Coupling Constants

Hi,

This may be a basic question but I am stuck on this and I cant find any help on the net about it, possbily looking for the wrong thing.

So I have a molecule its Sqaure Planar for example:

[Pt(CO)(Cl)(PPh3)2].

I need to produce a splitting tree & diagram of what the NMR will look like for P.
So In Cis form the two P's are not equal so produce a two signals. There is no coupling to Cl but is to Pt & CO.
Pa will couple to Pt which will have the largest J valueas it is a J1(one bond way.

But this is where I get stuck. It couples to both C & P but which one has the highest J value, what is the way of estimating this?? They both will have cis relationshiip the Pa.
Is there a way to work it out. I know they splt to form doublets, but they are not that same atoms so the split cant be equal.

Thanks