The reaction of [Pt(hpip)Cl-2] (hpip = homopiperazine = 1,4-diazacycloheptane) with d(GpG) yields two apparently isomeric products, separable by HPLC. These have been characterized by a combination of 2D NMR and molecular mechanics modeling. NOESY correlations between the H8 protons show that both products are head-to-head isomers, and NOESY correlations between the d(GpG) dinucleotide and the diamine ligand show that the difference between the isomers lies in the orientation of the two and three carbon chains of the hpip ligand with respect to the heads (H8 protons) of the guanine bases. Molecular mechanics calculations yield total energies that are consistent with the observation of the two isomers in approximately equal amounts.