Complete three-dimensional X-ray crystal structure analyses have been carried out on both compounds by identical experimental and computational methods. Observations were collected a t 1 20" f 5OK and the intensities corrected for absorption. Hydrogen atoms were not located. After anisotropic refinement by least-squares, R is 0-091 for the cis-isomer and 0.077 for the trans-isomer. The bond lengths, corrected for rotational oscillation are Pt-CI = 2.33 rt 0.01 A and Pt-N = 2.01 f 0.04 A in the cis-compound, and Pt-CI = 2.32 f 0.01 A and Pt-N = 2.05 * 0.04 A in the trans-compound. The only angle significantly different from 90' is CI-Pt-CI = 91 -9" f 0.4' in the cis-co m p o u n d .The crystals 0f.ci.s-dichlorodiammineplatinum are triclinic with two molecules in a unit cell having a = 6.75, b = 6.55, c =6.23 A, u = 92.2". p = 84.6", and y = 11 0.7", and space group PT. The molecules lie approximately parallel to (100) a t x/a -& and -$, with platinum-platinum separations alternately 3.372 and 3.409 A perpendicular to the molecular planes. At low temperature the structure of the trans-isomer is essentially the same as that a t room temperature reported by Porai-Koshits.WE have undertaken the crystal structure analyses of cis-and trans-dichlorodiammineplatinum(II) , to obtain accurate and strictly comparable bond lengths. The same experimental procedures were used with both compounds; the crystals were cooled to 1 2 0 "~, and the observed intensities were corrected for absorption. Both were refined ultimately by full matrix leastsquares, including anisotropic vibration parameters, and the final co-ordinates were corrected for librational errors.The structure of the cis-isomer had not previously been determined, probably because of the difficulty in obtaining a suitable single crystal. It is triclinic with two molecules, related by a centre of symmetry, in the 1 M. Atoji, J. W. Richardson, and R. E. Rundle, J . Amer.Chem. SOL, 1957, 79, 3017. unit cell. As shown in Figures 1 and 2, the planar molecules are parallel to one another and stacked along the a-axis. Many of the crystals examined showed twinning or disorder in the bc-plane. The Pt Pt distances along the stack are alternately 3.372 and 3.409 &0.002 A, which may be short enough to indicate an interaction. The crystals are not pleochroic. In Magnus Green Salt, [Pt(NH3)4]2+[PtC14]2-, there are stacks of parallel alternate anions and cations with a Pt-Pt separation of 3.25 and its dichroism2has been attributed to platinum-platinum interaction.