[BrockaFloka]

Here's the question:

This graph



shows the observed first-order rate constants for the reaction of [PdBrL]+ with various Y- to give [PdYL]+ where L is Et2NCH2CH2NHCH2CH2NEt2. Propose a mechanism.


So far I am assuming that the mechanism is Dissociative because the rate has very little to do with the incoming ligands' concentration. But the exception to this is the S2O3 2- ion, and I cannot figure out why. I know the metal center is fairly sterically crowded, so could it be the size of this ion compared to the others?

Any insight would  be appreciated.

[BrockaFloka]

is there possibly a nucleophilic difference between the ligands?