Simplest Homoleptic Metal-Centered Tetrahedrons, [M(OH<sub>2</sub>)<sub>4</sub>]<sup>2+</sup>, in 1-Ethyl-3-methylimidazolium Tetrafluoroborate Ionic Liquid (M = Co, Ni, Cu)

Dissolution of a tetrafluoroborate or perchlorate salt of [M­(OH<sub>2</sub>)<sub>6</sub>]<sup>2+</sup> (M = Co, Ni, Cu) in 1-ethyl-3-methylimidazolium tetraluforoborate ionic liquid ([emim]­BF<sub>4</sub>) results in significant solvatochromism and increasing intensity of color. These observations arise from partial dehydration from the octahedral [M­(OH<sub>2</sub>)<sub>6</sub>]<sup>2+</sup> and formation of the tetrahedral [M­(OH<sub>2</sub>)<sub>4</sub>]<sup>2+</sup>. This reaction was monitored by the intense absorption band due to the d–d transition in the UV–vis absorption spectrum. The EXAFS investigation clarified the coordination structures around M<sup>2+</sup> {[Co­(OH<sub>2</sub>)<sub>4</sub>]<sup>2+</sup>, <i>R</i>(Co–O) = 2.17 Å, <i>N</i> = 4.2; [Cu­(OH<sub>2</sub>)<sub>4</sub>]<sup>2+</sup>, <i>R</i>(Cu–O) = 2.09 Å, <i>N</i> = 3.8}. <sup>1</sup>H and <sup>19</sup>F NMR study suggested that both [emim]<sup>+</sup> and BF<sub>4</sub><sup>–</sup> are randomly arranged in the second-coordination sphere of [M­(OH<sub>2</sub>)<sub>4</sub>]<sup>2+</sup>.