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Coordination Compounds - Stability of coordination compounds

Grade 12ICSEChemistry

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

πŸ”‘Concepts

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

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Thermodynamic stability is a measure of the extent to which the complex will form or be transformed into another species at equilibrium. Kinetic stability refers to the speed at which the transformation takes place.

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The formation of a complex MLnML_n occurs in stepwise reactions, each characterized by a stepwise stability constant K1,K2,…,KnK_1, K_2, \dots, K_n.

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The overall stability constant, denoted by Ξ²n\beta_n, represents the equilibrium constant for the total reaction M+nLβ‡ŒMLnM + nL \rightleftharpoons ML_n.

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The instability constant or dissociation constant (KdK_d) is the reciprocal of the cumulative stability constant Ξ²n\beta_n.

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Factors affecting stability: 1. Charge on the central metal ion (higher charge β€…β€ŠβŸΉβ€…β€Š\implies higher stability). 2. Size of the metal ion (smaller size β€…β€ŠβŸΉβ€…β€Š\implies higher stability). 3. Nature of the ligand (stronger basicity and chelating ability increase stability).

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The Chelate Effect: Coordination compounds containing chelate rings (formed by multidentate ligands) are significantly more stable than complexes with unidentate ligands of similar nature.

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The Macrocyclic Effect: Complexes formed by cyclic multidentate ligands (like porphyrins) are even more stable than those with open-chain chelating ligands.

πŸ“Formulae

Kn=[MLn][MLnβˆ’1][L]K_n = \frac{[ML_n]}{[ML_{n-1}][L]}

Ξ²n=K1Γ—K2Γ—K3Γ—β‹―Γ—Kn\beta_n = K_1 \times K_2 \times K_3 \times \dots \times K_n

log⁑βn=log⁑K1+log⁑K2+log⁑K3+β‹―+log⁑Kn\log \beta_n = \log K_1 + \log K_2 + \log K_3 + \dots + \log K_n

Kd=1Ξ²nK_d = \frac{1}{\beta_n}

M+nLβ‡ŒMLnβ€…β€ŠβŸΉβ€…β€ŠΞ²n=[MLn][M][L]nM + nL \rightleftharpoons ML_n \implies \beta_n = \frac{[ML_n]}{[M][L]^n}

πŸ’‘Examples

Problem 1:

Calculate the overall stability constant β4\beta_4 for the complex [Cu(NH3)4]2+[Cu(NH_3)_4]^{2+} if the stepwise stability constants are log⁑K1=4.0\log K_1 = 4.0, log⁑K2=3.2\log K_2 = 3.2, log⁑K3=2.7\log K_3 = 2.7, and log⁑K4=2.0\log K_4 = 2.0.

Solution:

Given: log⁑K1=4.0\log K_1 = 4.0, log⁑K2=3.2\log K_2 = 3.2, log⁑K3=2.7\log K_3 = 2.7, log⁑K4=2.0\log K_4 = 2.0. Using the formula: log⁑β4=log⁑K1+log⁑K2+log⁑K3+log⁑K4\log \beta_4 = \log K_1 + \log K_2 + \log K_3 + \log K_4 log⁑β4=4.0+3.2+2.7+2.0=11.9\log \beta_4 = 4.0 + 3.2 + 2.7 + 2.0 = 11.9. Therefore, Ξ²4=1011.9β‰ˆ7.94Γ—1011\beta_4 = 10^{11.9} \approx 7.94 \times 10^{11}.

Explanation:

The overall stability constant is the product of individual stepwise constants. In logarithmic terms, these values are additive.

Problem 2:

Which of the following complexes is expected to be more stable and why? [Fe(C2O4)3]3βˆ’[Fe(C_2O_4)_3]^{3-} or [Fe(H2O)6]3+[Fe(H_2O)_6]^{3+}.

Solution:

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

Explanation:

The oxalate ion (C2O42βˆ’C_2O_4^{2-}) is a didentate (chelating) ligand. It forms five-membered rings with the central metal ion Fe3+Fe^{3+}. Due to the chelate effect, complexes with ring structures are significantly more stable than those with unidentate ligands like H2OH_2O.

Stability of coordination compounds Revision - Class 12 Chemistry ICSE