Matrix isolation and IR spectroscopic study on late-transition fluorides, metal oxo complexes and actinide halides

High-valent late transition metal complexes bearing multiple bonded, terminal M-O groups persistently attract attention. They are reactive intermediates in various oxidation reactions, and in hydrogen and oxygen atom transfer processes. Additionally, due to their peculiar electronic structures they often show an electrophilic radical oxyl (O${}^{\cdot -}$) character than that of a closed-shell terminal oxo ligand (O${}^{2-}$). Many early transition metal complexes with multiple M-O bonds are common, however, oxo complexes of group 11 and 10 metals are rarely investigated due to experimental and computational challenges. In this work, the target molecules were synthesized by the matrix-isolation technique and characterized by infrared (IR) spectroscopy combined with quantum-chemical methods to analyze their electronic structures and harmonic frequencies. The linear oxo monofluoride OMF (C${}_{\infty v}$, ground electronic state ${}^3$Σ${}^{\text{–}}$ for M = Au, Ag, and ${}^4$Σ${}^{\text{–}}$ for M = Ni, Pd, Pt), the T-shaped oxo difluorides OMF${}_2$ (C${}_{2v}$, ground state ${}^2$B${}_2$ for M = Au, Ag, and ${}^3$A${}_2$ for M = Ni, Pd, Pt), and the planar oxo trifluoride OPtF${}_3$ (C${}_{2v}$, ${}^4$A${}_1$ ground state) as well as the hypofluorite FOPdF (C${}_s$) are observed and spectroscopically assigned. A contradiction reported by Wei et al. [Inorg. Chem. 2019, 58, 9796-9810] regarding the ground state of ONiF${}_2$ has been solved experimentally (low-spin ${}^3$A${}_2$ rather than high-spin ${}^5$A${}_1$) by the observation of an additional stretching band. For OMF${}_2$, the singly occupied antibonding π* orbitals show significant oxygen (2p orbital) character, and these M=O π-bonds show a gradual conversion from a covalent to an inverted ligand field case going from the oxo metal group 10 to group 11 complexes. The high spin densities at the oxygen atoms of these oxo compounds indicate their high oxyl radical character. According to a simplified three-electron π-bonding scheme the spin population at the oxo ligand increases with increasing covalence of the M=O bond, and further increases with inversion of the M-O π-orbital space. The trend from an ionic to a covalent and even to an inverted ligand field has been analyzed also for the isoelectronic linear species of group 9 (MF${}_2$), group 10 (OMF) and group 11 (OMO). High-valent nickel fluorides are strong oxidizing agents and often take part in fluorination reactions. However, there is no available experimental structural data and spectroscopic information about molecular NiF${}_3$ and NiF${}_4$. On the one hand, their high reactivity leads to challenges in their synthesis; on the other hand, their open shell nature and low-lying excited electronic states make quantum-chemical calculation more complex. In this work, the distorted tetrahedral NiF${}_4$ (${}^3$A${}_2$, D${}_{2d}$) and the planar NiF${}_3$ (${}^4$A${}_{2^{\prime }}$, D${}_{3h}$) molecules were produced by the reaction of thermal evaporation and laser ablation of elemental nickel with F${}_2$ and trapped in solid rare gas matrices (Ne and Ar). The similarities of the experimental M-F stretching bands of the high-valent 3d metal fluorides of Fe, Co and Ni were particularly instructive for their identification. Furthermore, the expected shortening and strengthening of the M-F bonds with increasing the metal oxidation states, which was experimentally observed for the binary fluorides of the 3d predecessors FeF${}_n$ and CoF${}_n$ (n = 1-4), was not observed for the higher nickel fluorides NiF${}_3$ and NiF${}_4$. Their weak Ni-F bonds and a considerable fluorine radical character make these fluorides to very powerful fluorination and oxidation reagents. Uranium and thorium hydrides attract great interest due to 5f electron participation in their bonding. Owing to their high carcinogenicity, matrix isolation is an appropriate technique to synthesize and investigate the related species. In this work, isolated molecular HAnX and H${}_2$AnX${}_2$ (An = U and Th, X = Cl and Br) have been produced in solid argon using laser-ablated U and Th atoms with HCl and HBr. All these novel molecular species have been explicitly assigned based on characteristic isotopic shifts and quantum-chemical calculations. The analysis of Mulliken partition shows an increasing f-orbital participation in the H-An bonds of the HAnX (X = F < Cl < Br) molecules, while the opposite trend was observed in the An-X bonds. Moreover, the f-orbital participation is found to be larger for the Th analogues. Interestingly, the U-H stretching frequency increases in the series HUF < HUCl < HUBr < UH, as less electronic charge is removed from the U-H bond by the less electronegative substituent. A similar trend is found for the Th counterparts.