Note that these complexes are hardly formed in aqueous solu-Complexation of the cadmium(II) ion with thiocyanate and iodide tion, and interestingly their formation in micelles is associated ions have been studied by precise titration calorimetry in micellar with a relatively large and exothermic heat. With nickel(II) solutions of a nonionic surfactant Triton X-100 containing 0.4 mol and copper(II) ions, however, such complexes are not formed dm 03 NaClO 4 as a constant ionic medium at 298 K. With regard in micelles. to the Cd(II) -SCN 0 system, the formation of [Cd(NCS)] / and In order to obtain further insight into the specific interaction [Cd(NCS)(SCN)] in aqueous solution and [Cd(NCN) 2 (SCN)] 0 of metal complexes with a nonionic surfactant, we examined in micelles was established, though the complexation in micelles the complexation behavior of other types of metal and ligand is not extensive. With the Cd(II) -I 0 system, the formation of ions. Here we report the Cd(II) -SCN 0 and Cd(II) -I 0 sys-[CdI] / , [CdI 3 ] 0 , and [CdI 4 ] 20 was established in aqueous solution together with the formation of [CdI 2 ], [CdI 3 ] 0 , and [CdI 4 ] 20 tems in a nonionic surfactant Triton X-100 solution. It has in micelles. It is revealed that the complexation in micelles is been established that the thiocyanate ion binds to a rather soft accompanied by relatively large and negative enthalpies, which is cadmium(II) ion through either the N or S end to successively ascribed to the enthalpies of transfer of [CdI 3 ] 0 and [CdI 4 ] 20 form [Cd(NCS)] / , [Cd(NCS)(SCN)], [Cd(NCS) 2 (SCN)] 0 , from aqueous solution to micelles. This suggests that a hydrophoand [Cd(NCS) 2 (SCN) 2 ] 20 in an aqueous solution (14), bic complex with no capacity to form strong hydrogen bonds in where the [Cd(NCS) 2 (SCN) 2 ] 20 complex, for instance, inwater is favored in micelles. α§ 1997 Academic Press volves two Cd-N and two Cd-S bonds. The coordination Key Words: complexation; cadmium(II) ions; thiocyanate ions; geometry of the cadmium(II) ion, an octahedral six-coordiiodide ions; nonionic surfactant; Triton X-100; calorimetry.
nation when fully hydrated, changes to tetrahedral four-coordination upon formation of [Cd(NCS) 2 (SCN)] 0 . Also, the metal ion forms a series of mono-to tetraiodo complexes in