Searching for Monomeric Nickel Tetrafluoride: Unravelling Infrared Matrix Isolation Spectra of Higher Nickel Fluorides

Abstract Binary transition metal fluorides are textbook examples combining complex electronic features with most fundamental molecular structures. High‐valent nickel fluorides are among the strongest known fluorinating and oxidizing agents, but there is a lack of experimental structural and spectroscopic investigations on molecular NiF3 or NiF4. Apart from their demanding synthesis, also their quantum‐chemical description is difficult due to their open shell nature and low‐lying excited electronic states. Distorted tetrahedral NiF4 (D 2d) and trigonal planar NiF3 (D 3h) molecules were produced by thermal evaporation and laser ablation of nickel atoms in a fluorine/noble gas mixture and spectroscopically identified by a joint matrix‐isolation and quantum‐chemical study. Their vibrational band positions provide detailed insights into their molecular structures.


Part 1. Experimental and Computational Details and Details about Spectral
Assignments and Reaction Mechanism.

Experimental details
Matrix samples were prepared by co-deposition of laser-ablated excited nickel atoms or NiF2 molecules with different concentration diluted F2 in neon (99.999%, Air Liquide) and argon (99.999%, Sauerstoffwerk Friedrichshafen). The bulk NiF2 target was prepared in a hydraulic lab press and mounted on a target holder. The gases were mixed in a custom-made stainless steel mixing chamber equipped with a manometer. The stainless steel F2 storage cylinder was cooled in liquid nitrogen to avoid impurities. The mixing chamber was connected to the matrix chamber by a stainless-steel capillary. The reactants were condensed onto a gold mirror cooled to 6 K (neon) and 5-15 K (argon) using a closedcycle helium cryostat (Sumitomo Heavy Industries, RDK205D) inside the matrix chamber. [1] For the laser ablation of targets, the 1064 nm fundamental of a Nd:YAG laser (Continuum, Minilite II, repetition rate: 10 Hz, pulse width: 10 ns, pulse energy up to 40 mJ) was focused onto the rotating target through a hole in the gold mirror. Selective radiations of the matrix were used  = 617, 470, 455, 273 nm (LEDs) and 266 nm (laser), respectively. Another radiation of excimer laser  = 193 nm was also used. Matrices were annealed to different temperatures, as well as using a mercury lamp (Osram HQL 250) cooperating with different wavelength edge filters. Infrared (IR) spectra were recorded on a Bruker Vertex 80 FT-IR spectroscopy with 0.5 cm -1 resolution in the region between 4000-450 cm -1 using a liquid-nitrogen cooled MCT detector.
The thermal evaporation studies (performed at the University of Hull) used five strands of 0.5 mm nickel wire (99.9% Aldrich) tightly wound together and made into a coiled filament and heated using ca. 30 A at 1 V. To avoid heating of the window and matrix deposit, and to limit the reaction of the fluorine with the heated filament, a copper disc with a 5 mm aperture was placed between the furnace and the vacuum chamber containing the deposition window. Details of the matrix-isolation setup used in these experiments are published elsewhere. [2] The F2/Ar mixtures were prepared using standard manometric procedures from 10% F2/Ar (Air Liquide) and Ar (99.999% Energas) using a metal vacuum line. The vacuum line, reservoirs and vacuum chambers were well passivated. The reactants were condensed onto a CsI (IR) or CaF2 (UV-vis-NIR) window held at ca. 10 K by an APD DE-204 cryostat. Matrices were annealed to different temperatures using a Scientific Instruments SI 9600-1 digital controller and silicon diodes. Broadband photolysis (λ > 250 nm) was carried with a LOT-Oriel 200 W Hg(Xe) lamp, which was also used with 400-700 nm and 200-410 nm filters. IR spectra were recorded using a KBr beam splitter and DTGS detector on a Bruker EquNox55 FTIR instrument. Separate electronic absorption spectra were recorded on a Varian Cary 5E UV-vis-NIR spectrometer.

Computational Details
The MOLPRO program19 package [3] was used for quantum-chemical ab-initio calculations on several neutral nickel fluoride species. Scalar relativistic all-electron calculations were performed using the second-order Douglas-Kroll-Hess (DK) Hamiltonian [4] and corresponding (augmented) correlationconsistent polarized valence n-tuple Gaussian basis sets, denoted as (aug-)cc-pVnZ-DK and abbreviated as (A)VnZ-DK (n = D, T, Q). [5] In all tables, the basis set label suffix -DK always implies the use of the DK Hamiltonian. Calculations were done at different levels of theory up to coupled-cluster (CC) level. For each molecule, several possible spin states of all possible spatial symmetries (irreducible representations within a chosen point group) were initially considered by running state-averaged CASSCF calculations. Candidates for the molecular electronic ground state were thus identified and further studied. In every single case, the most important single configuration found at the CASSCF level was selected as reference for subsequent RHF-RCCSD(T) calculations, in order to obtain a fully optimized molecular structure for the chosen electronic state. Normal mode analysis in harmonic approximation was done to confirm that the optimized structure represents a minimum on the potential energy hypersurface. Selected results from the quantum-chemical calculations are presented below.
In order to calculate the relative IR intensities of the vibrational modes of NiF4, additional calculations at density functional theory (DFT) level were carried for this molecule using the Gaussian16 program package and the B3LYP functional as implemented therein. [6] All DFT calculations were performed within the restrictions of the D2d point group.

Reaction of Laser Ablated Cobalt Atoms with Fluorine
The IR spectrum obtained from laser ablation of metallic cobalt in a fluorine/argon gas mixtures ( Figure  S6), shows in addition to molecular CoF ( 3 Фi, 637.8 cm −1 ; in gas phase 662.6 cm −1 [7] ), CoF2 ( 4 g, 3band at 722.5 cm −1 ) [8] the higher fluorides CoF3 ( 5 A1ʹ, 736.9 cm −1 with matrix sites at 739.4 and 733.3 cm −1 ) [9] and CoF4 ( 6 A1, medium band at 767.8 cm −1 ). Their band positions agree very well with those from previous studies, [8][9][10] in which CoF3 vapour was investigated at 800 K and CoF4 was obtained at 650 K from solid mixtures of CoF3 and TbF4 as atomic fluorine source in a perfluorinated nickel effusion cell. [8] Thermal evaporation experiments also contained bands due to CoF (637.9 cm −1 ), CoF2 (722.8 cm −1 ) and a collection of bands at 740.5, 737.1 and 733.2 cm −1 due to CoF3 in a variety of sites, but with no evidence for the CoF4 band at 767.8 cm −1 .

Assignment of Binary Nickel Fluorides Isolated in Solid Rare-Gas Matrices
In the solid argon deposit obtained from thermally evaporated nickel atoms and elemental fluorine the antisymmetric stretching vibration of molecular NiF2 [11,12] appeared at 779.5 cm −1 for the 58 Ni isotopomer (Figures S1, S2, S3). Band positions for the 58/60/62 Ni isotopes are well resolved for this vibrational mode (Table S3.1) from which a bond angle of the argon matrix-isolated NiF2 molecule is estimated to 165º using a simple valence force field (SVFF), [13] compared to 165º, 154º and 152º reported previously. [8,12,14] However, for bond angles close to linearity a difference of only ca. 0.1 cm −1 in the isotope shifts already results in a change in bond angles from 180° to 160°, [15] it can be concluded that this estimate for argon-matrix isolated NiF2 is consistent with a linear structure. [16] Two set of bands with the highest wavenumber band at 800.2 cm −1 and a distinct Ni isotope splitting deserves special attention. Their ( 58/60 Ni) isotope splitting is close to that of molecular NiF2 (Δ = 5.1 cm −1 , Table S3.1, Figure S2), and the bond angle estimates from the isotope splitting (163 and 160º, respectively) indicates also for these bands a carrier with an almost linear F−Ni−F unit. It is tempting to assign these features at 800 cm −1 to a higher nickel fluoride such as NiF4, since higher fluorides can be assumed to have higher frequency Ni−F stretching modes. However, quantum-chemically predicted ground-state structures of molecular NiF4 (see Part 7) are inconsistent with the experimental observation of linear F−Ni−F units and with vibrational modes higher in frequency than NiF2. We therefore conclude that the features close to 800 cm −1 are due to matrix sites of NiF2 in F2/Ar matrices. It should be noted that these sites are not formed when NiF2 is thermally evaporated and trapped in solid argon, [11,12] or by photolysis of argon-matrix isolated NiF2, but they are present when NiF2 is generated by photolysis from Ni atoms and F2, indicating that fluorine is probably present in the trapping site. Similar site effects for other difluorides have been observed previously. [1,17] The bands observed in the region 736-729 cm −1 (Figures S1, S2, S3) grow on annealing on the expense of the NiF2 matrix sites, remain constant under broadband photolysis (see difference spectra in Figure  S1), and appear at relatively low annealing temperatures of 15 K. Since formation of dimers by annealing to 15 K would be unusual and the isotopic spacing observed for these bands is consistent with ca. 120º bond angle, these bands were finally assigned in good agreement with our CCSD(T) calculation (Table  S6.4) to the antisymmetric NiF3 stretching vibration of molecular NiF3 (D3h). At least two different matrix-sites were observed for this species in these experiments, one with a main peak at 733.2 cm −1 and a pair at 730.1 and 726.1 cm −1 .
In further experiments studying the reaction of IR-laser ablated nickel atoms with elemental fluorine all four nickel fluorides NiFn, n = 1-4, were also observed in solid neon matrices ( Figure S7). As expected, their Ni−F stretching bands revealed a significant blue-shift compared to the argon matrix (Table S3. 3). The antisymmetric NiF3 stretch shows at least two well separated Ne-matrix sites, which are strongly depleted by selective λ = 266 nm laser radiation with formation of NiF2 ( Figure S7c). The photodecomposition of NiF3 is associated with the depletion of only a single IR band in the Ni−F stretching region which clearly supports its quantum-chemically predicted D3h structure. Another general feature of metal-laser-ablation and Ne-matrix isolation is the formation of the polyfluorine monoanions [F3] − and [F5] − . [18] Their characteristic bands are indicated in Figure S7. We note that the laser-ablation process is associated with a hot plasma plume and a bright broad-band radiation. In these experiments there is photolysis all the time and the high wavenumber matrix sites of NiF2 are always present. We also studied the reaction of laser-ablated NiF2 molecules with elemental fluorine using fluorine/neon mixtures and a fluorine/argon mixtures. These experiments produced two remarkable results. We first noticed unprecedented high intensities of the E stretching vibration of NiF3 that were achieved despite the low fluorine content of the gas mixture (0.05 % F2 in Ne) used in these experiments ( Figure S8). The second observation is that the high-wavenumber matrix-site of NiF2 in solid argon were not observed in these experiments. From this observation it is assumed that these high-wavenumber features are likely formed by UV photolysis from Ni atoms and F2, which are both present in the solid matrices.

Electronic Absorption spectra
The electronic absorption spectra of Ni atoms in rare gas matrices have been well studied. [19][20][21][22][23][24][25][26][27][28][29][30][31][32] In the original reports [19][20][21] it was assumed that the atomic 3 F4 (3d 8 4s 2 ) ground state was also the ground state of the matrix isolated atoms. However, later work [22][23][24][25][26][27][28][29][30][31][32] showed that there were both 3 F4 and 3 D3 (3d 9 4s 1 ) states within Ar, Kr and Xe matrices, but only 3 F4 in solid Ne. In the gas phase the 3 D3 excited state is ca. 205 cm −1 above the 3 F4 ground state, and it was suggested that the inversion was the result of greater matrix induced repulsion of the 4s compared to the 3d electrons. Our spectra of thermally produced Ni atoms in argon matrices are shown in Figure S9 and they are in very good agreement with the earlier reports. [19][20][21][22][23][24][25][26][27][28][29][30][31][32] On deposition at ca. 10 K there are features due to both the 3 D3 state and the 3 F4 state as given in Table S3.2, but no evidence for dimers and trimers which have features in the range 18900-27000 cm −1 [21] On broadband photolysis the intensity of the 3 D3 features at 29000-38000 cm −1 and 45500-50000 cm −1 decreased, whilst the 3 F4 features at 42000-45500 cm −1 increased markedly. Similar photolysis behaviour has been observed for Ni atoms in solid Kr, although the 3 F4 features were initially assigned to dimers [24] before subsequent work identified them to be due to 3 F4 [30] and that the photolysis involved z-type 3 P2 0  3 D3 excitation, followed by decay to the 3 F4 ground state. This photolysis mechanism was also supported by Vala et al. [26] On annealing, the features associated with the 3 F4 state decay much more quickly than those of the 3 D3 state, which has also been observed previously. [25] Although the photolysis and annealing behaviours have been identified previously, the data in Figure  S9 is the first time both have been presented for argon matrices, and are necessary to follow the changes in Figure S10, where F2 has been introduced into the matrix.
In the presence of F2 ( Figure S10), there is the characteristic spectrum of Ni atoms in both 3 F4 and 3 D3 states on deposition. After broadband photolysis there is a dramatic reduction in the intensity of the features associated with the 3 D3 state, but a less marked reduction in the 3 F4 bands. This is in contrast to pure argon where there was a moderate transfer of intensity from the 3 D3 to 3 F4 features. On annealing, the remaining 3 D3 bands reduce slightly, but the 3 F4 peaks decay quite markedly as observed in pure argon. There are also two new broad bands at 45660 and 48950 cm −1 not observed in the pure argon spectra, which slightly increase in intensity after annealing to 15 and 20 K, followed by a slight decrease on annealing to 25 and 30 K. Since the 45660 and 48950 cm −1 bands are only observed in the presence of fluorine, this indicates that they belong to a nickel fluoride species. In previous studies, no UV absorptions were observed for NiF2, but this was carried out with photographic plates. [14] For NiCl2 in argon matrices an intense charge transfer absorption has been observed at 28350 cm −1 . [16] Using an optical electronegativity value of 3.0 for Cl, allows for an estimate of Ni of 2.1 in triatomics such as NiCl2 and NiF2, which is in good agreement with values of 2.0-2.1 reported for tetrahedral Ni(II). [33] This value of Ni in combination with a F value of 3.9 predicts charge transfer transitions of ca. 55000 cm −1 for NiF2. [33] For higher oxidation state nickel fluorides, the lowest energy charge transfer transition would be expected at significantly lower energies. Therefore, it is reasonable to assign the bands at 45660 and 48950 cm −1 to NiF2 charge transfer transitions, which have not been reported previously.
The observation of high wavenumber "sites" of NiF2 located in the IR spectra in the region from 800-791 cm −1 , which appeared after photolysis of solid argon matrices containing nickel atoms and F2 (Figures S1, S2, S3), but not by photolysis of argon matrices containing NiF2 and F2, lets us assume, that these less stable matrix sites are likely formed from excited nickel atoms and F2 after photoexcitation. Once formed, the less stable site species then decay to the stable NiF2 site. Further detailed studies to support this preliminary assumption are necessary, but are not the subject of this work.

Proposed Reaction Mechanism
From the matrix-isolation experiments we conclude that the reactivity of molecular NiF2 is markedly different from that of CoF2. As shown in Figure S6, the amount of CoF4 produced in the laser-ablation process can be considerably higher than that of CoF3. This observation suggests that initially formed CoF2 (Eq. 1) further reacts with F2 to yield CoF4 (Eq. 3), while CoF3 is formed either by decomposition of CoF4 or by the reaction of CoF2 and fluorine radicals (2). Interestingly, the relative yield of CoF4 strongly depends on the temperature of the matrix support ( Figure S6): At slightly higher deposition temperatures significantly higher amounts of CoF4 were obtained, indicating that reaction (3) likely takes place during the deposition in the "condensed" phase prior to the complete confinement of the reaction products in the cryogenic matrix. To the contrary, initially formed NiF2 (Eq. 4) can be regarded as chemically inert to elemental fluorine, but react rapidly with atomic fluorine radicals to form NiF3 (Eq. 5) and further to NiF4 (6). It has also been shown that the reaction of Ni atoms with elemental F2 to produce NiF2 (Eq. 4) requires UV photolysis to yield appreciable quantities of product under the cryogenic conditions applied here. Given the highly exothermic reaction energy of this reaction ( Table  2, main text), this observation indicates a considerable reaction barrier and the necessity for fluorine radicals.          Matrix site a Relative band intensity (in parentheses) are described as very strong (vs), strong (s), weak (w) and very weak (vw), respectively.   The molecular trifluorides of M = Fe, Co, and Ni all adopt D3h structures, and their stretching fundamentals were found in Ne matrices in a very narrow range between 743.6 cm −1 (Fe), [3] 748.2 cm −1 (Co), [6] and 743.8 cm −1 (Ni, this work). For a series of molecules with similar structures one expects a strong correlation between stretching frequencies and bond lengths, [7] a trend that also applies to CoF3 with similar computed bond length at the CCSD(T) level. Due to a strong Jahn Teller-induced spin crossover, [5,8] this series cannot be extended to the 3d 8 electron configuration of CuF3 with its singlet T-type (C2v) molecular structure.  ), equilibrium distance (r eq ), dipole moment (µ) and harmonic vibrational wavenumber (ω e ) for the 2 Π state of NiF computed at different levels of theory using the second-order Douglas-Kroll-Hess (DK) Hamiltonian. ), bond length (r eq ), and relative term energy (T e ) for the 3 Σand 1 Σ + terms of NiF 2 computed at different non-relativistic (NREL) levels of theory. ), bond length (r eq ), and relative term energy (T e ) for the 3 Σand 1 Σ + terms of NiF 2 computed at different levels of theory using the secondorder Douglas-Kroll-Hess (DK) Hamiltonian. Table S5.3. NiF 2 ( 1 Σ + ) Vibrational Analysis at the non-relativistic (NREL) CISD level of theory. Table S5.4. NiF 2 ( 1 Σ + ) Vibrational Analysis at the CISD level using the secondorder Douglas-Kroll-Hess (DK) Hamiltonian. Table S5.5. NiF 2 ( 1 Σ + ) Vibrational Analysis at the RCCSD(T) level of theory. Table S5.6. NiF 2 ( 3 Σ -) Vibrational Analysis at the non-relativistic (NREL) CISD level of theory. Table S5.7. NiF 2 ( 3 Σ -) Vibrational Analysis at the CISD level using the secondorder Douglas-Kroll-Hess (DK) Hamiltonian. Table S5.8. NiF 2 ( 3 Σ -) Vibrational Analysis at the RCCSD(T) level of theory.