Diferro nonacarbonyl, Fe₂(CO)₉

Gaseous hydrogen chloride and bromide are without action upon liquid iron pentacarbonyl, but hydrogen iodide reacts with it, yielding ferrous iodide, carbon monoxide, and hydrogen. When exposed to sunlight, the pentacarbonyl, either alone or in solution, decomposes, yielding: -

Diferro nonacarbonyl, Fe₂(CO)₉, which separates out as orange-red crystals, which are stable in dry air. The reaction is as follows: -

2Fe(CO)₅ = Fe₂(CO)₉ + CO.

The electric arc only induces the above change slowly, whilst the acetylene flame has very little influence. Blue light is most effective, the activity falling as the red end of the spectrum is approached. Variation of pressure up to 125 atmospheres is without influence, whilst heating to 60° or 100° C. entirely inhibits the change. The reaction induced by light is slowly reversed in the dark. The reaction proceeds more readily in ether, alcohol, or light petroleum solution than in benzene.

To obtain the nonacarbonyl in a pure condition, the penta derivative is dissolved in dry ether or light petroleum, and exposed to sunlight. The resulting crystals are dried over sulphuric acid and solid paraffin. The nonacarbonyl is practically insoluble in ether, Ьепгепё, or light petroleum; slightly more soluble in ethyl alcohol or acetone; and more so in pyridine. Its density at 18° C. is 2.085. On warming, it decomposes at about 100° C., yielding the pentacarbonyl, free iron, and carbon monoxide. Thus: -

2Fe₂(CO)₉ = 3Fe(CO)₅ + Fe + 3CO.