Magnesocene, also known as bis(cyclopentadienyl)magnesium(II) and sometimes abbreviated as MgCp2, is an organometallic compound with the formula Mg(η5-C5H5)2. It is an example of an s-block main group sandwich compound, structurally related to the d-block element metallocenes, and consists of a central magnesium atom sandwiched between two cyclopentadienyl rings. Properties Magnesocene is a white solid at room temperature. It has a melting point of 176 °C, though at atmospheric pressures it sublimes at 100 °C. Unlike ferrocene, magnesocene displays slight dissociation and subsequent ion association in polar, electron-donating solvents (such as ether and THF). MgCp2 <=> MgCp+ + Cp- MgCp2 + MgCp+ <=> Mg2Cp3+ MgCp2 + Cp- <=> MgCp3- While ferrocene is stable at ambient conditions, magnesocene decomposes rapidly on exposure to oxygen or moisture, and as such must be synthesized and stored under inert conditions. Structure & Bonding As revealed by X-ray crystallographic refinement, solid-phase magnesocene exhibits an average Mg-C and C-C bond distance of 2.30 Å and 1.39 Å, respectively, and the Cp rings adopt a staggered conformation (point group D5d). Gas-phase electron diffraction has shown similar bond lengths, albeit with the Cp rings in an eclipsed conformation (point group D5h). The nature of Mg-Cp bonding has been hotly contested as to whether the interaction is primarily ionic or covalent in character. Gas-phase electron diffraction measurements have been invoked to argue for a covalent model, while vibrational spectroscopy measurements have offered evidence for both. Hartree-Fock calculations have shown that, in contrast to transition metal metallocenes, the Mg 3d orbitals play no role in metal-ring bonding; instead, favorable bonding interactions with the Cp π system are accomplished by promotion of the two 3s electrons to the 3px,y orbitals. Further stabilization is afforded by back-donation from the Cp rings to the Mg 3s orbital. Such interactions afford a lesser degree of orbital overlap as compared to ferrocene, resulting in a comparatively weak metal-ring bond and a fairly high effective local charge on Mg. Experimental evidence in favor of an ionic bonding model can thus be explained by the very weak, highly polar Mg-Cp interactions. The weak nature of this bonding mode is responsible for magnesocene's relative instability and vigorous reactivity when compared to ferrocene. Synthesis High-temperature synthesis The first synthesis of magnesocene, as reported by F. A. Cotton and Geoffrey Wilkinson in 1954, involved the thermal decomposition of the cyclopentadienyl Grignard reagent. A similar procedure was offered by W. A. Barber