Two-electron reduction occurs when the Re(V) precursors ReOX3(PPh3)(2) and ReO(OEt)X-2(PPh3)(2) are reacted with biimidazole (biimH(2)) in boiling chloroform, affording rhenium(III) cationic complexes of the type cis,trans-[ReX2(PPh3)(2)(biimH(2))]X with X = Cl, Br, and I. Crystal structures are determined for the compounds with the three halogens, as well as for the [ReCl2(PPh3)(2)(biimH(2))](benzoate) salt. In all cases, the counterion is attached to the complex cation via hydrogen bonding with the N-H groups of coordinated biimidazole. Variable-temperature H-1 NMR spectroscopy shows that a mixture of [ReCl2(PPh3)(2)(biimH(2))](benzoate) and [ReCl2(PPh3)(2)(biimH(2))]Cl is in slow exchange below -50 degreesC in CD2Cl2, indicating that ion pairing is retained in solution. Both N-H groups can be deprotonated with sodium methoxide, and their acidities are evaluated from UV-visible spectra. Competition between monodeprotonated [ReCl2(PPh3)(2)(biimH)] and various carboxylic acids reveals that the acidity of the first N-H proton corresponds to that of acetic acid (pK(a)(aq) similar to 4.8). By a similar competitive reaction between bis-deprotonated [ReCl2(PPh3)(2)(biim)](-) and phenols, the second acidity is estimated to be close to that of phenol (pK(a)(aq) similar to 9.8).