Chemistry - Lewis Structure of OF+

Solution 1:

A strategy different from the one described by Yusuf Hasan would be to look at the neutral isoelectronic molecule. The oxygen atom is isoelectronic to the fluorine cation, so $\ce{OF+}$ is isoelectronic to $\ce{O2}$.

It is difficult to write a nice Lewis structure for $\ce{O2}$, see https://chemistry.stackexchange.com/a/15061. Experiments show that dioxygen is paramagnetic, i.e. it has unpaired electrons. If you try to write this as Lewis structure, you end up with 7 electrons on each oxygen (counting electrons in bonds twice), i.e. no octets either.

For $\ce{OF+}$, you should be able to write the same Lewis structure (and the same MO diagram). Because fluorine has a different number of protons than oxygen, however, the energy levels will be different and "the best" Lewis structure might be different as well.

As Yusuf Hasan mentioned, the struggles to write a "nice" Lewis structure for this species hints at high reactivity, so I think this is the main lesson of trying to write a Lewis structure. Which of the Lewis structure is best at approximating the real situation becomes secondary, and it makes sense to move on from a Lewis structure model to more comprehensive models of bonding for species such as this one.

Solution 2:

A good way to think about stuff like this, is to imagine a possible synthesis for the given intermediate. One way of making this species would be to cleave the H-OF bond of HOF against it's electronegativity, that is, heterolytically cleaving the bond to make H- and [OF]+

As we are going against electronegativity, you get the idea that the intermediates are quite unstable, and so your structure must represent a reactive species

Then, we can simply find out the structure by the curly arrow diagram, assuming that making the structure of HOF is easier than [OF]+

Another reason why your double-bonded structure won't work is that due to the close electronegativities of O and F, the MO picture would probably resemble that of O2 and F2, and so the pi bonding orbitals would experience at least some cancellation by filling of the antibonding pi orbitals(and even the O-F bond would be destabilized a little by the so-called 'antibonding effect')

As a final note, if you are not willing to evoke MO theory, then you can simply say that due to small size and close electronegativities of O and F, a pi bond will mostly not be favorable between them.