The gas-phase reactivity of C5H5Mg+ complexes generated from ionized magnesocene (Cp)(2)Mg.+ have been investigated by means of tandem mass spectrometry. Decomposition of metastable C5H5Mg+ ions involves the metal-ligand dissociation and a hydrogen atom elimination leading to the formation of Mg+ and C5H4-Mg.+ ions. Collisional activation induces further fragmentations, yielding C3H3Mg+ and C3H2Mg.+ ions. To have some insight into the structure and energetics of these four series of complexes, high-level ab initio calculations (G2(MP2)) have been performed. In addition, the nature of the metal ion-ligand interaction, the charge distribution, as well as the characterization of orbitals involved in relevant systems have been analyzed by using the atoms in molecules theory (AIM) of Bader and the natural bond orbital (NBO) analysis. The Cp-.... Mg+ binding energy has been estimated to be 74.0 kcal/mol, over the cyclopentadienyl geometry optimized at the CASSCF level both for B-2(1) and (2)A(2) electronic states.