Why does Pb(II) promote ligand substitution?

I have heard of an article where divalent lead $\ce{PbL2}$ ($\ce{L}$ is ligand) reacts with something (again not sure, as I dont know the article) and what it reacts with gains a ligand.


In less confusing words:


$\ce{PbL2 + MXn}$ ($\ce{X}$ is another ligand, not necessarily halogen) ==> $\ce{PbL(n-1)X + MX(n-1)L}$
Or something similar. That is what I got from hearing about the article. I was wondering if you guys can share your thoughts as to why happens?


