| Summary: | The non-toxic inorganic antimicrobial agents iodine (I<sub>2</sub>) and copper (Cu) are interesting alternatives for biocidal applications. Iodine is broad-spectrum antimicrobial agent but its use is overshadowed by compound instability, uncontrolled iodine release and short-term effectiveness. These disadvantages can be reduced by forming complex-stabilized, polymeric polyiodides. In a facile, in-vitro synthesis we prepared the copper-pentaiodide complex [Cu(H<sub>2</sub>O)<sub>6</sub>(12-crown-4)<sub>5</sub>]I<sub>6</sub> ´ 2I<sub>2</sub>, investigated its structure and antimicrobial properties. The chemical structure of the compound has been verified. We used agar well and disc-diffusion method assays against nine microbial reference strains in comparison to common antibiotics. The stable complex revealed excellent inhibition zones against <i>C. albicans WDCM 00054</i>, and strong antibacterial activities against several pathogens. [Cu(H<sub>2</sub>O)<sub>6</sub>(12-crown-4)<sub>5</sub>]I<sub>6</sub> ´ 2I<sub>2</sub> is a strong antimicrobial agent with an interesting crystal structure consisting of complexes located on an inversion center and surrounded by six 12-crown-4 molecules forming a cationic substructure. The six 12-crown-4 molecules form hydrogen bonds with the central Cu(H<sub>2</sub>O)<sub>6 </sub>. The anionic substructure is a halogen bonded polymer which is formed by formal I<sub>5</sub><sup>− </sup>repetition units. The topology of this chain-type polyiodide is unique. The I<sub>5</sub><sup>− </sup>repetition units can be understood as a triodide anion connected to two iodine molecules.
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