delta_o$ in a group

1) (correct)Increases

2)Decreases

3)Remains same

4)None of the above

Solution

The magnitude of $\Delta_o$ increases on going down a group, i.e., an ion of an element in the first transition series has a smaller value of $\Delta_o$ than the ion of a heavier member of the same group.

Hence, the answer is the option (2).

Example 5

Which metal has the higher crystal field splitting ($\Delta$), with other factors remaining the same?

1)Co(I)

2)Co(II)

3)Co(III)

4) (correct)Co(V)

Solution

Factors Affecting CFSE -

1. Nature of central metal atom: As we move down the group, the CFSE increases. From 3d to 4d, there is a 30% increase in CFSE, and from 4d to 5d, there is a 50% increase in CFSE.
2. The oxidation state of the central metal atom: The oxidation state is directly proportional to CFSE. The CFSE of [Fe(CN)₆]^3- is greater than [Fe(CN)₆]^4-.
3. Nature of ligand: In the case of a strong field ligand, the magnitude of CFSE is high. In the case of a weak field ligand, the magnitude of CFSE is low.
4. Nature of complex: In an octahedral complex, 6 ligands approach the central metal atom and thus repulsion is higher, due to which the CFSE is higher. But, in a tetrahedral complex, 4 ligands approach the central metal atom and thus repulsion is lower, and thus the CFSE is lower.

The higher the Oxidation State of the metal, the greater the crystal field splitting.

Hence, the answer is the option (4).

Summary

The concept of stability of complexes is hence quite complex, with many other factors at stake like those of the metal atom and type of ligand. Truly, knowledge of factors like these shall help one in making proper predictions of coordination compounds' behavior under different chemical contexts.
The difference between thermodynamic and kinetic stability was considered with respect to how different metal ions and various ligands contributed to overall stability. Some basic considerations, such as charge, size, basic strength, and steric factors, were discussed in showing the role played by those factors in either improving or decreasing stability.