Fluoroborylene Complexes FBMF₂ (M = Sc, Y, La, Ce): Matrix Infrared Spectra and Quantum Chemical Calculations

Laser-ablated group 3 transition metal and cerium atom reactions with boron trifluoride were explored in excess solid neon at 4 K through matrix isolation infrared spectroscopy and quantum chemical calculations. The fluoroborylene complexes FBMF₂ (M = Sc, Y, La, Ce) were trapped in inert gas and identified by the isotopic substitutions. The observed frequencies of FBMF₂ were reproduced by DFT, NEVPT2, and CASSCF calculations. From Sc to La, the observed F-¹¹B stretching mode has been observed at 1391.9 cm^-1 (Sc), 1370.8 cm^-1 (Y), and 1337.1 cm^-1(La); however, for Ce this mode shifts up to 1340.8 cm^-1, which is due to relativistic effects. The electron localization function (ELF) analysis and the theory of atoms in molecules (AIM) were applied to investigate the character of the B-M bond in FBMF₂ molecules, which favors bond order 1.5.