Periodatonickelates

The per&shy;iodato&shy;nickelates are a series of anions and salts of nickel complexed to the periodate anion. The most important of these salts are the di&shy;periodato&shy;nickelates, in which nickel exhibits the +4 oxidation state: these are powerful oxidising agents, capable of oxidising bromate to perbromate.

The first per&shy;iodato&shy;nickalates discovered were sodium nickel periodate (NaNiIO6·0.5H2O) and potassium nickel periodate (KNiIO6·0.5H2O). P. Ray and B. Sarma obtained these dark purple double salts in 1949, mixing nickel sulfate with potassium or sodium periodate and (as oxidant) a boiling aqueous solution of an alkali persulfate salt. It is now known that ozone can replace the persulfate salt, and that similar solids exist for other alkali (RbNiIO6·0.5H2O, CsNiIO6·0.5H2O, and NH4NiIO6·0.5H2O), as well as certain other tetravalent metals (including manganese, germanium, tin and lead).

The crystalline salts are insoluble in water, acid, or base. The colour is due to absorbance of visible light shorter than 800 nm, with a peak at 540 nm. The crystal structure of each has space group P312. The structure is built from hexagonal oxygen layers; in every other layer, alkali atoms fill one-third of the hexagon centers and iodine and nickel fill the remainder; in the other layers, the centers are vacant.

The di&shy;periodato&shy;nickelates, also known as dihydroxy&shy;diperiodato&shy;nickelates, contain nickel in the +4 oxidation state along with two periodate anions. A solid mono&shy;periodato&shy;nickelate salt KNiIO6·0.5H2O will dissolve in a solution of potassium hydroxide and potassium periodate to yield a di&shy;periodato&shy;nickelate solution.

The ion can form a brown salt with sodium ·6H2O), another acidic sodium salt ·H2O) and an orange salt with cobalt ). Di&shy;periodato&shy;metalates with the same formula also exist for palladium and nickel, and similar di&shy;periodato&shy;metalates can be made for Cu, Ag, Au, Ru and Os.

A di&shy;periodato&shy;nickelate will dissolve in alkaline water. Depending on pH and concentration, the resulting solution is an equilibrium between and. It is also a strong oxidiser: like very few reagents, it oxidises bromate to perbromate. In the reaction, NiIV reduces to NiIII with the release of a hydroxyl radical. The radical then oxidises bromate to a BrO42− radical, which NiIII then converts to perbromate BrO4−.