Yay! I know....

how to apply dissociative ligand substitution reaction...

first... rates of reaction is how much of something is consumed/produced in a certain range of time...

Given ML5X + Y --> ML5Y + X (overall reaction)

Proposed mechanism :
ML5X --> ML5 + X (k1); ML5 + X --> ML5X (k-1)
ML5 + Y --> ML5Y (k2)

ML5 is a intermediate state and should be gotten rid
so we assume steady-state approximation which means production rate=consumption rate (intermediate)

d[ML5]/dt=0 =k1[ML5X]-k-1[ML5][X]-k2[ML5][Y]

Then [ML5]=k1[ML5X]/(k-1[X]-k2[Y])

Overall rate of product formation, d[ML5Y]/dt = k2[ML5][Y] = k2[Y]k1[ML5X]/(k-1[X]-k2[Y])
so then if concentration of k2[Y] is very large, k-1[X] is negligible, thus overall rate of product formation does not depend on concentration of Y as it has cancelled out.

So now, I believe I have understood this part of Chemistry... hope it sticks in my brains... or at least in my blog... :P