Why we can't see 3-d structures of certain compounds on PubChem?
Since it's been a couple of days, I guess I convert my comment into an answer.

MMFF94s is Merck Molecular Force Field.
From what I understand, the issue is that this FF has certain limitations when it comes to electrostatic interaction, estimating polarizability and intramolecular interactions, which confuses the method trying to predict 3D conformers for ionic compounds, metal complexes or molecules with twisted geometry.

From the Concluding Discussion section of the original publication [1, p. 515]:


  Despite encouraging success, certain limitations are evident.
  One of particular importance arises from the fact that MMFF94 uses static atom-centered charges. As such, it neglects both higher order multipoles and electrostatic effects that arise from molecular polarizability.
  Because of these simplifications, MMFF94, like a number of other force fields, employs “enhanced” charge distributions that emulate the effect of polarizability in amplifying electrostatic interactions for favorable contacts in a high-dielectric medium. Unfortunately, these enhanced charge distributions also amplify electrostatically unfavorable iteractions, whereas proper account of polarizability would
  diminish them.
  They also improperly enhance electrostatic interactions in gas-phase or low-dielectric environments. Furthermore, they may not be optimal for describing intramolecular interactions, and may thereby limit the ability of the force field to account for differences in conformational energies.
  Indeed, compounds containing two or more strongly polar functional groups in close
  proximity have proven to be the most problematic in this respect, though good results have been obtained in most cases to date. …  Other significant limitations include: the overly simplistic nature of the bond-charge-increment scheme used to assemble the partial atomic charges …; the lack of conformational dependence of the resultant charges …;
  and the omission of bond-torsion (and certain other) cross terms needed to account for significant geometrical changes that can occur when a torsion angle varies,… an example being the elongation of an amide partial C−N double bond by up to 0.1 Å when conjugation is broken.
  A further significant limitation is that no account is taken of metal-ligand interactions beyond that afforded by a relatively simplistic model that includes only electrostatic and van der Waals nonbonded interactions.


References


Halgren, T. A. Merck Molecular Force Field. I. Basis, Form, Scope, Parameterization, and Performance of MMFF94. Journal of Computational Chemistry 1996, 17 (5–6), 490–519. DOI: 10.1002/(SICI)1096-987X(199604)17:5/6&lt;490::AID-JCC1>3.0.CO;2-P.