A series of six long chain alkyl mono- and bi-cationic imidazolium based salts with bis(trifluoromethylsulfonyl)imide (NTf2(-)) as the anion were synthesized and characterized. The single crystal structure of 1-methyl-3-octadecylimidazolium bis(trifluoromethylsulfonyl)imide could be obtained by X-ray analysis. All these long chain alkyl imidazolium based ILs were applied in the synthesis of nickel nanoparticles via chemical decomposition of an organometallic precursor of nickel. In these media, spontaneous decomposition of Ni(COD)2 (COD = 1,5-cyclooctadiene) in the absence of H2 occurred giving small NPs (≤4 nm) with narrow size distributions. Interestingly, formation of regularly interspaced NP arrays was also observed in long chain ILs. Such array formation could be interesting for potential applications such as carbon nanotube growth.