[Borek]

One of many halogen compounds of a metal known for toxicity of its compounds (high enough to make it a poison of choice in many famous cases, both in fiction and real life) has a molar mass of 1030.3 g/mol.

What is the metal, what is the compound?

[Borek]

No one interested in diophantine chemistry?

[fledarmus]

You got me. The heaviest metal I can think of that has been deliberately used for poisoning is polonium, and even if PoI₆ existed (which I am not sure of), that only gets to around 972. I'm not an inorganic chemist though - perhaps there is a dimetal halide that fits this description?

[DrCMS]

OsAt₄ ?

[Borek]

You don't need any fancy elements 

[fledarmus]

I'm assuming there is nothing in the halogen compound except the halogen and the metal?

Thallium, polonium, mercury, and lead are the metals I know have been used in poisonings. I can't find a sufficiently heavy halide for any of them, however. Cadmium and lead might be added to the list, although I don't know specifically of intentional poisonings with either of these, only accidental ones. Does this jog anybodies thought processes?

[Borek]

I'm assuming there is nothing in the halogen compound except the halogen and the metal?

Yes, just a metal and halogen.

[DrCMS]

Thallium Chloride Tl₄Cl₆ or Tl^I₃Tl^IIICl₆

[AWK]

Journal of Materials Science, vol. 18, issue 10, pp. 3087-3091

[Borek]

Thallium Chloride Tl₄Cl₆ or Tl^I₃Tl^IIICl₆

Yes!

When I wrote about "diophantine chemistry" it was a hint - I checked (by brute force) that there are not many compounds fitting the description and having molar mass close to that listed. Of all other Tl_nX_m type formulas the closest one to given molar mass is Tl₃F₂₂ with 1031.11 g/mol, but the formula doesn't look convincing Similarly with other metals, and list of those I checked is very similar to the one posted by fledarmus.

Journal of Materials Science, vol. 18, issue 10, pp. 3087-3091

I found the compound in Physicochemical handbook published in 1962 in Poland by WNT, so it must be known for quite some time. I have a gut feeling it could be easily synthesized in 19^th century.

[fledarmus]

I even looked for that one! I missed the subscript when I checked the formula though...

Tl₂Cl₃
          This yellow compound is formulated Tl^I₃ Tl^IIICl₆.^[7]

http://en.wikipedia.org/wiki/Thallium_halides