[snorkack]

Does anyone know the geometry of perchloric acid (and the isoelectronic hydrogen sulphate anion)?
Routinely described as tetrahedral.
Obviously absurd.
For one thing, one of the oxygen is bonded to hydrogen.
For another, the bonds at OH are not linear. Therefore the other three oxygens cannot be equal either - only two of them are. The third is coplanar with Cl-O-H.
For the coplanar bonds O=Cl-O-H, is the geometry cis or trans?

[jeffmoonchop]

I see what you're getting at, due to the H there must be a slight distortion of the bond angle between Cl and O because of attraction between the H and the O on the Cl=O. I would say its cis based off how I know H acts in the presence of an electron dense =O, hydrogen bond like, although I dont know the distance between the two atoms, if its above 4Å, it may not be very attractive.

[mjc123]

It seems to be usually drawn as trans, whether based on fact I don't know.
I found these papers, but I can't access the full text:
https://onlinelibrary.wiley.com/doi/abs/10.1002/anie.198813391
https://pubs.acs.org/doi/abs/10.1021/j100085a013

[Flatbutterfly]

The HClO4 molecule in the gas phase consists of an approximate ClO4 tetrahedron with H bonded to one O at an angle that is not given.  The Cl-O(H) length is 163.5 pm; the other Cl-O lengths 140.8 pm. The Cl-O-OH angle is 105.8°; the other O-Cl-O angles are 112.8°.
“This molecular structure persists in the liquid phase, with some H bonding, and also in the crystalline phase, X-ray study at -160°C.”
N. N. Greenwood, A. Earnshaw, Chemistry of the Elements 2nd ed. (1997) p 866

[wildfyr]

Flatbutterfly that's some above and beyond sleuthing!