Multimetallocenes. A Theoretical Study Alejandro Velazquez, ² Israel Ferna ´ndez, ‡ Gernot Frenking,* ,‡ and Gabriel Merino* ,² Facultad de Quı ´mica, UniVersidad de Guanajuato, Noria Alta s/n, CP 36050, Guanajuato, Gto, Me ´ xico, and Department of Chemistry, Philipps-UniVersity Marburg, Hans-Meerwein-Strasse, D-35042 Marburg, Germany ReceiVed May 14, 2007 Quantum chemical calculations using gradient-corrected density functional theory at the BP86 level in conjunction with TZ2P basis sets have been carried out for the multimetallocenes CpM n Cp, where M ) Be, Mg, Ca, and Zn with n ) 2-5. The equilibrium geometries and energetics with respect to loss of one metal atom are theoretically predicted. The nature of the metal-ligand interactions between the M n 2+ and (Cp - ) 2 moieties was investigated with energy decomposition analysis (EDA). The calculations predict that the CpM n Cp species with n > 2 are thermodynamically unstable with respect to loss of one metal atom except for the beryllium compounds. The beryllocenes exhibit unusual stabilities in the gas phase for the whole series CpBe n Cp up to n ) 5. The calculations suggest that the energy for loss of one metal atom from CpBe 2 Cp is significantly higher than from CpZn 2 Cp. The energy for the metal extrusion reaction of CpBe 3 Cp is much less endothermic than for CpBe 2 Cp but it is still more endothermic than the reaction of CpZn 2 Cp. The thermodynamic stability of the higher members CpBe 4 Cp and CpBe 5 Cp toward loss of one metal atom is only slightly less than for CpBe 3 Cp, while the other multimetallocenes, CpM 3 Cp, CpM 4 Cp, and CpM 5 Cp (M ) Mg, Ca, Zn), possess little extra stabilization with respect to the dimetallocenes. The calculated reaction energies which include the heats of sublimation of the metals indicate that CpBe 2 Cp might become isolated in the condensed phase, while the prospect for CpCa 2 Cp and CpMg 2 Cp and for the higher members CpM 3 Cp, CpM 4 Cp, and CpM 5 Cp is less likely. The analysis of the metal-ligand bonding in CpM n Cp using the EDA method suggests that the interactions between M n 2+ and (Cp - ) 2 have a larger electrostatic than covalent character. The beryllocenes are more covalently bonded than the other multimetallocenes. The orbital interactions in the lower members of CpM n Cp come mainly from π orbitals, but the σ contribution continuously increases when n becomes larger and eventually may become stronger than the π contributions, which become weaker in the higher members of the series. Introduction The synthesis of the first stable molecular compound contain- ing a Zn-Zn bond unsupported by bridging ligands (deca- methyldizincocene) was reported by Carmona and co-workers in 2004. 1 The crystal structure of decamethyldizincocene reveals a pair of zinc atoms which are sandwiched between two permethylcyclopentadienyl rings (Cp*) in such a way that the Zn-Zn bond is collinear with the C 5 axes of the organic rings. The discovery of the dizincocene, the first dimetallocene, has triggered the interest of several experimental and theoretical groups in finding analogous compounds Cp* 2 M 2 , where M is any metal other than zinc. 2-21 Until today the search for stable species with the formula Cp* 2 M 2 has not been successful except for M ) Zn. However, there is a related question regarding the change of element M which has not been addressed so far: How many atoms M n can be sandwiched by two Cp* rings, yielding a stable multimetallocene, Cp* 2 M n ? 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