Theoretical study of the hexahydrated metal cations for the understanding of their template effects in the construction of layered double hydroxides

A series of octahedral hexahydrated metal cations as model formula [M(OH2)6]n+ (M = metal cation, n = 2, 3 and 1 for M = Li) have been investigated by density functional theory (DFT) to shed light on their template effects in the construction for layered double hydroxides (LDHs) layers. The metal cations were classified to three types according to their calculated structural distortion angle θ as follows: Type I (canonical structure, θ: 0-1°), Type II (slightly distorted structure, θ: 1-10°) and Type III (heavily distorted structure, θ: >10°), respectively. The structure and properties of the hydrated cations such as bond distance, O-M-O bond angle distortion, binding energy, the valence electronic configuration, ligand field and Jahn-Teller effect and natural bond orbital (NBO), which are related to the construction of LDHs layers, were systematically investigated. It was found that in the case that the introduced cations which have close ionic radii to that of Mg2+, the distortion angles of their octahedral hexahydrated metal cations (which are governed by the coordination environment of the metal ion) play more significant role of structure directing for the LDHs layers than the ion size. These cations can be introduced into LDHs layers with the availability in the following order: Type I > Type II > Type III. The coordination preference of the cations with much larger size in the formation of LDHs are also been discussed. The calculation-based rule is in good agreement with the experimental results.