Herein　we　report　on　the　facile　and　large-scale　syntheses　and　characterizations　of　a　series　of　transition　metal-containing　ionic　liquids　(MILs),　namely　[C12MMIm]n[MCl4]　(M=Fe3+,　n=1;　M=Cu2+,　Co2+,　Zn2+,　Cd2+,　n=2;　C12MMIm=1-decyl-2,3-dimethylimidazolium)　by　simply　stoichiometrically　mixing　metal　chlorides　and　the　IL　[C12MMIm]Cl.　All　of　the　compounds　could　be　obtained　in　crystalline　state　at　room　temperature　in　nearly　100%　yield.　Single-crystal　X-ray　diffraction　analysis　reveals　that　all　the　MIL　crystals　belong　to　the　space　group　of　P21/c,　consisting　of　[C12MMIm]+　cations　and　halometallate　anions　that　interact　with　each　other　giving　rise　to　supramolecular　layered　structures　via　C−H⋅⋅⋅Cl　hydrogen　bonding　and　anion-π　interactions.　Thermal　properties　of　the　five　compounds　were　studied　by　differential　scanning　calorimetry　(DSC)　and　thermogravimetry　(TG),　revealing　reversible　phase　transitions　near　room　temperature.　Interestingly,　these　MILs　are　air-　and　moisture-stable　when　stored　in　ambient　conditions.　The　[C12MMIm]n[MCl4]　(M=Cu2+,　Co2+　and　Fe3+)　crystals　exhibited　obvious　colour-change　when　exposed　to　ammonia　(NH3)　gas,　indicating　potential　applications　in　ammonia　detection.　©　2018　Wiley-VCH　Verlag　GmbH　&　Co.　KGaA,　Weinheim