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Coordination Compounds - Stability of Coordination Compounds

Grade 12CBSEChemistry

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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The stability of a complex in solution refers to the degree of association between the metal ion and the ligands involved in the state of equilibrium. It is expressed in terms of the stability constant (KK).

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Thermodynamic stability deals with the bond energy and stability of the species at equilibrium, while kinetic stability deals with the speed with which the complex undergoes transformation.

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Formation of a complex occurs in stepwise reactions. Each step has its own stability constant, denoted as K1,K2,K3,...KnK_1, K_2, K_3, ... K_n.

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The overall stability constant, denoted by βn\beta_n, is the product of the stepwise stability constants: βn=K1×K2×K3×...×Kn\beta_n = K_1 \times K_2 \times K_3 \times ... \times K_n.

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The instability constant or dissociation constant (KiK_i) is the reciprocal of the cumulative stability constant: Ki=1βnK_i = \frac{1}{\beta_n}.

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Factors affecting stability: 1. Charge on the central metal ion (Higher charge →\rightarrow Higher stability). 2. Size of the metal ion (Smaller size →\rightarrow Higher stability). 3. Nature of the ligand (Stronger basic character →\rightarrow Higher stability).

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The Chelate Effect: Coordination of a metal ion with a polydentate ligand to form a ring structure results in a much more stable complex than a similar complex with monodentate ligands.

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Macrocyclic Effect: Multidentate ligands which are cyclic (like porphyrin) form even more stable complexes than linear multidentate ligands.

📐Formulae

M+L⇌ML;K1=[ML][M][L]M + L \rightleftharpoons ML; K_1 = \frac{[ML]}{[M][L]}

βn=K1×K2×K3×...×Kn\beta_n = K_1 \times K_2 \times K_3 \times ... \times K_n

log⁡βn=log⁡K1+log⁡K2+...+log⁡Kn\log \beta_n = \log K_1 + \log K_2 + ... + \log K_n

Kinst=1βnK_{inst} = \frac{1}{\beta_n}

M+nL⇌MLn;βn=[MLn][M][L]nM + nL \rightleftharpoons ML_n; \beta_n = \frac{[ML_n]}{[M][L]^n}

💡Examples

Problem 1:

Calculate the overall complex dissociation equilibrium constant for the [Cu(NH3)4]2+[Cu(NH_3)_4]^{2+} ion, given that β4\beta_4 for this complex is 2.1×10132.1 \times 10^{13}.

Solution:

The overall stability constant β4=2.1×1013\beta_4 = 2.1 \times 10^{13}. The dissociation constant (KiK_i) is the reciprocal of the stability constant. Ki=1β4K_i = \frac{1}{\beta_4} Ki=12.1×1013≈4.7×10−14K_i = \frac{1}{2.1 \times 10^{13}} \approx 4.7 \times 10^{-14}

Explanation:

The instability constant represents the equilibrium constant for the reverse reaction (dissociation of the complex). Higher βn\beta_n implies a lower KiK_i, indicating a very stable complex.

Problem 2:

Between [Fe(C2O4)3]3−[Fe(C_2O_4)_3]^{3-} and [Fe(H2O)6]3+[Fe(H_2O)_6]^{3+}, which complex is more stable and why?

Solution:

[Fe(C2O4)3]3−[Fe(C_2O_4)_3]^{3-} is more stable than [Fe(H2O)6]3+[Fe(H_2O)_6]^{3+}.

Explanation:

This is due to the Chelate Effect. The oxalate ion (C2O42−C_2O_4^{2-}) is a bidentate ligand that forms five-membered rings with the central metal ion Fe3+Fe^{3+}. Complexes involving ring formation are thermodynamically more stable than those formed with monodentate ligands like H2OH_2O.

Problem 3:

The stepwise stability constants for a complex ML2ML_2 are log⁡K1=4.0\log K_1 = 4.0 and log⁡K2=3.0\log K_2 = 3.0. Calculate the overall stability constant β2\beta_2.

Solution:

We know the relationship: log⁡β2=log⁡K1+log⁡K2\log \beta_2 = \log K_1 + \log K_2 log⁡β2=4.0+3.0=7.0\log \beta_2 = 4.0 + 3.0 = 7.0 β2=107\beta_2 = 10^7

Explanation:

The overall stability constant is the product of stepwise constants. In logarithmic form, they are additive.