9.6 Bonding in Metal Carbonyls 23
9.7 Stability of Coordination Compounds In general, the coordination complexes are highly stable complexes, where the stability arises from the interaction between the metal ion and the ligands. The stronger the interaction, higher will be the stability of the complex. Now considering the general reaction for the formation of the coordination complex, Ma+ + nLb− → [ML n ]c + where a+, b- and c+ are the respective charges on the metal, ligand and complex. The stability constant is represented as K =
[(ML n )c + ] [Ma+ ][Lb− ]n
The reaction may proceed through various steps: M + L → ML ML + L → ML 2
[ML] [M][L] [ML 2 ] K2 = [ML][L] K1 =
ML n−1 + L → ML n K n =
[ML n ] [ML n − 1][L]
In these steps of the reaction, K1, K2, … are referred to as “stepwise stability constants” and overall stability constant is given by K = K1 × K 2 × K 3 K n Now, let us again consider the formation of the complex MLn by the following pathway [M(H2O)n ] + nL → [ML n ] + H2O Here, the overall stability constant is written as ML H O β n = n 2 n M(H2O )n ][L Since concentration of water is high, it can be considered constant and the stability constant can be written as
βn =
ML n n M(H2O )n L
This reaction actually proceeds in steps and can be written as [M(H2O)n ] + L → [M(H2O)n−1L] + H2O
K1 =
[M(H2O)n−1L] [M(H2O)n ][L]
[M(H2O)n−1L] + L → [M(H2O)n−2L2 ] + H2O
K2 =
[M(H2O)n−2L 2 ]2 [M(H2O)n−1L][L]
Kn =
[ML n ] [M(H2O)L n−1][L]
[M(H2O)L n−1] + L → [ML n ] + H2O
The overall stability constant can be related to the stepwise stability constants by the relation:
βn = K1 × K 2 × K 3 K n
Chap_09.indd 23
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