Reactions of the anticancer active compound cis-[Rh-2(DToIF)(2)(CH3CN)(6)](BF4)(2) with 9-ethyladenine (9-EtAdeH) or the dinucleoticle d(ApA) proceed with bridging adenine bases in the rare imino form (A*), spanning the Rh-Rh bond at equatorial positions via N7/N6. The inflection points for the pH-dependent H2 and H8 NMR resonance curves of cis-[Rh-2(DToIF)(2)(9-EtAdeH)(2)1(BF4)(2) correspond to N1 H deprotonation of the metal -stabilized rare imino tautomer, which takes place at pK(a) approximate to 7.5 in CD3CN-d(3), a considerably reduced value as compared to that of the imino form of 9-EtAdeH. Similarly, coordination of the metal atoms to the N7/N6 adenine sites in Rh-2(DTolFW{d(ApA)} induces formation of the rare imino tautomer of the bases with a concomitant substantial decrease in the basicity of the N1 H sites (pKa 7.0 in CD3CNd3), as compared to the imino form of the free dinucleotide. The presence of the adenine bases in the rare imino form, due to bidentate metalation of the N6/N7 sites, is further corroborated by DQF-COSY H2/N1 H and ROE N1H/N6H cross-peaks in the 2D NMR spectra of Rh-2(DToIF)(2){d(ApA)} in CD3CN-d(3) at -38 degrees C. Due to the N7/N6 bridging mode of the adenine bases in Rh-2(DToIF)(2){d(ApA)}, only the anti orientation of the imino tautomer is possible. The imino form A* of adenine in DNA may result in AT-CG transversions or AT-GC transitions, which can eventually lead to lethal mutations. The HH arrangement of the bases in Rh-2(DTolF)(2){d(ApA}1 is indicated by the H8/H8 NOE cross-peaks in the 2D ROESY NMR spectrum, whereas the formamidinate bridging groups dictate the presence of one right-handed conformer HH1 R in solution. Complete characterization of Rh-2(DTolF)(2){d(ApA)} by 2D NMR spectroscopy and molecular modeling supports the presence of the HH1 R conformer, anti orientation of both sugar residues about the glycosyl bonds, and N-type conformation for the 5'-A base.