Side-On Sulfur Monoxide Complexes of Tantalum, Niobium, and Vanadium Oxyfluorides

Side-on sulfur monoxide complexes of tantalum, niobium, and vanadium oxyfluorides OMF₂(η²-SO) were prepared via the reactions of metal atoms and SO₂F₂ upon UV-vis irradiation in a cryogenic matrix. The product structures were identified by the characteristic infrared absorptions and isotopic frequency ratios of terminal M-O, F-M-F, and M-(SO) stretches, which were further supported by density functional theory calculations at the B3LYP level. All of the three complexes were predicted to have doublet ground states with the S-O bond nearly perpendicular to the terminal metal-oxygen bond. Although end-on bonded isomers with either M-OS or M-SO geometry are stable as well, they are higher in energy than the side-on isomers, and their vibrational frequencies do not match the experimental values. For the OMF₂(η²-SO) complexes, the S-O bond length approaches that of SO^-, but it is longer than that of neutral SO due to the electron transfer from the metal d orbital to the in-plane π* orbital of SO. The metal-SO bonding in the side-on complex is mainly ionic, but covalent interactions play some role between the two parts. On the basis of the calculation results, the OMF₂(η²-SO) complexes can be considered as (OMF₂)⁺(SO)^- in which the unpaired electron is mainly located in the out-of-plane π* orbital of SO. In addition to the complexes containing SO ligand, a series of other stable isomers were obtained by the B3LYP calculations, and they were proposed as intermediates involved in the formation of the OMF₂(η²-SO) products via fluorine and oxygen transfer reactions between metal atoms and SO₂F₂ under UV-vis irradiation.