Reaction of truns-[a2PtL212+ (a = NH3 or CH3NH2; L = 1-methylcytosine, CSHTNJO, or l,S-dimethylcytosine, CsH9N3O) with HgO in water yields novel heteronuclear Pt-Hg compounds containing two bridging cytosyl ligands L-, binding to Pt via N3 and to Hg via the deprotonated exocyclic amino group N4. Three representative examples have been characterized by X-ray crystallography: transrs-[(CH3NH2)2Pt( 1-MeC-)2Hg](NOs)2 (ld), truns-[(CH3NH2)2Pt(l,5-DimeC-)2Hg](N03)~0.5H20 (M), and truns-[(CH3NH2)2Pt(l-MeC-)2Hg]Cl(NO3) (le). Crystal data are as follows: Id, triclinic system, space group Pi, u = 10.157(4) A, b = 10.494(4) A, c = 11.452(4) A, cy = 101.33(2)O, B = 102.44(2)O, y = 104.65(2)O, Y = 11 11.8(7) A3,Z = 2; le, monoclinic system, space group a / m , u = 13.876(4) A, b = 16.254(3) A, c = 10.358(3) A, p = 112.02(1)O, V = 2166(1) A3, Z = 4; M, monoclinic system, space group n l / n , u = 8.614(3) A, b = 20.610(6) A, c = 13.984(5) A, fl = 97.67(2)O, V = 2461(2) A3, Z = 4. In all three compounds, Pt displays a square-pyramidal coordination sphere, with Hg in the apical position.Hg has severely distorted coordinations with two short Hg-N(4) distances, a Hg-Pt contact, and longer Hg-03Ndistances in Id (distorted octadedron) and 2d (distorted bipyramid), whereas in l e the coordination around Hg is completed by C1-(distorted bipyramid). Pt-Hg separations are 2.785(1) A (ld), 2.765(1) A (2d), and 2.835(1) A (le), and interactions are interpreted in terms of weak Pt-Hg bonds. The I95Pt NMR spectra of the Pt-Hg compounds display 199Hg satellites due to IJ coupling, consistent with Pt-Hg bonding. lH NMR spectra of the various Pt-Hg compounds show 195Pt and 199Hg coupling, as determined by decoupling and spectral editing techniques.Unlike related Pt-Pt compounds, the mixed-metal Pt-Hg species, when reacted with a large variety of N and S donor ligands X, tend to decompose with formation of trans-[a2PtL2l2+ and HgX2. IntroductiooMetal-metal bonding in heteronuclear complexes containing both d* and d10 metal ions has been known for some 25 yeam2s3 A rich chemistry has been established for such species, in particular for the following combinations: Irl and Au1,4 Irl and Ag1,'ls6 P t l l and Hg11,7-9 P t l I and Ag1,lO-I3 and P t I I and Au1,14Js as well as Rhl

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Nucleobase rotation about bonds with transition metal ions has been intensively studied for purine complexes of d s-a2Pt(II) (with a = N H 3 or amine) [1][2][3][4][5][6][7][8][9]. Results obtained from these studies are considered meaningful with regard to the second step (chelate closure) of reactions of the antitum or agent d s-a2PtCl2 with DNA. Favorable or unfa vorable interactions between the am(m)ine li gands at the platinum atom and exocyclic groups of the nucleobases in the vicinity of the metal co ordination site are crucial as far as rotational b ar riers are concerned [5,7,8]. Although this aspect has been studied in detail for /V7-platinated p u rines only, it is of even greater significance to N3-platinated pyrimidine nucleobases. Qualitatively, we have dem onstrated in many instances that binding of a second metal ion to a TVJ-platinated pyrimidine base (uracil, thymine, cytosine) has a We have now applied the very same principle to a related system, namely to fra«s-[a2Pt(l-M eC -N3)2]2+ (1-MeC = 1-methylcytosine) for the following reasons: First, the aspect of nucleobase rotation has hardly been studied in complexes of trans-a2Pt(II) [13]; second, we have recently been able to characterize a series of heteronuclear com plexes of this compound with the two (deproto nated) nucleobases arranged head-head [14][15][16]; third, we found that removal of the heterom etal by a suitable nucleophile can be sufficiently fast, and subsequent ligand rotation sufficiently slow, to permit crystallization of the minor rotam er (headhead) of rra/7s-[a2Pt(l-MeC-/V3)2]2+ from aqueous solution. To the best of our knowledge, this is the

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