Direct contact versus solvent-shared ion pairs in saturated NiCl2 aqueous solution: a DFT, CPMD, and EXAFS investigation.

In this work, a systematic investigation of the competition coordination of H2O and Cl(-) with Ni(2+) 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 NiCl(x)(H2O)(n) (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-6)Cl) 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-6)Cl2) is more preferable than its CIP (NiCl2(H2O)4(H2O)(n-4)) and solvent-shared ion pair with single dissociated Cl(-) (SSIP/s) (NiCl(H2O)5(H2O)(n-5)Cl) isomers as n ≥ 11. The six-coordinated SSIP/d (Ni(H2O)6(H2O)(n-6)Cl2) 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 Ni(2+) 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 Ni(2+) 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 Ni(2+) and Cl(-), even in saturated NiCl2 aqueous solution at room temperature.