The volatile compound used in the Mond process for refining nickel is:

In the Mond process impure nickel reacts with $\mathrm{CO}$ at low temperature to form volatile $\mathrm{Ni}(\mathrm{CO}){}_4$, which is separated from non-volatile impurities. Heating the gas at a higher temperature decomposes it to pure nickel and $\mathrm{CO}$.