Pt[BrCl(PR₃)₂] cis, trans isomer nmr - Which is cis and which is trans?
Look at the two structures drawn below



In the trans-isomer each phosphorous atom is in the exact same chemical environment.  Each one has a chlorine on one side, a bromine on the other side and a phosphorous diagonally across.  Therefore only one signal is observed for the phosphorous.

In the cis-isomer, each phosphorous is in a different chemical environment.  One phosphorous has a chlorine diagonally across from it, the other has a bromine diagonally across from it.  In this case we observe two phosphorous signals, one for each of the two different phosphorous environments.