Molecular oxofluorides OMFn of nickel, palladium and platinum: oxyl radicals with moderate ligand field inversion

Abstract

Terminal oxo compounds of high-valent late transition metals attracted attention because of their peculiar metal–oxygen bond in which the oxo ligand exhibits an electrophilic and distinct radical oxyl (O˙⁻) rather than the commonly expected nucleophilic (O²⁻) character. These properties confer the late transition metal oxo compounds their promising high reactivity in oxidation reactions, as well as H-atom transfer (HAT) and O-atom transfer (OAT) reactions. On the other hand, because of their highly reactive nature, direct spectroscopic evidence especially for high-valent nickel oxo compounds is rare. In this work, we report on linear oxo monofluorides OMF (C_∞v) and difluorides OMF₂ (C_2v) of the group 10 metal atoms M = Ni, Pd and Pt, as well as on the hypofluorite FOPdF (C_s) and the oxo trifluoride OPtF₃ (C_2v). These molecules were prepared by the reaction of the metal atoms with gaseous OF₂ and isolated in solid Ne and Ar matrices. They have been investigated by a joint analysis of IR matrix isolation spectroscopy and electronic structure calculations at the DFT, CCSD(T), as well as CASPT2 and MRCI levels of theory. The linear OMF molecules have ⁴Σ⁻ ground states and bear a terminal oxyl radical ligand. The oxo difluorides have planar T-shaped structures in the ³A₂ electronic states. The terminal oxo ligand in these difluorides also carries large spin densities of ≥1.0. A contradiction about the electronic ground state for the recently reported ONiF₂ [Wei et al., Inorg. Chem., 2019, 58, 9796–9810] could be solved experimentally by the observation of a more complete vibrational spectrum. The relationship between the electron and spin population on the oxo ligand in the group 10 transition metal oxofluorides and related compounds will be discussed. Remarkable differences were found for the metal–oxygen bonds in ONiF₂ and OCuF₂. The continuous transition from a predominantly ionic to an inverted ligand field is shown by comparing the linear oxo monofluorides of group 10 (O[10]F), with the difluorides of group 9 (F[9]F) and the isoelectronic dioxides of group 11 (O[11]O).