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