A bulky platinum triamine complex, [Pt(Me 5 dien)(NO 3 )]NO 3 (Me 5 dien=N,N,N′,N′,N′′-pentamethyldiethylenetriamine) has been prepared and reacted in D 2 O with N-acetylmethionine (N-AcMet) and guanosine 5′-monophosphate (5′-GMP); the reactions have been studied using 1 H NMR spectroscopy. Reaction with 5′-GMP leads to two rotamers of [Pt(Me 5 dien)(5′-GMP-N7)] + . Reaction with N-AcMet leads to formation of [Pt(Me 5 dien)(N-AcMet-S)] + . When a sample with equimolar mixtures of [Pt(Me 5 dien)(D 2 O)] 2+ , 5′-GMP, and N-AcMet was prepared, [Pt(Me 5 dien)(5′-GMP-N7)] + was the dominant product observed throughout the reaction. This selectivity is the opposite of that observed for a similar reaction of [Pt(dien)(D 2 O)] 2+ with 5′-GMP and N-AcMet. To our knowledge, this is the first report of a platinum(II) triamine complex that reacts substantially faster with 5′-GMP than with N-AcMet; the effect is most likely due to steric clashes between the methyl groups of the Me 5 dien ligand and the N-AcMet.