Trends in successive formation constants:

Normally, we will see for a series of stepwise constants the numerical order: 

K₁ > K₂> K₃ > …..> K_n

As a reaction proceeds fewer ligands are available in solution to coordinate, so this order is further enhanced. However, sometimes one of the higher-order constants is found to be significantly greater than an earlier one. This usually means that some significant structural change has occurred, such as the transition from high-spin to low-spin configurations. This occurs, for example in the step: 

The constant for this step is much greater than for the first two ligand-attachment reactions,accompanying the change from high spin d⁶ to low-spin d⁶. Another example is the conversion of Cd²⁺ to [CdBr₄]^2-, involving the change from octahedral to tetrahedral geometry. =>See Previous Post Also