[Rutherford]

I got an idea here. It is obvious that A/B and D/F are isomers. As the compounds are trigonal bipyramidal it was hard to find the appropriate names of these isomers, they can't be cis/trans nor mer/fac. Then I found that stereoisomers of trigonal bipyramidal molecules can be axial and equatorial, so I believe that they much differ in stability and therefore in their chemical properties. The only problem is that I don't know which form is more stable and which is covalent and ionic.

[Dan]

The trigonal bypyramidal compounds will always have 2 axial and 3 equatorial substituents, so you can't get isomers for XY₅.

The trigonal bipyramidal compound could be represented as:


Think about the polyhedra in the hint I posted.

[Rutherford]

I didn't mean for XY₅. I meant it for compounds A/B and D/F. There is more that one possible space arrangement for XY₄Z.
I can't see how the tetrahedron and the octahedron are related to the geometry of these molecules, so I will leave this.

[Dan]

I can't see how the tetrahedron and the octahedron are related to the geometry of these molecules, so I will leave this.

Hint: empirical vs molecular.

[Rutherford]

As no one tried this and as I am really curios about the answer, I will try it again.
You said empirical vs molecular, and on the picture there were a tetrahedron and an octahedron. I came to the idea that the molecules combine, so that B and F make molecular crystal cells which is the reason why they are solids.

[Dan]

I came to the idea that the molecules combine

You're so close now. How could XY₅ (represented as a trigonal bipyramid) combine to form a tetrahedron and an octahedron?

Hint: How many vertices do these polyhedra have?

[Rutherford]

Trigonal bipyramid has 5, tetrahedron 4 and octahedron 6. From two trigonal bipyramids, one tetrahedron and one octahedron can be formed. If  XY₅ would have to make a tetrahedron, then it will lose one Y so it will become a cation (XY₄⁺).  If XY₅ would have to make a octahedron it has to accept a Y so it will become an anion (XY₆^-). These two can combine to make an ionic compound XY₄*XY₆ or X₂Y₁₀.

[Dan]

Correct, phosphorous(V) halides can undergo autoionisation.

Se for example: http://en.wikipedia.org/wiki/Phosphorus_pentachloride#Structure

So, can you propose structures for B and F?

[Rutherford]

Do they overlap?

[Dan]

Do they overlap?

I don't understand your question.

[Rutherford]

Because of the positive charge on one P atom and the negative on the other one, a ionic bond will be formed. As the ionic bond is shorter than the covalent bond, the tetrahedron and the octahedron should overlap, but then the electron pairs would repel each other much, so I am not sure about the structure.

[kaliaden]

It's easy to find the empirical formula of each compound but how exactly did you calculate the molecular masses of X, Y and Z?

So far, i get that C is PCl₅ and E is PF₅

A and D are covalent and could auto-ionise to form B and F (which are ionic) respectively
I think it would be easier for a chlorine atom to move between two molecules of A and F since the phosphorus-fluorine bond would be stronger (PF₅ doesn't auto-ionise but PCl₅ does)
So B would be [PClF₃]⁺[PCl₃F₃]^- and F would be [PCl₃F]⁺[PCl₅F]^- right?