Complex formation of nickel(ii) with dimethyl sulfoxide, methanol, and acetonitrile in a TFSA−-based ionic liquid of [C2mim][TFSA]Electronic supplementary information (ESI) available: Spectra of n for [Ni(ml)n] complexes, overall stability constants, log βn for [Ni(ml)n] at 30.0, 35.0, 40.0, and 45.0 °C, and van't Hoff plots for stepwise stability constants for [Ni(ml)n]. CCDC 1427034-1427036. For ESI and crystallographic data in CIF or another electronic format see DOI: 10.1039/c7cp06469a

The thermodynamics of complex formation of Ni 2+ with molecular liquids (ML), dimethyl sulfoxide (DMSO), methanol (MeOH), and acetonitrile (AN) in the ionic liquid (IL) of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([C 2 mim][TFSA]) has been elucidated using ultraviolet (UV)-visible spectroscopy. X-ray structural analyses for single crystals grown from Ni 2+ -[C 2 mim][TFSA]-DMSO and −AN solutions at high ML contents have shown that six DMSO oxygen or AN nitrogen atoms coordinate with Ni 2+ to form octahedral structures of [Ni(dmso) 6 ](TFSA) 2 and [Ni(an) 6 ](TFSA) 2 , respectively. This is the same in the case of the Co 2+ complex of [Co(dmso) 6 ](TFSA) 2 . UV-visible spectroscopic experiments have revealed that the TFSA − anions that initially combine with Ni 2+ in the IL are replaced with ML molecules in the IL-ML systems in three steps with increasing ML content. The electron donicities of the three MLs are larger in the order of DMSO > MeOH > AN. However, the stability of each complex does not simply depend on this order; the stability is higher in the order of [Ni(dmso) n ] > [Ni(an) n ] > [Ni(meoh) n ]. In other words, the stability of the MeOH complexes is lower than that of the AN ones, despite the higher electron donicity of MeOH. The reasons for the order of the complex stabilities have been interpreted on the molecular scale, according to the stepwise enthalpies and entropies determined, together with the strength of the hydrogen bonding between the MLs and the imidazolium ring and the formation of MeOH clusters in [C 2 mim][TFSA]. When methanol molecules coordinate with Ni 2+ , the hydrogen bonds among them and those with the imidazolium ring are disrupted.