[Au(en)Cl-2]Cl center dot 2H(2)O, where en = ethylenediamine (1,2-diaminoethane), has been synthesized, and its structure has been solved for the first time by the single-crystal X-ray diffraction method. The complex has square-planar geometry about Au-III, and the anionic Cl- is located in the apical position and at a distance of 3.3033(10) A compared to 2.2811(9) and 2 2836(11) angstrom for the coordinated Cl-. [Au(en)Cl-2]Cl center dot 2H(2)O belongs to the space group Pbca with a = 11.5610(15) angstrom, b = 12.6399(17) angstrom, c = 13.2156(17) angstrom, alpha = beta = gamma = 90 degrees, and Z = 8. Bond lengths of Au-N are 2.03 angstrom. [Au(en)Cl-2]Cl center dot 2H(2)O is less thermally stable than [Au(en)(2)]Cl-3 because of the replacement of two Cl ligands by a second en ligand in the latter. Cyclic voltammetry shows that the formal potential of Au-III/Au-0 becomes more negative in the series [AuCl4](-), [Au(en)Cl-2](+), and [Au(en)(2)](3+). H-1, C-13, and P-31 NMR reveal that in an aqueous solution [Au(en)Cl-2](+) bonds to guanosine 5'-monophosphate, 5'-GMP (1:1 mole ratio), via N(7), although the stability is not very high. NMR data also indicate that N(7)-O(6) or N(7)-phosphate 5'-GMP chelation, as found in some gold(Ill) nucleotide complexes, is not present. The gold(111) complex undergoes hydrolysis at pH > 2.5-3.0 and, therefore, N1 coordination to 5'-GMP is not observed. No direct coordination between 5'-GMP and (Au(en)(2)]Cl-3 is observed.