Dioxygenyl

The dioxygenyl ion, O₂⁺, is a rarely encountered oxycation in which both oxygen atoms have a formal oxidation state of +½. It is formally derived from oxygen by the removal of an electron:

O₂ → O₂⁺ + e⁻

The energy change for this process is called the ionization energy of the oxygen molecule. Relative to most molecules, this ionization energy is very high at 1175 kJ/mol.^[1] As a result, the scope of the chemistry of O₂⁺ is quite limited, acting mainly as a 1-electron oxidiser.^[2]

Structure and molecular properties

O₂⁺ has a bond order of 2.5, and a bond length of 112.3 pm in solid O₂[AsF₆].^[3] It has the same number of valence electrons as nitric oxide. The bond energy is 625.1 kJ mol⁻¹ and the stretching frequency is 1858 cm⁻¹,^[4] both of which are high relative to most molecules.

Synthesis

The reaction of oxygen, O₂, with platinum hexafluoride, PtF₆, yields dioxygenyl hexafluoroplatinate, O₂[PtF₆]⁺:

O₂ + PtF₆ → O₂⁺[PtF₆]^–

PtF₆ is one of the few oxidising agents sufficiently powerful to oxidise O₂.

Dioxygenyl hexafluoroplatinate played a pivotal role in the discovery of noble gas compounds. After Neil Bartlett found that PtF₆ could oxidise O₂ to O₂⁺, he investigated its reaction with noble gases and discovered "xenon hexafluoroplatinate".

O₂⁺ is also found in similar compounds of the form O₂MF₆, where M is arsenic (As),^[5] gold (Au),^[6] niobium (Nb), ruthenium (Ru), rhenium (Re), rhodium (Rh),^[7] vanadium (V),^[8] or phosphorus (P).^[9] Other forms are also attested, including O₂GeF₅ and (O₂)₂SnF₆.^[8]

The tetrafluoroborate and hexafluorophosphate salts may be prepared by the reaction of dioxygen difluoride with boron trifluoride or phosphorus pentafluoride at −126 °C:^[9]

2 O₂F₂ + 2 BF₃ → 2 O₂BF₄ + F₂

2 O₂F₂ + 2 PF₅ → 2 O₂PF₆ + F₂

These compounds rapidly decompose at room temperature:

2 O₂BF₄ → 2 O₂ + F₂ + 2 BF₃

2 O₂PF₆ → 2 O₂ + F₂ + 2 PF₅

Reactions

The reaction of O₂BF₄ with xenon at 173 K produces a white solid believed to be F–Xe–BF₂, containing an unusual xenon-boron bond:^[10]

2 O₂BF₄ + 2 Xe → 2 O₂ + F₂ + 2 FXeBF₂

The dioxygenyl salts O₂BF₄ and O₂AsF₆ react with carbon monoxide to give oxalyl fluoride, F–(C=O)–(C=O)–F, in high yield.^[11]

References

1. ^ Michael Clugston; Rosalind Flemming (2000). Advanced Chemistry, Oxford University Press, ISBN 0199146330, ISBN 9780199146338, p. 355.
2. ^ Foote, Christopher S.; Valentine, Joan S. (1995). Active oxygen in chemistry. Joel F. Liebman, A. Greenberg. Springer. ISBN 0412034417. 
3. ^ Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Oxford: Butterworth-Heinemann. ISBN 0080379419.  p. 616
4. ^ J. Shamir, J. Binenboym, H. H. Claassen (1968). "The vibrational frequency of the O₂⁺ cation". J. Am. Chem. Soc. 90 (22): 6223−6224. doi:10.1021/ja01024a054. 
5. ^ A. R. Young, T. Hirata, S. I. Morrow (1964). "The Preparation of Dioxygenyl Salts from Dioxygen Difluoride". J. Am. Chem. Soc. 86 (1): 20−22. doi:10.1021/ja01055a006. 
6. ^ Nakajima, Tsuyoshi (1995). Fluorine-carbon and fluoride-carbon materials: chemistry, physics, and applications. CRC Press. ISBN 0824792866. 
7. ^ Vasile, M. J.; Falconer, W. E. (1975). "Vapour transport of dioxygenyl salts". Journal of the Chemical Society Dalton Transactions (4): 316–318. doi:10.1039/DT9750000316.  edit
8. ^ ^{a b} Holleman, Arnold F.; Wiberg, Egon (2001). Inorganic chemistry. Academic Press. p. 475. ISBN 0123526515. 
9. ^ ^{a b} Solomon, I. J.; Brabets, R. I.; Uenishi, R. K.; Keith, J. N.; McDonough, J. M. (1964). "New Dioxygenyl Compounds". Inorganic Chemistry 3 (3): 457. doi:10.1021/ic50013a036.  edit
10. ^ Goetschel, C. T.; Loos, K. R. (1972). "Reaction of xenon with dioxygenyl tetrafluoroborate. Preparation of FXe-BF₂". Journal of the American Chemical Society 94 (9): 3018–3021. doi:10.1021/ja00764a022.  edit
11. ^ Pernice, H. (2001). "The reaction of dioxygenyl salts with 13
    CO Formation of F13
    C(O)13
    C(O)F". Journal of Fluorine Chemistry 112 (2): 277–590. doi:10.1016/S0022-1139(01)00512-7.  edit