The rates of exchange of CO on cis square-planar Ir complexes have been observed for the...

The rates of exchange of CO on cis square-planar Ir complexes have been observed for the following reaction at 298 K:

The observed rate constants were:

All three reactions have large negative values of entropy of activation

a. Is this reaction associative or dissociative?

b. On the basis of the data, which halide ligand exerts the strongest trans effect? (See R. Churlaud, U. Frey, F. Metz, A. E. Merbach, Inorg. Chem ., 2000 , 39 , 304.)