[Borek]

There are potentially a very large number of candidates for A any number of which could be yellow.

Care to list some of them?

Before starting the thread I was aware of two - A and B. Now I know there is a chance there exists also a third one (although I wonder if it really does - but we will get there once someone names A). If the list goes further - I would love to see other compounds.

[fledarmus]

But with some more effort you can probably square this question up

37354-82-8?

If so, sir, I must say I admire your clever use of English! I wasn't able to find the synthesis, however. Would you happen to have a reference?

[Borek]

Yes, A was intended to be an anhydrous nickel squarate.

See http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19730063214_1973063214.pdf - they started with dihydrate and were able to produce anhydrous salt.

[stewie griffin]

I struggle through this all week, then, once I'm a few steps from the end, folks jump in and help  Ah well better late than never. It would have taken me too long to get to the squarate anyway. Plus it was fun to think about during the workday
.
Dan I was aware that NiCO₄ didn't fit the description given of A...but I couldn't find any obvious alternative left at that point.

Nice problem Borek 

[Wastrel]

Now the cat is out of the bag I'll lay my cards on the table.  Is that metaphor mixed enough?  The candidate I was most happy with was nickel mellitate, which is normally hydrated and I don't know how easy it is to make in anhydrous form.  The aluminium salt is a rare mineral called honeystone and the structures may ring bells in fans of obscure oxides of carbon.  I thought about Dan's compound and the dimer.  On a more theoretical basis I also considered nickel cubane octacarboxylate and C₆₀(CO₂)₆₀Ni₃₀ and a few others but once you get it down to salts of C-CO₂ it's pretty much Lego.

[Borek]

I am an idiot. I visited mellitic acid when looking for ideas, but I got so fascinated by deltate, squarate, croconate and rhozidonate I forgot about it completely. One of the versions of the question contained determination of the molar mass through the freezing point depression, but I removed it as it could be confusing and I had no time to check details properly (does the squarate hydrolyze? does the Ni²⁺ hydrolyze? what is value of the Van 't Hoof factor?).

Initially I wanted to prepare a question about croconate, but decided putting nickel squarate and nickel tetracarbonyl side by side makes the question much more interesting.

Oh well, time to start to think about next week's problem. I am afraid it may be not that surprising - good ideas don't grow on trees