Iron carbonyl

Iron pentacarbonyl reacts with cyclopentadiene at 135° in an autoclave to give the binuclear complex [(π-C5H5)Fe(CO)2]2 as deep reddish-purple crystals which are stable to air and water. The crystal structure determination shows this molecule to be centrosymmetric with two bridging carbonyls forming a plane with the two iron atoms [Fig. 19(a)]. The rather short Fe-Fe distance of 2-49 ? and the diamagnetism of the compound indicate the presence of the Fe-Fe bond. In solution, however, it tautomerizes to the structure believed to be that shown in Fig.19(b),in which the two rings lie on one side of the mean plane formed by the bridging carbonyl groups and the iron atoms and the two terminal CO groups lie on the other side.

Mobile, yellow liquid.Evolves carbon monoxide on exposure to air or light. Soluble in nickel tetracarbonyl and most organic solvents; soluble with decomposition in acids and alkalies; insoluble in water.

Catalyst in organic reactions, carbonyl iron for high-frequency coils.

Iron pentacarbonyl is a useful dehalogenating and carbonylating agent.

Although nickel carbonyl can be obtained from nickel and carbon monoxide at atmospheric pressure and moderate temperature, the production of iron pentacarbonyl requires a pressure of 5– 30 MPa, a temperature of 150–200 C, and the presence of reactive iron. Even at high temperature and pressure, massive iron reacts sluggishly with carbon monoxide, so iron sponge, with its greater surface area, is used as starting material.

Flammable, dangerous fire risk. Toxic by ingestion, inhalation, and skin absorption.

Iron pentacarbonyl is an easily combustible substance. It does not react with water or with weak or dilute acids. With concentrated acids, the corresponding iron salts are formed with the evolution of carbon monoxide and hydrogen. Reactions with halogens yield iron halides. Iron pentacarbonyl also reduces organic compounds; for example, nitrobenzene is reduced to aniline; ketones to alcohols; and indigo to indigo white.

The trigonal bipyramidal structure has been preferred to the square pyramidal one in the interpretation of electron diffraction, Raman and infrared and thermodynamic data; the apparently contradictory dipole moment of 0-8 D being attributed to atom polarization. A single crystal X-ray structure determination at - 80°C has confirmed the trigonal bipyramidalstructure.