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Treatment of the anionic Ru III precursor X[trans-RuCl 4 (DMSO-S) 2 ] (X = protonated DMSO, Na + , NH 4 + ) and acyclovir (acv) in various solvents yields two new products mer-[RuCl 3 (acv)(DMSO-S)(CH 3 OH)]·0.5CH 3 OH (1) and mer-The antitumor properties of simple ruthenium(III) coordination compounds were discovered over 30 years ago, [1] and since then several tumor-inhibiting ruthenium complexes have been isolated and characterized, [2] with DMSOruthenium-N donor ligand ternary complexes being amongst the most promising. The typical example of the group, NAMI-A, [ImH] [trans-RuCl 4 (DMSO-S)Im] (Im ϭ imidazole), has already entered clinical phase I trials. [3] The main advantages of ruthenium-DMSO complexes are the selectivity for solid tumor metastases and the lack of significant host toxicity.Acyclovir [9-(2-hydroxyethoxymethyl)guanine, acv] is a well-known antiviral drug used in clinical practice. The crystal structures of acv complexes with Cu, [4] Pt, [5] Ni, Co, Cd and Zn, [6] have appeared in the literature. It was found that the coordination of acv to the metal occurs in most cases through N(7) only, whereas O(6) is not directly involved in the bonding. The sole exception is [Cu(acv) 2 (H 2 O) 2 ](NO 3 ) 2 , in which a pseudo-chelate N(7)/ O(6) bonding of a purine to the metal was found. [4c] Recently, the interactions of two Ru II -DMSO precursors, cis-and trans-[RuCl 2 (DMSO) 4 ], with acv were studied by NMR spectroscopy in aqueous solution. The authors proposed the formation of mono and bis adducts with N(7) as the preferred binding site. [7] Our basic idea was to prepare an NAMI-A type compound of acv with the formula [trans-RuCl 4 (acv)(DMSO-S)] Ϫ . The main chemical feature of the Ru III anionic precursor of NAMI-A type complexes, X[trans-RuCl 4 (DMSO-S) 2 ] (X ϭ protonated DMSO, Na ϩ , NH 4 ϩ ), is the facile dissociation of one of the two trans S-bonded DMSOs, at- [a]

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