Abstract. Organometallic complexes hosted inside porous frameworks are particularly interesting because of possible application in heterogeneous catalysis or electronic devices and optical materials. We present here a detailed spectroscopic investigation on the structure and reactivity towards simple reagents (such as CO) of chromocene molecules (Cp₂Cr) encapsulated into the nanovoids of two different matrices: a non polar polystyrene (PS) and a polar NaY zeolite. We demonstrate that stable Cp₂Cr(CO) complexes are formed in PS, while in NaY the presence of high internal electric fields confers to the Cp₂Cr molecules a much higher reactivity towards CO. In situ EXAFS data, combined with other spectroscopic techniques and ab initio theoretical calculations, allowed the structural determination of the reaction products and afforded the full comprehension of the complex reactivity of Cp₂Cr molecules inside the cavities of PS and NaY hosts.