195Pt and 15N NMR Data in Square Planar Platinum(II) Complexes of the Type [Pt(NH3)aXb]n(Xb= Combination of Halides): "NMR Effective Molecular Radius" of Coordinated Ammonia

By studying a model set of square-planar [Pt(NH3)(a)X-b](n)(a+b= 4; X-b= combination of b halido ligands;n= 2 - b) complexes, we found that their delta(Pt-195) NMR chemical shift decreases proportionally to the platinum bonded halido ligands' ionic radii overall sum. This confirms also for these systems, the already observed NMR shielding attributed topseudoring currents, circulating around the M-X bond axis. Moreover, the present data show that also the NH(3)ligands are characterized by a constant NMR shielding ability toward the central metal. This could be rationalized in term of a "NMR effective molecular radius" of the NH(3)ligand, affecting the observed delta(Pt-195) as previously found for halido ligands. Interestingly, a delta(N-15) decrease is observed in Pt bonded NH(3)ligands if the ionic radius of acishalido ligand is increased. The opposite occurs if the ionic radius of atranshalido ligand is increased. The two contrasting effects stem from both shielding electric ring currents affecting thecisligands and prevailingtrans-influence due to coordinated halido ligands.