Exploring the Superhalogen Properties of Polynuclear Structures without Halogen Ligands: A Combined Ab Initio and DFT Study on Triple-Bridged [Mg2L5]-1 (L = -OCN, -SCN) Anions

Via combined ab initio and DFT calculations, 16 [Mg-2(OCN)(5)](-1) and [Mg-2(SCN)(5)](-1) isomers are studied to explore their potentials as superhalogens. The results of high-level OVGF calculations verify that these systems are superhalogen anions due to their high VDE values which lie within 6.74-7.04 eV and 5.73-6.27 eV for [Mg-2(OCN)(5)](-1) and [Mg-2(SCN)(5)](-1), respectively. The results of low-cost methods, either ab initio or DFT, are generally consistent with those of OVGF, and some of them demonstrate promising accuracy. The best performance of low-cost ab initio methods comes from (HF+MP2)/2, which represents the average value of the MP2 and HF results. In the aspect of DFT, five functionals (CAM-B3LYP, omega B97X-D3, M06, M06-2X, and B2PLYP) provide the most accurate results when compared to OVGF. Thus, these low-cost methods could be used to calculate the VDE value of the future systems of larger size. Useful information about the VDEs of [Mg-2(OCN)(5)](-1) and [Mg-2(SCN)(5)](-1) could be obtained from the analysis of the extra electron density and HOMO as well as spin density.

Automated summary

By combining ab initio and DFT calculations, it was found that [Mg-2(OCN)(5)](-1) and [Mg-2(SCN)(5)](-1) are superhalogen anions with high VDE values. Low-cost methods, both ab initio and DFT, showed promising accuracy and could be used for calculating VDE values of larger systems in the future.