Ab initio study on the coordination structures of M +XY 3− and M +XY 4− ion pairs

Abstract

The optimized geometries and vibrational frequencies of the T d symmetry anions BF 4 −, AlF 4 −, BH 4 − and AlH 4 − symmetry ones BeF 3 −, MgF 3 −, BeH 3 − and MgH 3 − are reported. The association of these anions with the alkali-metal and Na + was studied at the HF-SCF and Mp n levels ( n = 2, 3, 4). Particular attention was focused on the bidentate (C 2v) and tridentate (C 3v) structure of MBF 4, MAlF 4, MBH 4, MAlH 4, MBeF 3, MBeH 3, MMgF 3 and MMgH 3 (M = Li + and Na +). Li and NaBF 4 prefer tridentate binding, however, the stability of the bidentate structure of LiBF 4 is comparable to that of the tridentate isomer. On the contrary, LiAlF 4 and NaAlF 4 prefer bidentate coordination; however, the stability of the tridentate isomers, and particularly of NaAlF 4, is high. Tridentate and bidentate MBH 4 and MAlH 4 ion pairs are both stable species; tridentate LiBH 4 and NaBH 4 are more stable than the respective bidentate forms and the stability order is reversed for the MAlH 4 species.