This article describes the synthesis, characterization, and kinetic studies of a new class of stable ribonuclease (RNase) inhibitors. Herein the first synthesis of oxorhenium(V) complexes of diaminonucleosides and diaminoaldopentofuranoses is reported. Stability studies have shown that the complexes are stable between pH 3 and 13 at temperatures of 25-80 degrees C for 24 h with no occurrence of ligand exchange or epimerization at the Re=O core. Kinetic studies show that both the syn (1a) and anti (1b) diastereoisomers of oxorhenium(V) complexes of 9-(2’,3’diamino-2’,3’-dideoxy-beta-D-ribofuranosyl) (13) are excellent inhibitors of the purine specific ribonuclease, RNase U-2. Dixon and Lineweaver-Burke analyses suggest that the inhibition is competitive, with inhibitory constants (K(i)s) of 15 and 93 mu M, respectively. These complexes are of equivalent inhibitory potency to previously reported nucleoside vanadium alkoxide inhibitors with the added advantage of being completely stable in aqueous solution. The oxorhenium(V) complexes of 2,5-anhydro-3,4-diamino-3,4-dieoxy-D-ribitol (2a-Re) and (2b-Re) display much poorer inhibition (K-i > 2 mM), highlighting the critical nature of the purine base for successful inhibition.