In some occasions, if the ligand acts on Pd it do so as a monodentate ligand. I am attempting to explain this phenomenon but came into a block.
So basically, you're comparing a soft base and a hard base. In HSAB theory, a soft acid like Pd2+, would preferably react with a soft base like R-PH2. Soft acids and bases are easily polarized because of their size. Although, the possibility of the ligand acting as a bidentate can't be ignored, the P to Pd2+ interaction is thermodynamically favored than N to Pd2+ interaction.