Rapid rotation of guanine base derivatives about Pt-N7 bonds results in fluxional behavior of models of the key DNA intrastrand G-G cross-link leading to anticancer activity of Pt(II) drugs (G = deoxyguanosine). This behavior impedes the characterization of LPtG(2) models (L = one bidentate or two cis-unidentate carrier ligands; G = guanine derivative not linked by a phosphodiester group). We have examined the formation of LPtG(2) adducts with G = 5'- and 3'-GMP and L = sp(2) N-donor bidentate carrier ligands [5,5'-dimethyl-2,2'-bipyridine (5,5'-Me(2)bipy), 3-(4'-methylpyridin-2'-yl)-5,6-dimethyl-1,2,4-triazine) (MepyMe(2)t), and bis-3,3'-(5,6-dialkyl-1,2,4-triazine)R(4)dt)]. NMR spectroscopy provided conclusive evidence that these LPt(5'-GMP)(2) complexes exist as interconverting mixtures of head-to-tail (HT) and head-to-head (HH) conformers. For a given G, the rates of G base rotation about the Pt-N7 bonds of LPtG2 models decrease in the order Me(4)dt > Et(4)dt > MepyMe(2)t > 5,5'-Me(2)bipy. This order reveals that the pyridyl ring C6 atom + H atom grouping is large enough to impede the rotation, but the equivalently placed triazine ring N atom + N lone pair grouping is sterically less impeding. For the first time, the two possible HH conformers (HHa and HHb) in the case of an unsymmetrical L have been identified in our study of (MepyMe(2)t)Pt(5'-GMP)(2). Although O6-O6 clashes involving the two cis G bases favor the HT over the HH arrangement for most LPtG(2)-type complexes, the HH conformer of (R4dt)Pt(5'-GMP)(2) adducts has a high abundance (similar to 50%). We attribute this high abundance to a reduction in O6-O6 steric clashes permitted by the overall low steric effects of R(4)dt ligands. Under the reaction conditions used, 3'-GMP forms a higher abundance of the LPt(GMP)(2) adduct than does 5'-GMP, a result attributable to more favorable second-sphere communication in the LPt(3'-GMP)(2) adduct than in the LPt(5'-GMP)(2) adduct.