In this work, a systematic investigation of the competition coordination of H2O and Cl– with Ni2+ in saturated NiCl2 aqueous solution at room temperature was conducted using density functional theory (DFT), Car–Parrinello molecular dynamics (CPMD) simulations, and extended X-ray absorption fine structure (EXAFS) spectra. The calculated results reveal that the six-coordinated structure is favorable for [NiClx(H2O)n]2–x (x = 0–2; n = 1–12) clusters in the aqueous phase. The hydration energy calculation shows that the six-coordinated solvent-shared ion pair (SSIP) ([Ni(H2O)6(H2O)n?6Cl]+) is more stable than its contact ion pair (CIP) ([NiCl(H2O)5(H2O)n?5]+) isomer as n ≥ 9 in the aqueous phase, and the six-coordinated solvent-shared ion pair with a dissociated double Cl– (SSIP/d) ([Ni(H2O)6(H2O)n?6Cl2]0) is more preferable than its CIP ([NiCl2(H2O)4(H2O)n?4]0) and solvent-shared ion pair with single dissociated Cl– (SSIP/s) ([NiCl(H2O)5(H2O)n?5Cl]0) isomers as n ≥ 11. The six-coordinated SSIP/d ([Ni(H2O)6(H2O)n?6Cl2]0) conformers are the dominant structures in a saturated NiCl2(aq)solution (NiCl2 concentration:
5.05 mol·kg–1, corresponding to n ≈ 11). The CPMD simulations exhibited that there are six water molecules with Ni–O distance at
205.0 pm on average around each Ni2+ in the first hydration sphere, even in the saturated NiCl2 aqueous solution (
5.05 mol·kg–1) at room temperature, and no obvious Ni–Cl interaction was found. The EXAFS spectra revealed that the first solvation shell of Ni2+ is an octahedral structure with six water molecules tightly bound in the NiCl2(aq) solution with a concentration ranging from 1.00 to 5.05 mol·kg–1, and there is no obvious evidence of Ni–Cl contact ion pairs. A comprehensive conclusion from the DFT, CPMD, and EXAFS studies is that there is no obvious direct contact between Ni2+ and Cl–, even in saturated NiCl2 aqueous solution at room temperature.
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Original paper link:http://pubs.acs.org/doi/abs/10.1021/jp405168r