Cis-trans isomerization in crystalline [(η5-C5H5)Fe(μ-CO)(CO)]2

The cis-trans isomerization of [(eta(5)-C5H5)Fe(mu-CO)(CO)](2), well known in solution, has been studied also in the solid state. The solid-state process has been investigated on polycrystalline samples by C-13 CPMAS NMR spectroscopy and variable-temperature powder diffraction (VTXPD). It has been shown that an irreversible isomerization takes place at 80 degrees C. The reaction was also monitored by heating the sample at four different temperatures for periods of time and by determining the conversions by a C-13 CPMAS spectrum. The transformation has been shown to obey a first-order Avrami-Erofe'ev rate law typical of a nucleation and growth mechanism. By the Sharp-Hancock plot of the isotherms it has been demonstrated that the process is not isokinetic, and the Avrami exponentials indicate that it passes from a one- to a three-dimensional growth. An activation energy of 164 +/- 7 kJ/mol for the isomerization process has been found.