[Big-Daddy]

What are the bond angles in a pentagonal bipyramidal molecule in general? If they vary from molecule to molecule then just the bond angles for IF₇ would be ideal. I'd like the same thing for square antiprismatic molecules (any complex ion with coordination number 8 which is square antiprismatic would do the trick!). I simply can't find any of this info anywhere online. 

[Schrödinger]

Well there are two sorts of bond angles in pentagonal bipyramidal. If 'a' represents the atom along the axial plane and 'e' represents the atoms in the equatorial plane and 'c' stands for central atom, then a-c-e bond angle is 90^o. And the e-c-e (adjacent e's) is just the central angle of a pentagon, which is 72^o.

[Big-Daddy]

Well there are two sorts of bond angles in pentagonal bipyramidal. If 'a' represents the atom along the axial plane and 'e' represents the atoms in the equatorial plane and 'c' stands for central atom, then a-c-e bond angle is 90^o. And the e-c-e (adjacent e's) is just the central angle of a pentagon, which is 72^o.

Thank you

Should I be confident that all complexes with a coordination number of 6 are octahedral? And how does one differentiate between which metal cations will form square planar complexes, and which will form tetrahedral ones, with a coordination number of 4?

[Schrödinger]

And how does one differentiate between which metal cations will form square planar complexes, and which will form tetrahedral ones, with a coordination number of 4?

Well that depends upon the strength of the ligands (see : spectrochemical series) and electronic configuration of the metal ion.