Communication pubs.acs.org/IC
Computational Gas-Phase Formation Enthalpy and Electron Affinity for Platinum Hexafluoride: Is Gaseous PtF6 Diamagnetic because of a Relativistic Effect? Robson Fernandes De Farias* Universidade Federal do Rio Grande do Norte. Cx. Postal 1664, 59078-970 Natal-Rio Grande do Norte, Brazil For such kinds of compounds, the so-called computational thermochemistry could make an important contribution. It was proposed, for example, that platinum fluorides with PtVIII are nonviable (from a thermodynamic point of view) compounds.6 In this context/connection, in the present work, a computational thermochemistry study of PtF6 is performed. The gasphase formation enthalpy (ΔfH°) and electron affinity (EA) are calculated and the results compared with the experimental data.4 All computations were performed by using Spartan14 (version 1.1.8).7 Thermochemical calculations were performed by a semiempirical (PM6) method. The thermochemical results are summarized in Table 1. As can be verified, the calculated gaseous formation enthalpy value is in
ABSTRACT: In the present work, a computational thermochemistry study (semiempirical, PM6 method) for platinum hexafluoride (PtF6) is performed. The gas-phase formation enthalpy (ΔfH°) and electron affinity are calculated and the results compared with the experimental data. A calculated ΔfH°(g) value of −669.47 kJ mol−1, in very good agreement with the experimental data from the literature, was obtained by Knudsen cell mass spectrometry. However, such a value was obtained when a PtF6 molecule with no unpaired electrons (diamagnetic) was modeled. Such a fact is in contrast with the four-unpairedelectron configuration (t2g3eg1) generally accepted for gaseous (and solid) PtF6 but is in agreement with the fact (based on quantum relativistic calculations) that the triplet state t2g is split in the tetragonal field into a singlet and a lower-lying doublet, with four 5d4 electrons occupying the 5dxz and 5dyz atomic orbitals in the spin paired state. The modeled compound exhibits a distorted structure and a dipole moment of 0.30 D. The calculated electron affinity is 7.6 eV, in very good agreement with the experimental and calculated data. The computed zeropoint energy, G°, H°, and S° values for such a structure are 34.71 kJ mol−1, −725.59 kJ mol−1, −612.11 kJ mol−1, and 380.31 J K−1 mol−1. The positive S° and negative G° and H° values show that, from a thermodynamic point of view, the structure it stable. A working hypothesis is proposed in which the coordination number in PtF6 is 8 and the fluoride has a polymeric nature.
Table 1. Calculated (Semiempirical, PM6 Method) Gas-Phase Formation Enthalpy and EA for PtF6 experimental
−669.47
−676 ± 28,4 −6758
EA/eV
7.6
7.00 ± 0.358
very good agreement with the experimental data.4,8 Some experimental data4,8 were obtained by Knudsen cell mass spectrometry, searching for the equilibrium constant to the reaction PtF4 + F2 ↔ PtF6. With the experimental data, the K value and also other thermochemical data and suitable thermodynamic cycles (shown in Table 1) were determined. At this point, a very important statement must be made: ΔfH°(g) (−669.47 kJ mol−1) shown in Table 1, which is in very good agreement with the experimental data, was obtained by assuming (in the modeling parameters) a compound with no unpaired electrons. Such a compound exhibits a distorted structure, as shown in Figure 1. Such a structure exhibits Pt−F
P
latinum hexafluoride (PtF6) is a dark-red volatile solid (octahedral crystals). It is a unique example of platinum in the 6+ oxidation state. Because of its four unpaired d electrons, it is a paramagnetic compound, with a triplet ground state.1,2 It is used as a strong oxidizing agent (can oxidize oxygen from air). PtF6 forms compounds with molecular oxygen, xenon, and fullerenes, such as [O2+][PtF6−], XePtF6, and C60F18, respectively.1−3 There are relatively little thermochemical data available in the literature for platinum fluorides, probably because of their chemical instability (they are highly reactive) and high volatility, making them difficult compounds to study by traditional thermochemical techniques, such as calorimetry. Hence, there are little experimental thermochemical data available in the literature about this class of compounds, most of them obtained by Knudsen cell mass spectrometry,4,5 and no other measurements (by other techniques) to be compared with. © XXXX American Chemical Society
calculated ΔfH°(g)/kJ mol−1
Figure 1. Distorted structure exhibited by PtF6 in the gaseous phase (if modeled with no unpaired electrons). Received: October 28, 2016
A
DOI: 10.1021/acs.inorgchem.6b02618 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry
This proposal is in agreement with the fact that the calculated energy spectra and electron distributions for PtF6 (taking into account the relativistic contributions) suggest that the triplet state t2g is split in the tetragonal field into a singlet and a lowerlying doublet, with four 5d4 electrons occupying the 5dxz and 5dyz atomic orbitals in the spin paired state.11 In PtF4, the coordination number (CN) of platinum is 6; that is, each platinum cation is surrounded by six F− ions. So, as a working hypothesis, it will be supposed here that, analogously, in PtF6, the CN is 8; that is, each platinum cation is surrounded by eight F− ions: four of them bonded to only one platinum atom and the other four bonded (bridging) to adjacent platinum atoms. Such kinds of CN 8 environments to platinum have been previously modeled6 in order to verify whether higher platinum fluorides (with more than six F− ions) are feasible. Despite the fact that it was concluded that the existence of platinum fluorides higher than PtF6 is highly unlikely,6 it must be pointed out that such a conclusion was obtained for “monomeric” platinum fluorides. So, also as a working hypothesis, it is proposed here that a polymeric structure of PtF6 is probable, taking into account the polymeric nature of PtF4 and PtF5 (a tetramer).9
bond lengths ranging from 1.955 to 2.018 Å, and six different bond angles: 55.47°, 70.12°, 77.80°, 88.53°, 142.74°, and 166.46°. The point group is C1, and the dipole moment is 0.30 D. The calculated EA for such a structure is 7.6 eV, a value in very good agreement with the experimental EA of PtF6 (7.00 ± 0.35 eV)8 as well as in very good agreement with the previously obtained EA values6 from density functional theory and ab initio quantum-chemical calculations. Such very good agreement with the experimental4,8 and previously calculated6 values for ΔfH°(g) and EA shows that (a) the use of a semiempirical method is suitable and (b) the computed structure has effectively a “real” existence. Such a structure, it is said, “demands” zero unpaired electrons. The computed zero-point energy, G°, H°, and S° values for such a structure are 34.71 kJ mol−1, −725.59 kJ mol−1, −612.11 kJ mol−1, and 380.31 J K−1 mol−1, respectively. The positive S° and negative G° and H° values show that, from a thermodynamic point of view, the structure it stable. So, it is proposed here that such a structure exists, even as a transient species under the experimental conditions in the Knudsen cell.4,8 However, as is well-known, PtVI has four unpaired electrons (t2g3eg1) exhibiting a Jahn−Teller distortion.9 On the other hand, it is was considered (despite some controversy) that PtF62− is diamagnetic,10 and so with a t2g6eg0 configuration. In fact, because PtVI has a d4 configuration, in an octahedral ligand field, it is possible to suppose a t2g3eg1 (high-spin, four unpaired electrons) configuration or a t2g4eg0 (low-spin, two unpaired electrons) configuration. Of course, F− is a weak ligand, but the cation charge is 6+; therefore, one could expect a Δ0 energy gap higher than the pairing energy and a t2g4eg0 configuration. In fact, the energy-level diagram obtained for PtF6 (using nonrelativistic basis sets) shows a two-unpairedelectron configuration.11 As a comparison, PtF4, with the field of only four ligands and PtIV, is diamagnetic9 (d6: t2g6eg0 configuration). Initially, It was assumed that PtF4 exhibited a tetrahedral geometry, but it is now known that the platinum chemical environment in such a compound is, in fact, octahedral, with PtF4 being a polymeric compound (with four out of the six fluorine atoms on each platinum bridging to adjacent platinum centers). Using the crystal-field-theory approach, it is proposed that (at least in the gaseous phase), PtF6 is diamagnetic, with the proposed energy diagram shown in Figure 2.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The author declares no competing financial interest.
■
REFERENCES
(1) Patnaik, P. Handbook of inorganic chemicals; McGraw-Hill: New York, 2002. (2) Hagenmuller, P. Inorganic solid fluorides; Academic Press: Orlando, FL, 1985. (3) Nakajima, T.; Zemva, B.; Tressaud, A. Advanced inorganic fluorides; Elsevier: Amsterdam, The Netherlands, 2000. (4) Korobov, M. V.; Nikulin, V. V.; Chilingarov, N. S.; Sidorov, L. N. Enthalpy of formation of platinum hexafluoride. J. Chem. Thermodyn. 1986, 18, 235−240. (5) Weinstock, B.; Malm, J. G.; Weaver, E. E. The Preparation and Some Properties of Platinum Hexafluoride. J. Am. Chem. Soc. 1961, 83, 4310−4317. (6) Riedel, S. J. Platinum fluorides beyond PtF6? J. Fluorine Chem. 2007, 128, 938−942. (7) Spartan14; Wavefunction Inc.: Irvine, CA, 2014. (8) Korobov, M. V.; Kuznetsov, S. V.; Sidorov, L. N.; Shipachev, V. A.; Mit'kin, V. N. Gas-phase negative ions of platinum metal fluorides. II. electron affinity of platinum metal hexafluoride. Int. J. Mass Spectrom. Ion Processes 1989, 87, 13−27. (9) Livingstone, S. E. The chemistry of ruthenium, rhodium, palladium, osmium, iridium and platinum, Comprehensive inorganic chemistry; Pergamon Press: Oxford, U.K., 1973; Chapter 43. (10) Gabuda, S. P.; Kozlova, S. G. NMR, magnetic behavior and structural effects of spin−orbit interactions in PtF6 and in related octahedral molecules and fluorocomplexes. In Handbook of Inorganic Chemistry Research; Chemistry Research and Applications Series; Morrison, D. A., Ed.; Nova Science Publishers: Hauppauge, NY, 2010; p 69. (11) Gabuda, S. P.; Ikorskii, V. N.; Kozlova, S. G.; Nikitin, P. S. Relativistic effects in PtF6 and isoelectronic octahedral groups: magnetic susceptibility, 19F NMR, and ab initio Calculations. JETP Lett. 2001, 73, 35−38.
Figure 2. Proposed energy-level diagram to explain the diamagnetic nature of gaseous PtF6. B
DOI: 10.1021/acs.inorgchem.6b02618 Inorg. Chem. XXXX, XXX, XXX−XXX
