Side-On Coordination in Molecular Alkaline Earth Metal Hypofluorites FM(η²-OF) (M = Mg, Ca, Sr, and Ba)

Side-on coordinated molecular hypofluorites FM(η²-OF) (M = Mg, Ca, Sr, and Ba) were prepared by co-deposition of laser-ablated alkaline earth metal atoms with diluted OF₂ in cryogenic matrices. The structures parallel the non-VESPR structures of alkaline earth metal difluorides and dihydrides, such that FMg(η²-OF) is planar, FCa(η²-OF) is planar but with a shallow bending-potential curve, while FSr(η²-OF) and FBa(η²-OF) are non-planar. Bonding analyses show that the interactions between the metal center M²⁺ and the negatively charged ligands F^- and OF^- are mainly electrostatic. Moreover, for FMg(η²-OF), donation of electron density from OF^- to empty s and p orbitals of magnesium is involved. However, for the heavier alkaline earth metal compounds, the donation to empty d orbitals of the metal center becomes dominant in orbital interactions, which contributes to their non-planar structures.